• New Demonstration: The Paramagnetism of a Dysprosium Pendulum

     Pendula of different materials are suspended here from a horizontal rod. As a large magnet is moved in next to them, we see an unsurprising response. The steel pendulum leaps over to the magnet; the wooden pendulum is entirely unaffected. There is a third pendulum, however, that appears to be slightly, but not very strongly, attracted to the magnet. The bob on this pendulum is a lump of dysprosium, as seen in our new demonstration, J5-15.

     three pendulum bobs hang near a magnet. The steel one clings to the magnet, the wooden one hangs straight down, the dysprosium lean leans slightly towards the magnet

    Dysprosium is a rare earth element that historically has been noteworthy primarily for being difficult and expensive to refine, and for having very few real world applications. It has been joked about as being one of the most useless of elements. This is not really true, though. Late 20th century chemical processes have made extracting it somewhat more practical, and it does have a few uses when in combination with other materials – it is a minor component used in making neodymium magnets, compounds of dysprosium are used in high powered floodlights (remember gas discharge lighting from last month’s blog post?), and it even has uses in making industrial radiation detectors. Mostly, though, these are often forgotten about because dysprosium appears as a small component of another chemical or system.

     Dysprosium on its own does have one interesting property, though: it is a paramagnetic material.

     Paramagnetic materials are those which are only weakly attracted to magnetic fields. The reason for this is complex, and relates to the quantum behaviour of electrons.

    Every atom in a substance has its own tiny magnetic field, determined largely by the electrons in its outer shell. In a ferromagnetic material, like iron, these tiny fields tend to spontaneously align with each other, giving the larger object a permanent, measureable magnetic field. In a paramagnetic material, however, they mostly do not interact with each other, and are oriented randomly in the material… until an external magnetic field affects them and temporarily causes them to orient themselves in line with it. Thus, a paramagnetic material will act like a weak magnet when in a magnetic field, but won’t retain that magnetism when the external field goes away.

     Interestingly, materials can switch from being ferromagnetic and paramagnetic depending on temperature. The temperature at which they make this transition is called the Curie temperature, after Pierre Curie, who performed important early research on paramagnetism in the 19th century.

     Learn more about paramagnetism 

  • New Demos: Buoyancy and Electromagnetic Forces

    We have introduced several new demonstrations recently, in a variety of topic areas. Be sure you take some time to visit our website occasionally and see what's new! Here are brief introductions to two of them.

    Electricity and Magnetism: Forces Between Current-Carrying Coils

    K1 07: two wires coils mounted side by side on a horizontal rod

    Check out K1-07 Interacting Coils, developed by student (now alumna) Sarah Monk! This is a new way of illustrating the forces between parallel current-carrying coils. This attractive new tabletop demonstration will be easier to use in many classes. Challenge your students to predict how changing the direction of the current will change the motion of the coil.

    This demonstration was itself developed in conjunction with classwork, as part of a final project for PHYS411 last semester. There are many different ways to explore incorporating demonstrations and hands-on learning activities into your classes – not only using demonstrations to illustrate concepts, but advanced students can learn a great deal by developing systems and devices themselves!

    Buoyancy, Density, and Pressure

    f2 27: a balance bears two beakers of water. In one, a hollow plastic ball floats in water; in the other, a steel ball hangs suspended in water.

    At the suggestion of Mark Eichenlaub, we have also added F2-27 Buoyancy Paradox: Two Spheres. Two identical beakers of water sit on the pans of a pan balance. In one, a ping-pong ball is tethered to the bottom of the beaker so the ball floats submerged in the water. In the other, a steel ball if the same size hangs from an overhead hook, submerged at the same height. Invite your students to predict whether the pans will remain balanced, or will show one as heavier!

    This demonstration was first developed for use in a presentation for AAPT's US Physics Team. Every year AAPT trains high school students for the International Physics Olympiad, and UMD Physics hosts their training camp in the early summer, with demonstrations and problem solving exercises.

    Physics demonstrations can enhance learning and student engagement in a wide variety of contexts. What experiments could you use to expand student interaction in your classes?

  • New Portable Ripple Tank

    UMD Faculty: Are you interested in showing wave phenomena in class, but unsure if the familiar lecture-hall sized Shive Wave Machine and Ripple Tank will fit in your seminar or discussion room? Here’s another demonstration you may want to check out! 

     portable ripple tank


    The new portable ripple tank is a small desktop-sized device we use in our physics classes here at the University of Maryland that produces waves in a small tabletop tank. It can be used hands-on for small groups to gather around in the classroom, or displayed on screen with a digital camera for larger audiences.

