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PHYS141

  • D1-40: CENTRIPETAL FORCE ON ROTATING RUBBER BAND

    D1-40
    Demonstrate centripetal force and centrifugal reaction.
    A rubber band is stretched around a disc which can be rapidly rotated. Use stroboscope to stop the motion, showing that the rubber band stretches when the disc is rotated. The stretch becomes greater as the angular velocity is increased.
    D1, N2, LS1

  • D1-41 ROTATING WATER BUCKET

    D1-41
    Demonstrates centripetal force and centrifugal reaction
    Put some water in the bucket and rotate the bucket in a vertical circle over your head. The centripetal force provided by your arm keeps the water bucket moving in a circle, while the centrifugal reaction keeps the water in the bottom of the bucket, as long as the rotational velocity is sufficient.
    D1

  • D1-52: FAIRGROUND ROTOR

    D1-52
    Illustrate the application of rotational forces
    Place the little people onto their platform against the wall of the rotor and start the rotor in motion. A trip switch (just to the left of the peole in the close-up photo) removes the floor, but the people remain pinned against the wall due to centrifugal reaction, and do not fall due to friction with the wall of the rotor. When the rotor slows down, the frictional force of the people against the wall becomes less than component of the gravitational force down the wall and the people fall off the rotor. The entire assembly may be tilted while the rotor is in motion without causing the people to fall off.
    OS1

  • D1-53 LOOP-THE-LOOP

    D1-53
    Demonstrates centripetal force and conservation of energy in a rotating object

    This track can be described as three segments: the long upright segment, the loop, and the shorter upright segment. If you begin by placing the ball on the long upright segment at a height equal to the height of the loop (2R), the ball will roll down the track, begin to climb the loop, and then fall off and roll away. You can then repeat this at increasingly higher positions until the ball makes it all the way around the loop and begins to climb the shorter upright segment. In either case, be ready to catch it as it falls off afterwards!

    This is a good demonstration to encourage students to make predictions about its behaviour. Invite students to make arguments about what starting height will allow the ball to complete the full loop. A meter stick can be additionally provided upon request to aid in measuring the height.

    Background
    Motion of the ball down the track and around the loop-the-loop can be described in terms of gravitational potential energy, rotational and translational kinetic energy, and centripetal force. A ball of mass m and radius r must be released at some minimum height h above the bottom point of the track so that it will not leave the track while passing around the loop-the-loop.

    In order to stay on the track at the top of the loop the centrifugal reaction of the ball on the track must be equal to or greater than the gravitational force on the ball: mv^2/R = mg, or v^2 = gR, where v is its linear velocity at the top of the loop, R is the radius of the loop, and g is the acceleration of gravity. Conservation of energy dictates that at the top of the loop Iw^2/2 + mv^2/2 +2mgR = mgh, where the moment of inertia of the ball I = 2mr^2/5 and w is the angular velocity of the ball at the top of the loop.

    From these considerations we obtain the minimum starting height for the ball above the bottom of the loop-the-loop in order that the ball remain in contact with the track at all times: h = 2.7 R. In the case of an object sliding along a frictionless loop-the-loop, the height would be h = 2.5 R. Marks have been made at the points 2.5 R and 2.7 R. The ball remains in contact with the track at the top of the loop only when the height 2.7 R is reached, demonstrating the effect of the rotation of the rolling ball.
    FS2

  • D1-55: ROTATING ELASTIC RINGS

    D1-55
    Demonstrate "centrifugal reaction" and to indicate why the earth is oblate.

    We have a pair of thin steel rings mounted on a rotating base. The top of the rings is free to slide along its axis, while the bottom is fixed to the rotating base.

    Turning the crank causes the elastic rings to rotate about the vertical axis. The rotation mechanism here uses the mechanical advantage of a large cranked wheel driving a smaller pulley to give the rotating rings a very high angular velocity.

    Engagement Suggestion
    Before rotating at high speed, invite students to predict what will happen to the rings when you get it spinning as fast as you can. Will they:
    • a) keep their circular shape
    • b) flatten at the top and bottom and bulge in the middle
    • or c) extend upwards and grow narrower in the middle?
    Afterwards, encourage students to relate this to other physical phenomena.
    Background
    As the rings rotate, their form distorts, growing wider around the center and flattening at the top and bottom. Interestingly, this is not due to a true outward force acting on the metal at this point, but is an artifact of its rotating reference frame and the forces acting to keep it moving in a circle. This is often termed a centrifugal reaction or centrifugal force, though it is technically a pseudo-force arising from the reference frame.

