This little toy, C7-18 in our collection and sold in many shops as the Astro Blaster, is a fun way to demonstrate some interesting and complicated collision physics. John Ball presents it in this video:

 This device has four balls of graduated masses on a central shaft. When the whole assembly is dropped, the smallest ball on the end flies with considerable velocity, potentially rising to significantly greater than the initial height.

 The balls are highly elastic, and when they collide, they transfer much of their energy to the smallest ball, which has a slightly larger hole in it and thus is the only one free to move off the shaft. Since it now carries the kinetic energy of the greater mass of falling balls, it can bounce higher than it started! Meanwhile, the rest of the balls fall quietly (more or less) to the surface.

It is believed that the use of this kind of collision in physics classes was initiated by Stirling Colgate of Lawrence Livermore National Laboratory. It was also popularized by an article in The Physics Teacher by Richard Mancuso and Kevin Long,  https://aapt.scitation.org/doi/abs/10.1119/1.2344238 .

 Once you’ve seen it in action in the video, you can also try it out in this simulation. Or try it at home with a couple of elastic balls, one high mass and one low mass, such as a small rubber ball and a well inflated basketball. If you can drop them together (this takes practice), you should see the smaller ball bounce away with much greater velocity. Just try not to break anything!

Today we’re looking at an exciting demonstration of electromagnetic induction: The Eddy Current Pendulum, seen in this video starring physics student Dan Horstman:

We have a very strong permanent magnet mounted at the bottom of an aluminum stand. We can install a variety of pendula to swing from the top of this stand. As the pendulum swings, the bob passes between the poles of the magnet. With a wooden bob, the pendulum swings freely, just like we would expected it to do without the magnet there; this is unsurprising.

A conductive copper pendulum bob, though, shows very different behaviour. While copper is not innately attracted to a magnet the way iron is, it is an electrical conductor. As the copper plate passes through the magnetic field, it experiences a changing magnetic flux. The laws of electromagnetism tell us that a conductor in this situation will have an induced electric current.

Loops of current, called eddy currents, form in the pendulum bob. These currents have their own magnetic fields, which interact with the magnetic field of the permanent magnet and slow the pendulum’s motion. The energy of the pendulum’s motion is gradually dissipated in this way, and the pendulum slows and stops.

This is a nice way to see electromagnetic induction in action. This effect also has many practical uses. Just like the magnetic field slows and stops the swing of the pendulum, eddy currents can be used to make brakes for vehicles! Automobiles, trains, and even roller coasters can use this process to slow their wheels without friction, reducing wear.

But there are other cases where you actively want to prevent eddy currents – if you’re trying to avoid losing energy! For example, conductive components of electrical transformers might be made with insulating gaps to make it harder for eddy currents to form, so you lose less of your electrical energy to heating up the transformer. Metal pendula with interruptions can model this behaviour as well.

Read more:

• Visit the demonstration page

• Video: Dan Horstman

• Learn more about industrial applications of eddy current braking at Wikipedia

Welcome to the latest Demo Highlight of the week! This week, we’re taking a look at a popular demonstration used to introduce the concept of energy: C8-04, the Hill Track. Dave Buehrle introduces it in the video below.

The ball starts out with a certain amount of gravitational potential energy based on its height above the base. As it rolls down the track, it converts this potential energy into kinetic energy. This includes both the kinetic energy related to its linear motion along the track, and the rotational kinetic energy of the ball spinning as it moves. So if the ball is released on the high end of the track from a height exactly equal to the height of the hill in the middle, it doesn’t quite make it over the hill, it doesn’t quite reach that same height. Some energy has been lost to friction, but importantly, some energy is still in the form of kinetic energy as the ball is still rotating. Thus, for the ball to get over the hill, it has to start out slightly higher than the hill to compensate for this.

The PhET Collection at the University of Colorado has a simulation related to this demonstration. The Energy Skate Park simulator (https://phet.colorado.edu/en/simulation/energy-skate-park) is a simpler (non-rotating) system that lets you experiment with a simulated skateboarder on a variety of tracks, including one with a hill in the middle. Try it out at home, and see how the initial position affects how your skater moves!

This demonstration, though simple, is also used in advanced classes. You can see it cross-listed as demonstration P2-41, and it is often used in the teaching of quantum mechanics to illustrate the concept of potential wells.

This week marks the birthday of Émilie du Châtelet, French philosopher and scientist best remembered today for first developing the concepts of kinetic energy and the conservation of energy in physical systems.