    We fill a small tank with water, and illuminate it with the built-in LED strobe. A small stepper motor drives vibrating needles at the edge of the tank, generating ripple patterns that propagate through the tank. The strobe rate is adjustable, allowing you to seemingly slow down or freeze the movement of the waves to highlight particular interference effects, or you can switch it over to steady illumination to show wave propagation in real time.

    Waves themselves are an important physical phenomenon regardless of medium. A demonstration like this shows how waves form in water as a result of a driving force, and are propagated through the water – but, interestingly, while the wave as a structure is moving across the tank, the water is not! At any given point, most of the water is only moving up and down in place, not across the tank. The wave is a physical phenomenon independent of the molecules making up the water. It carries energy across the tank, but mostly not the water itself.

    With two sources of wave motion, we can see interference patterns form. Where the peak of one wave meets the peak of another, a larger wave is formed; where the peak of one wave meets the trough of another, they cancel out, forming an oddly calm spot amid the rippling surface. Two waves that line up with their peaks overlapping perfectly are in phase with each other; waves where the peak of one exactly matches the trough of another are out of phase.Where this kind of interference occurs repeatedly across the surface as wavefronts interact, in phase and out of phase at different points, an interference pattern is formed, as the individual waves interfere with each other.

    waves in phase: Constructive interference (image: wikimedia commons, public domain)waves in phase: Destructive interference (image: wikimedia commons, public domain)

    Challenge your students to predict how the interference patterns will change as you vary the vibration rate!

    A valuable trait of waves is that many of the properties of wave phenomena are the same for all waves, in any medium. Just like these waves of moving water, other waves light sound waves, radio, and light propagate through space, and can interfere and form interference patterns. The Ripple Tank is a good way to introduce these concepts in class in a familiar form, and analogues can then be drawn to how these effects appear in other situations. The way waves of water diffract around a barrier is much like the way sound waves diffract around a sound-baffling wall by the highway; the calm spots formed by two out of phase sources interfering are much like the quiet zone formed by noise-cancelling headphones as they repeat a soundwave with its phase inverted to remove background noise.

    Try out a demo sometime and explore the word of wave physics. But don't get seasick!

  • Phun with Electrons: Particle or Wave?

    Welcome back to the Physics Demonstrations Blog! We’re back from the winter deep freeze and here to share more fun ideas about physics. This week, a brief look into the quantum world with electron beams!

    The electron is a fundamental particle, one of the earliest discovered in modern physics, which has a small but finite rest mass and carries a negative electrical charge.
    However, quantum mechanics has taught us that, under certain circumstances, the electron, which we conventionally think of as a particle, can also behave like a wave.

    a cathode ray tube with power supply

    A stream of electrons traveling through space means a stream of charges is traveling through space – this is, in effect, what an electrical current is. And an electrical current can be deflected by a magnet – so, too, can a beam of electrons. If you hold a magnet near a beam of electrons, like the one in this cathode ray tube, the beam will deflect in a direction perpendicular to the magnetic field.

    glowing cathode ray tube, an electron beam is green against a white screenglowing cathode ray tube, green electron beam deflected by magnet

    Likewise, if you run two electrical currents parallel to each other, the electric field of each will deflect the other slightly, which can be seen in either two wires running parallel, or by running a wire parallel to the electron beam.two parallel wires being pushed apart by the force of the currents within themglowing cathode ray tube, green electron beam deflected by the current in a parallel wire

    This beam is made up of electrons; the glow in the screen behind it is made when a few individual electrons passing by interact with the phosphors in the screen.

    a cathode ray tube with power supply, facing end-on towards the viewer

    But what happens if we take that beam and pass it through a narrow opening? Conventionally, if you throw a bunch of particles at a hole, either they pass through in a straight line, or they miss and bounce off. But instead, look what happens when we do it with electrons:

    cathode ray tube with green electron beam diffracted into ringscloseup of diffraction rings of an electron beam

    The electrons are not passing through in a straight line, but instead are diffracting, forming a pattern of rings. This is an interference pattern formed by the interaction of the peaks of a wave. We can see this frequently when light passes through a narrow opening, forming such a pattern.

    red laser light forming diffraction rings after passing through a pinhole

    But here, it is being formed by electrons – very small particles, but particles nonetheless. Thus, here we see an electron behaving as a wave.

  • Physics Teatime 2: On The Making Of Tea

    We talked before about the shape of the teapot; now, another question has come up online: Why does your tea water heat differently in the microwave than in an electric kettle?

     Several years ago, Nadia Arumgam mentioned this problem in an article for Slate on making tea. One thing that contributes to the taste of tea is how hot the water is when it is brewing. When you heat water with an electric kettle or on a stove, the heat source is at the bottom. The hot water at the bottom rises, forming convection currents, drawing the cold water downwards to be heated in turn. Over time, the entire pot of water reaches the same temperature. When you see the water boiling at the surface, you know that the entire container is boiling.

    In a microwave oven, though, the molecules are heated by electromagnetic agitation at points throughout the container of water, with warm and cool spots. Some of these points may begin to boil while other areas of the water are still cool. When you take the water out and make tea, it may not be as hot as you intend.

     Let’s see what this looks like on a larger scale. We have a Dewar flask, an insulated glass container, filled with water. We lower a small heating element into the water so that it heats the top layer of water. We can lower a thermometer probe down to the bottom of the water.

    container of water with a heating element at the top and a thermometer probe at the bottom; top of water is boiling but temperature at bottom is displayed as twenty-three celsius

    As you can see in the picture, the bottom layer of water is still cold! 23°C, right about at room temperature, and not very appealing for a cup of tea.

    The top is boiling; when we move the probe to the top, the temperature is much higher, 97°C. (Not quite 100°C, though, and here we’re only a few centimeters from the heating element.)

    container of water with a heating element and a thermometer probe at the top; temperature at top is displayed as ninety-seven celsius

     This can be a surprising result. We are used to thinking of water as conducting heat well, and it certainly does conduct heat better than air or glass; but not nearly as well as metals or many other materials. In most circumstances, though, most of the heat transfer in water isn’t from conduction, but from convection. In a big container, if your heat source is near the top, there may not be enough convection to make up the difference.

    So put on the kettle, curl up with a nice cup of tea, and enjoy the snow!


  • Physics Teatime 3: Do Not Try This At Home

    This week has seen some chilly weather on campus here, and with it a return to warm beverages and the physics behind them. So, for this week, we’re going to look at a bit of physics we heartily recommend you do NOT try at home: superheating water.

     Title: Making Tea with Super Heated Water (overlain on a plate)

    In this video, taken by our own Don Lynch, you will see a mug of water that has just been heated in the microwave. Note that the mug is freshly cleaned, smooth, and cylindrical. The water is quite still; but when a teabag is dropped in, suddenly the water bursts into boiling! After a few seconds, the boiling stops, and we are left with what appears to be a cup of tea. (Pity about the mess on the plate beneath.)

     A cup of water, just heated but not visibly boiling

    Normally, when water reaches its boiling point, it bursts into bubbles as the liquid water begins to turn into a gas. These bubbles usually first form around nucleation sites, tiny (or not so tiny) impurities in the water. The bubbles expand as the force of the vapor pressure of the steam inside the bubble exceeds the external forces of atmospheric pressure and the surface tension of the water. As the bubbles expand, their internal forces exceed the external forces more rapidly, and so once the boiling process has begun it accelerates and the entire liquid boils.

     A cup of water with a teabag, boiling vigorously

    Occasionally, though, if there are no nucleation points and the water is not agitated, no single tiny bubble manages to overcome the atmospheric pressure and surface tension to start the liquid boiling, even though the temperature is at or even slightly above the usual boiling point. When the water is disturbed (in our case, by dropping the teabag in), suddenly there are lots of nucleation points, and the boiling begins in earnest.

     A cup of tea, with some spilled onto the plate beneath

    We don’t know exactly how hot the water is here; if we put a temperature probe in the container, that itself would trigger the boiling effect before we could make a measurement! But we suspect that it is probably only very slightly above the boiling point. It is certainly not as hot as the more conventional “superheated” water one might find by heating water under pressure – but it is still more than hot enough to cause some very bad burns, so please don’t try to make your tea this way! Take a few more seconds and do things the old fashioned way, and enjoy a relaxing cup of tea and a pleasant winter break.