Born Gabrielle Émilie Le Tonnelier de Breteuil in Paris in 1706, Émilie was the daughter of prominent couriters. She had an early talent for both languages and mathematics, and was fortunate to have parents who could provide her with tutors in a time when such topics were rarely available to women. At the age of 19 she married the older Marquis du Châtelet; his work kept him away on his travels, and she devoted much of her time to mathematics, philosophy, and the arts.

Émilie du Châtelet’s training in languages enabled to read Isaac Newton’s recently published Principia Mathematica, and she translated it into French (her translation remains the standard French version of the text to this day). In her studies and experiments on falling masses, she extended Newton’s concept of momentum to postulate a separate quantity that was not proportional to velocity, like momentum, but the square of the velocity – which we now know as kinetic energy. From this eventually developed the implication that energy is a constant quantity in a system that could be conserved, though the full mathematical understanding of this had to wait another two hundred years for the work of German mathematician Emmy Noether.

She also studied the physics and chemsitry of compusion, in part in collaboration with the philosopher (and her occasional partner) Voltaire, and published an essay on the topic in 1744.

Émilie du Châtelet was also famous in her day not only as a philosopher and scholar, but as a socialite and patron of the arts, fond of carousing, gambling, and drama. Perhaps some of her spirit lives on in our students today.

Happy Labor Day to all in the US!

(and belated greetings to readers elsewhere for whom Labor Day was in May.)

This holiday celebrates all kinds of work, but in physics Work is a more specific and mathematically defined quantity Today we’re taking a look at a couple of recent articles in The Physics Teacher that related to the concept of work in the classroom.

In physics, work is the energy transferred to or from a body via the application of force over a distance. Positive work is work done in the direction of motion, negative work is done in the opposing direction (or has components in those directions, in a 3-dimensional system). We sometimes, conversely, refer to energy as the ability to do work.

A recent paper in The Physics Teacher by G. Planinšič & E. Etkina, “Boiling water by doing work” (https://doi.org/10.1119/5.0049411), shows some of this relationship between work and energy. As work is done on a system, moving a rope, some of the energy in the system is dispersed as heat. This heat is then seen to boil water. In their videos (linked in the article), you can see the relationship between work done on the system and temperature, and calculate for yourself the efficiency of energy conversion.

A paper in The Physics Teacher last year by P. Gash looks at potential energy and work in a more familiar mechanical system: a Slinky. In “A Slinky’s Elastic Potential Energy” (https://doi.org/10.1119/1.5145416), we can see a detailed breakdown of the forces acting on the coils of a Slinky. You can check out the experimental data for yourself and calculate the work done by gravity on the spring. It’s a handy reminder that work in the physics sense doesn’t always mean human labor!

We have many demonstrations in our collection relating to physical work. Section C8 of our demonstration index is all about the mechanics of energy, power, and work; all are useful in the classroom, and many can be tried at home as well with materials you have on hand! There are many other demonstrations that explore energy and work as well, from thermodynamics experiments like the one in the paper above, to the newly repaired J4-31: Energy Stored in a Capacitor, which shows a capacitor that holds enough energy to let a motor do the work of lifting the capacitor itself against gravity. Now that’s a nice bit of work!

With the growing attention on energy and climate change, there has been some renewed interest in the uses of nuclear power. Here are some new educational resources available for exploring this subject in class or independently.

• The Navigating Nuclear website is a joint project of the American Nuclear Society and Discovery Education. They present the new Nuclear Reimagined Virtual Field Trip video, with a pdf educator guide and links to further resources.

• At the other end of the process, the Physics Central Physics Buzz blog presents The Steps to Closing a Nuclear Power Plant, exploring Pennsylvania's Three Mile Island plant as it’s one year into its multi-year project of shutting down and cleaning up. Physics Central is an outreach project of the American Physical Society

• Delft University of Technology (TU Delft) has released one of their OpenCourseWare online classes, Understanding Nuclear Energy.

This week we expect to see the launch of NASA’s latest mission to Mars! The launch is scheduled for the morning of July 30; see the countdown at undefined . The lander is expected to then touch down on Mars in February 2021.

This is an unusually complex mission, featuring two separate mobile units. The Perseverance rover is a heavier redesign of the previous Curiousity rover, with a similar plutonium-based radiothermal generator for power. In addition, Ingenuity is a new helicopter-style flying drone that is carried by the rover, with an array of sensors and cameras. This is an experimental model; if it works as anticipated more complex aerial units may be added to future missions.

Get ready to watch the launch Thursday, and keep your eyes on the sky!

Further reading:

NASA Mars 2020 mission page

 https://mars.nasa.gov/mars2020/

NASA  video

 https://mars.nasa.gov/mars2020/timeline/launch/watch-online/

Dr. Baranwal of the Department of Energy on the RTG power source for the rover 

Image credit: NASA