    See you next year!


  • Seeing Sound: Vibrations on a Plate

    This week, a tweet by Simone Schnall of Cambridge University went viral on Twitter, sharing a video by British science presenter Steve Mould

    Coincidentally, this week also marks the birthday of Ernst Chladni, for whom this phenomenon is named- Chladni Figures. 

    sand forms patterns on a square steel plate as it is stroked by a violin bow

    Sand is sprinkled on top of a plate. As it vibrates, the sand traces out the pattern of node and antinodes, accumulating along the lines where the plate is at rest, and being driven away from the areas where the plate is moving up and down with the sound wave. In its simplest form, the plate would be clamped at the center and driven by the bow at one edge; as you change the bow position, you can excite different vibrational modes of the plates and form different patterns. But pressure elsewhere on the plate, even something as simple as pressing your thumb against the edge, can form a node and thus change the pattern of standing waves.

    Of course, this can be made far more complex, and the formation of these patterns can be used to staudy how different objects vibrate in different conditions. This is still used today to help in the design of musical instruments.

    If you are teaching this topic at UMD, consider using our own set of Chladni plates, and invite your students to try it for themselves. Or, for greater complexity (and volume), try the oscillator driven version. By using an audio oscillator to drive the plate from the center, a wide range of modes can be observed by carefully varying the driving frequency. For this version, we can also offer a wider variety of plate shapes, including a model of a violin back. 

    g4 22hg4 22i

    To learn more about Ernst Chladni, Chladni Plates, and the history of acoustics, visit these pages at the Smithsonian and at Cambridge's Whipple Museum.


  • Space News: The Nancy Grace Roman Space Telescope

    NASA announced last month that their upcoming infrared observatory project had been named the Nancy Grace Roman Space Telescope. Today, we’re going to take a brief look at this project, Dr. Roman, and the role of physics in astronomy.

     NR-WFIRST spacecraft modelNR-WFIRST primary mirror assembly

    Nancy Grace Roman (1925-2018) was the first Chief of Astronomy for NASA, and the first woman ever to hold an executive-level position there. A graduate of Western High School in Baltimore, and later of Swarthmore College and the University of Chicago, she began her career in astronomy research specializing in the emission spectra of stars.

     spectrum of Oxygen - image credit McZusatz

    Spectroscopy is the study of the spectrum of light, the individual frequencies and colors that make up the light we see. Every element and compound emits its own distinctive pattern of frequencies of light, based on the structure and energy of the electrons within the atom. By analyzing the light from stars, we can use these distinctive patterns as a kind of fingerprint to identify the chemical makeup of distant stars and planets. Our demonstration collection at UMD Physics has many demonstrations about spectra and spectroscopy; be sure to click here and check them out!

     Not all light is visible to our eyes; lower frequency light is below the range that we can see, in infrared wavelengths. Space telescopes are often valuable for observing this invisible light without interference from our atmosphere, just as they are in the visible spectrum.

     Dr. Nancy Roman with an early satellite model

    Despite her skill as a researcher, the widespread discrimination against women in the sciences made it difficult for Dr. Roman to advance her career in academia. Eventually, she moved to working in government instead, first joining the Naval Research Laboatory and then NASA. She essentially created the astronomical science program at NASA, plotting its course for decades to come. She was a key player in the development of many of NASA’s research satellites, including Uhuru, the first X-ray astronomy satellite; and she was a leader in the creation of the Hubble Space Telescope. She advocated that NASA science should be for everyone, and ensured that their research and data were publicly available.

     Nancy Grace Roman in 2015

    Dr. Roman retired from NASA, and began a second career in scientific computing. She learned programming at Montgomery College and went on to work as a contractor specializing in scientific data management, eventually returning to NASA as a contractor to manage the Astronomical Data Center at Goddard Space Flight Center.

     Dr. Roman worked hard to inspire more women to become scientists and leaders in science, and we can hope to follow in her footsteps.

  • STEM News Tip: Children Learning About Gears

    This week we’re introducing a new feature of the blog: STEM News Tips. Every week we’ll be bringing you short introductions to recent publications and current events in physics, physics education, and adjacent fields.

     This week, we’re checking out a new paper from Timo Reuter and Miriam Leuchter in the Journal of Research in Science Teaching. ( In Children's concepts of gears and their promotion through play, Reuter &Leuchter surveyed how young people came to understand a simple machine through hands-on interaction. Interactive and experiential learning is a vital part of modern STEM education, and something we try to promote through our demonstrations and programs.

     The relationship between turning direction, turning speed, and the arrangement and size of interacting gears is a basic concept in introductory mechanics. Before students can truly comprehend the mathematics of force and torque, they need to have an intuitive familiarity with how such objects interact in the real world. It’s valuable to take a look at how young children begin to grasp these concepts years before they come to high school and university physics classrooms.

     Check out the article here!


  • STEM News Tip: Measuring the gravitational force of tiny masses

    A recent publication announced some remarkably fine new measurement of the gravitational attraction between extremely small objects. Also reported in Scientific American, the Aspelmeyer group at the University of Vienna has measured gravitational interactions between millimeter scale objects. You can read all the details in the papers below.

    B. Brubaker, Physicists Measure the Gravitational Force between the Smallest Masses Yet, Scientific American, 2021 March 10

    Westfal, Hepach, Pfaff, & Aspelmeyer, Measurement of gravitational coupling between millimetre-sized masses, Nature, 20201 March 10

    Aspelmeyer Group at the University of Vienna


    If you’re discussing this kind of measurement in class, check out our model of the classic Cavendish Experiment, E1-01 in the demonstration collection.


  • STEM News Tip: Meteor Showers and Rain Showers

    There's a lot happening overhead this week!

    The Perseid Meteor Shower is at its peak this week. Meteor showers are a stunning sight in the night sky, and the Perseids are one of the brightest of the year. The sparks of light you see in the sky are the burning debris from past passages of Comet Swift-Tuttle, which passes through the solar system every 133 years... but we pass through its path in August every year! Read more about the Perseids at NASA's Solar System Exploration pages.

    Ordinarily, this would not have been the best of years to see the Perseids anyway, since the moon is full, and its bright light would make them hard to see. But it has turned out to be a moot point in our region, as our skies are darkened by a phenomenon much closer to home: Tropical Storm Isaias. This storm has been making its way up the coast and is now dumping quite a lot of rain on us, so everyone please be cautious! These storms can be dramatic to watch, but also potentially dangerous.

    Hurricanes and tropical storms are driven by converging winds and moist air over warm water. Climate change in recent years has warmed the waters of the ocean - by only a small amount, but in a large and complex system, a small change in initial conditions can have a dramatic effect on the outcome! You can see this modeled in the classroom with demonstrations like our chaotic pendulum, G1-60; be sure to check out the simulation linked there that lets you experiment with tiny changes in the initial conditions. 

     Keep your eyes on the sky - but be careful out there!

    Read more:

    National Hurricane Center

    NOAA video: Fuel for the Storm

    NASA: In Depth on the Perseid Meteor Shower Perseid Meteors 2020

    Double Pendulum Simulator by Erik Neumann


  • STEM News Tip: New Gravitational Waves Discoveries!

    UMD Physicists are heavily involved with the LIGO collaboration, the Laser Interferometer Gravitational-Wave Observatory that detects and analyzes gravitational waves to study distant celestial phenomena. Several recent papers have announced important new findings. One highlight is the observation of merging black holes including the largest one yet observed in such a merger.

     The merger of these massive objects distorts spacetime around it, creating a ripple that we can detect here on earth through the use of extremely precise interferometery. Some of you may recall presentations we hosted a few years ago when LIGO announced their first detections. New research from this team is coming in all the time!

     Read more about recent discoveries:

    More places to visit:


  • Teatime in Physics

    A question that came in via Twitter recently is one that comes up a lot this time of year, as we tend to want to spend more and more time curled up with a warm beverage. How does my little round teapot fill up so many cups? And why is the tea in the pot still warm when the tea in my cup has gone cold? The answer comes down to geometry!


    Here’s a pretty ordinary sized teapot from the cabinet, and an official UMD Physics mug. We’ve tested it twice today and confirmed: This teapot can fill this mug six times. Sure, the pot is bigger than the mug, but it doesn’t look that much bigger, right?


    The teapot can even fill this bigger UMD Physics travel mug four times! How?


    The answer is related to what biologists call the Square-Cube Law. As an object grows in size, its volume increases faster than its surface area. If you take a cubical container and double its length, width, and height, multiplying by 2 in each direction, then its surface area is multiplied by 4, the square of 2. But its volume is multiplied by 8, the cube of 2. The exact numbers will change, though, depending on the shape of the container. Every shape has its own relationship among liner size, area, and volume. As it turns out, the most efficient shape, with the highest ratio of volume to surface, is a sphere.

    This sounds like just abstruse math, but it actually explains a lot about things we deal with every day, from teapots and fuel tanks to giraffes and polar bears. (OK, maybe not all of us deal with polar bears every day, but it’s good to know about them anyway.)


    Here’s an example from the demonstration collection. This round flask and this tall cylinder each hold the same volume of water, 500 milliliters. The cylinder is much longer and narrower than the sphere, so it looks bigger, but it has the same volume!

     A1 32 1A1 32 2

    One thing that makes this interesting is that, having a larger surface area, the cylinder is also heavier. It takes more glass to make a 500mL cylinder than to make a 500mL sphere. That might not matter much for our purposes, when we just want one container to sit on the table, but it can make a big difference in large storage containers, or in places where weight is important, like spacecraft.


    This is also why fluids in free fall, like raindrops, form into spheres. The surface tension of the liquid is pulling inwards, compressing the surface to the smallest area for that volume of water: a sphere. On a larger scale, this even happens to big accumulations of rock, pulled in by gravity over a long period of time. We call them planets – and luck for us, they do tend to end up round!


    That’s all interesting, but isn’t my tea getting cold after all this?


    No, and here’s why: The total amount of heat in the container is proportional to its volume. But the radiation of heat away from the container is proportional to its surface area. So my nearly spherical teapot loses heat a lot more slowly than that tall cylinder does. Plus, because there’s less surface area for the same volume, we can make the walls thicker for the same weight, giving it better insulation.


    And that’s where the polar bears come in. (Not literally, polar bears should not drink tea.) Ever wonder why so many animals in warm climates evolved long, lanky builds, while arctic animals tend to be rounder? A lot of it comes down to heat. A round polar bear loses heat a lot more slowly, so they can burn fewer calories to stay warm. That can be important in the long winters when there’s not much to eat. In a hot climate where the bigger problem is staying cool, many animals tend to be thinner. Others find other ways to increase their surface area, like the big ears on an elephant, to radiate heat away faster. There are lots of other factors at play in evolution as well, of course, but heat is always an important one.

    This relates to why animals only come in certain sizes, too. If you scale up an ant 100 times in each direction, its mass increases by one million - but the surface area of its legs doesn't, so it can't stand up!


    So sit back, make a pot of tea, and curl up with a good book about somewhere warmer. And spring will be here before you know it!


    (Note: No tea was harmed in the creation of this blog post. But quite a lot of it was consumed.)

  • The Physics Soda Can Returns: Electrostatic Induction

    Today we introduce another new demonstration*, this time on electrostatics. Remember that soda can from last month? It has now dried out and has returned for another adventure!  

    An empty soda can, a hard rubber rod, and a piece of woolen fabric

    Some materials, when rubbed together, build up an electric charge through contact; this phenomenon is known as triboelectricity. In the case of the materials we have here, after the rubber has extensive contact with the wool, the rubber is charged with an excess of electrons, giving it a net negative charge. If you touched the rod with your bare hand, you might feel a faint shock as that excess charge jumped to you. (Perfectly safe, just a little surprising!)

    In this case, though, we're going to be careful not to discharge the rod too soon. Instead, we hold the rod carefully near and parallel to the soda can, being careful not to touch the rod to either the can or the table. As we do, we see an unusual phenomenon: the can will slowly start rolling towards the rod! We can even slowly pull the can across the table as it seemingly chases the rod about. How can this be?

    What we are seeing is a property known as electrostatic induction. Left to its own devices, the can has no net charge - it has an equal number of positive and negative charges, distributed evenly throughout the material of the can. But because the aluminum can is an electrical conductor, some of those charges can be free to move around if acted upon by an outside force. When the negatively charged rod is brought near the can, the electrostatic force (or Coulomb force) that this negative charge exerts affects the atoms in the can. Some of those free electrons, also carrying negative charge, are repelled and move to the far side of the can. This leaves the side of the can nearest the rod with a net positive charge, which is now attracted to the rod! And so the whole can, being quite lightweight, can slowly start moving towards the rod. 

    Try this out in your own classes with demonstration J1-14: Electrostatic Induction - Attracting A Can.


     (*New to us; credit for this development goes to the Physics Instructional Resource Association)


  • The Tablecloth, the Coin, and Other Adventures with Inertia

    We’ve all seen this classic stage magic trick: You arrange a nice table setting, with plate and cup and silverware and maybe a nice vase of flowers, on a pretty silk tablecloth. Then you yank the tablecloth out from underneath, but the dishes all stay on the table! We even have a video clip of it here.

    A nice table setting on a red tablecloth. Someone is about to pull the tablecloth away.

     Newton’s First Law of Motion states that an object’s velocity is constant unless there is a net force acting on it. What this means is that if an object is not moving (at rest), it will not start moving until there is a force pushing or pulling on it. If an object is moving at a constant speed and direction, it will keep going with that same speed and direction unless a force pushes or pulls on it to change that. An object’s inertia is its resistance to changing its velocity, e.g. how difficult it is to start it moving from rest.

    So that’s what’s going on here. We are applying a force to the tablecloth, pulling it away. But so long as we have a smooth, unwrinkled tablecloth, we’re not applying any force to the dishes, just the cloth. So the cloth moves away quickly, but the dishes stay where they were. This is a very popular way of demonstrating inertia, and you can find it on our website.

    The dishes do have a force pulling straight down on them, of course – the force of gravity. They aren’t actually moving downwards because they can’t go through the table. Technically, the table and tablecloth exert an upward force on the dishes, which is formally called the normal force (“Normal” here is using an archaic definition that means “perpendicular to the plane,” not what we usually mean by normal). The normal force here is equal and opposite to the force of gravity, counterbalancing it and canceling it out, so the dishes don’t move. When the tablecloth goes away, the dishes are for a moment not touching anything, so gravity does pull them down a very short distance, the thickness of the tablecloth, and they hit the table with a clatter. But the point is that they don’t follow the tablecloth off the table.

    But there can be some more complex issues at work here. Why does it matter that the tablecloth is smooth and unwrinkled? Why do you have to pull quickly on the tablecloth, and not slowly? Why do we sometimes see this trick fail, and end up with the dishes all breaking on the floor? (Please don’t do that.) Perhaps the physics is more subtle than it first appears.

    One issue to consider is that force and acceleration are vector quantities; they have a magnitude and a direction. Remember the note above that the “normal” force from the table is called that because it’s perpendicular? That matters here. Newton’s law says that the object is at rest stays at rest until acted upon by a force. That force will then give the object an acceleration in a particular direction. The force we’re putting on the tablecloth is a contact force – we pull on the tablecloth, and it experiences a force in the direction we pull. The tablecloth and table are pushing straight up with a normal force so long as the dishes are touching them. Gravity is different – gravity acts at a distance, and pulls down on things regardless of whether or not they’re touching something.

    So gravity is pulling the dishes straight down, the table is pushing the dishes straight up. We pull sideways on the tablecloth. Nothing is pushing sideways on the dishes, though, so they only feel forces going up and down. But! If the tablecloth is wrinkled, then when that wrinkle is pulled against the side of the dishes, it’s now transferring that sideways force we put on the tablecloth into the side of the dishes, and pushes them along… which could mean they follow the tablecloth off the table and onto the floor. So the tablecloth needs to be flat, so that we’re only applying sideways forces to the tablecloth, and no sideways force vectors get a chance to interact with the dishes.

    But there’s something else in play here, that makes this complicated: Friction. Specifically, contact friction. This is a force that acts between two object that are touching each other. As we’re pulling the tablecloth past the plate, there is a frictional force between the bottom surface of the plate and the top surface of the tablecloth. How large this force is is partially dependent on the properties of the surfaces and how they interact, what we call the coefficient of friction. This is essentially a way of measuring how rough or smooth the junction between two things is. So using a smooth silk or rayon tablecloth will give a very low coefficient of friction, while a rough wool or linen tablecloth might result a higher one, and thus exert more sideways frictional force on the dishes. If this frictional force is too high, it might be enough to overcome the inertia of the dishes, and drag them along.

    Let’s look at a simpler demonstration in our collection, with fewer variables to worry about. Demonstration C3-01 is quite simple: A small steel sphere rests on a stiff piece of plastic, which is resting on a steel stand. Attached to the stand is a spring that you can pull back and release so that it hits the plastic. When it does, the plastic flies out from underneath the sphere, and the sphere drops down and rests on the stand, right below where it was before.

    cA small steel ball rests on a piece of plastic, atop a stand. A metal spring is mounted next to it.

     This demonstration can be valuable in the classroom if you want something a little easier to use than the tablecloth. And it’s also a bit more portable. And, crucially, you can try this at home with materials you probably ready have!

    Put a cup or glass on the table, with a playing card on top of it. The card can be from a regular deck of cards, or Yi-gi-oh or Pokemon, or a baseball card or a card from a boardgame, or whatever else is handy; it just needs to be a nice stiff card. Now, get a coin and put it on top of the card, so that the card is holding it above the mouth of the cup.

    A coin rests on a card, which is resting on a water glass.

    Now, flick the edge of the card with your finger, quickly and firmly, as seen in this video. You may need to practice this a few times, but once you get the hang of it, the card should fly straight out from under the coin and flutter to the table or the floor. And the coin drops straight down into the cup, just like the steel sphere, and just like the dishes on the table!

     Inertia wins again.


    Watch for more secrets of the physics behind these demonstrations in Part 2 of this series, coming soon!


  • Welcome to the Blog! - second example

    Physics (from Ancient Greekφυσική (ἐπιστήμη)translit. physikḗ (epistḗmē)lit. 'knowledge of nature', from φύσις phýsis "nature"[1][2][3]) is the natural science that studies matter[4] and its motion and behavior through space and time and that studies the related entities of energy and force.[5] Physics is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves.[a][6][7][8]

    Physics is one of the oldest academic disciplines and, through its inclusion of astronomy, perhaps the oldest.[9] Over the last two millennia, physics, chemistrybiology, and certain branches of mathematics were a part of natural philosophy, but during the scientific revolutionin the 17th century, these natural sciences emerged as unique research endeavors in their own right.[b] Physics intersects with many interdisciplinary areas of research, such as biophysics and quantum chemistry, and the boundaries of physics are not rigidly defined. New ideas in physics often explain the fundamental mechanisms studied by other sciences[6] and suggest new avenues of research in academic disciplines such as mathematics and philosophy.

    Advances in physics often enable advances in new technologies. For example, advances in the understanding of electromagnetism and nuclear physics led directly to the development of new products that have dramatically transformed modern-day society, such as televisioncomputersdomestic appliances, and nuclear weapons;[6] advances in thermodynamics led to the development of industrialization; and advances in mechanicsinspired the development of calculus.

  • Women Nobel Laureates in Physics

    Earlier this month, Prof. Donna Strickland, along with two other physicists, won the Nobel Prize in Physics for her work with lasers. She is the third woman to win the Nobel Prize in Physics in over 100 years of it being awarded. (Irene Joliot-Curie won the Nobel Prize in Chemistry, though she was primarily a physicist by profession by our modern standards.) Let's take this opportunity to look back at these three women (and hope that this list gets longer soon!)

     Nobel Prize Medal - image public domain in US

    Marie Skłodowska Curie is perhaps the best known woman in the history of physics. She shared the Nobel Prize in Physics in 1903 with Pierre Curie and Henri Becquerel for their research on radiation. She later went on to win the Nobel Prize in Chemistry in 1911, for her work on radium and polonium.


    Maria Goeppert-Mayer won the Nobel Prize in Physics in 1963 for her development of the nuclear shell model, sharing the prize with Eugene Wigner and Hans Jensen. Her work provided the first clear explanation for the stability or instability of different atomic nuclei, by showing that the nucleus could be modeled as a series of shells of nucleons with coupled spins. She is the only woman to win the prize for theoretical rather than experimental work.


    This year, 2018, Donna Strickland won the Nobel Prize in Physics for her development of chirped pulse amplification in lasers. This is the method used in most modern high-powered laser research installations to increase the energy available in ultrashort bursts.


    This is an appallingly short list for over a century of work, and provides an excellent opportunity to talk with our classes about the history of physics and about the challenges and discrimination women still face in our field. Consider: What can we as scientists and educators do to make our field more welcoming and inclusive?



    Some resources:

    UMD Women in Physics

    APS Women in Physics

    The Nobel Prize in Physics in 1903

    The Nobel Prize in Physics in 1963

    The Nobel Prize in Physics in 2018

    Nature: Donna Strickland on her work and on the under-representation of women in physics

    Particles for Justice