    This effect is seen in astronomy and geography, as rotating planets, stars, and other bodies take on similarly oblate spheroidal forms.
    D1

  • D1-61: Rolling versus Sliding

    D1-61
    Applies conservation of energy to a rolling object

    An aluminum cylinder rolls down an inclined plane. An identical aluminum cylinder has tiny bearings on one end, so that it slides without friction down the incline.

    Invite the students to make a prediction: If the two cylinders are started from the top at the same time, will the rolling cylinder or the sliding cylinder reach the bottom of the incline first?

    Background
    The two cylinders start at the same height with the same potential energy. As they slide or roll down the ramp, that potential energy is converted into kinetic energy. Linear kinetic energy is proportional to the mass of the cylinder and the square of its velocity. However, the rolling one also has rotational kinetic energy, which is proportional to the moment of inertia of the cylinder and the square of its angular velocity. So for the rolling cylinder, some of the potential energy is converted into rotational kinetic energy as it rolls, and only some of the potential energy is converted into linear potential energy, giving it a lower velocity as it goes down the ramp. So the sliding cylinder reaches the bottom first.
    D1, FS2

  • D1-83: SPOOL

    D1-83
    Illustrate a counterintuitive problem in rotational dynamics.
    The cord is wrapped around the smaller radius of a spool and placed on a horizontal surface such that the cord emerges over the top side of the spool, as shown at the left above. When the cord is pulled the spool will move toward the direction of the applied force, the forward direction. Q: When the cord emerges from the bottom side of the spool, as shown at the right, how will the spool move? A: Forward, just as in the previous example. This problem can be varied as follows. Imagine a line along the cord such that when the cord is held up at some large angle this line intersects the floor along the line of contact of the large radius rims of the spool with the floor. If the cord is held below this line, the spool will move toward the applied force, in the forward direction. If the cord is held above this line, the spool will move away from the applied force, in the backward direction. If the cord is pulled along this line, the spool will remain in place and spin. By holding the cord at this angle while you walk, the spool will slide along the floor as you move, so you can "walk the spool."

  • D2-01 RING AND DISC ON INCLINED PLANE

    D2-01
    Demonstrates effect of rotational inertia on acceleration of an object

    A solid disc and a thin ring having the same mass and the same radius roll down an incline starting at rest from the same position. The roller with the greater moment of inertia, in this case the ring, rolls more slowly.
    Engagement Suggestion
    • When presenting the demonstration, encourage students to make a prediction before showing the roll. Will the two objects reach the bottom at the same time, or will one get there first? Which one?
    Background
    The effect of rotational inertia on the speed of a rolling object can be confusing to new students, especially those who have just recently internalized the principles of linear motion. Having just un-learned misconceptions about objects in free fall and discovered that objects experiencing the same force fall at the same rate regardless of mass, it can be counterintuitive to realize that the corollary does not apply in rotational motion. The distribution of mass does itself affect the torque on an object and how fast it rolls.

  • D2-11: HINGED STICK AND FALLING BALL

    D2-11
    Application of the rotational analog of Newton's second law.

    The hinged stick is held in place as shown with the ball balanced on the end of the stick. When the stick is released, it accelerates faster than the ball, so the ball falls into the cup.

    Note that the initial position of the ball is directly above the final position of the cup!

    Download the mpeg below for a brief clip of the demonstration in action.
    D2

    d2-11a

  • D2-51: BICYCLE WHEEL PENDULUM

    D2-51
    Demonstrate the Parallel Axis Theorem.
    A bicycle wheel is suspended at its axis on a physical pendulum, as seen in the photograph above. Set it swinging, and invite students to predict how its motion will change if the wheel is given some initial rotation versus with it initially not rotating versus with it fixed and unable to rotate (cord for fixing wheel available upon request).

    This demonstration can be used to introduce the Parallel Axis Theorem.

  • D3-03 ROTATING CHAIR AND WEIGHTS

    D3-03
    Illustrates conservation of angular momentum

    A subject, holding the weights with their arms extended, is started into rotation. When the weights are pulled inward to the chest of the subject, the moment of inertia of the system is decreased, leading to significant increase in the angular speed of the rotating chair.

    Please take care when using this device, especially when accelerating. You can gain a significant increase in rotational speed, so hold on! And it is best not to have a person push the chair around very much, as it is very easy to hit them with a weight by accident.

    Engagement Suggestions
    • Consider inviting a participant from the class.
    • Encourage students to predict what will happen before performing the demonstration.
    • Once the demonstration has been performed, discuss the activity both in terms of angular momentum and its conservation, and in terms of kinetic energy.
    • For extended discussion, introduce the idea of friction. How does friction work in this system? How does it affect the angular momentum? Where does the kinetic energy go?
    Background
    This device illustrates the conservation of angular momentum. When the heavy weights are moved closer to or farther from the axis of rotation, the distribution of mass and thus the rotational inertia (or moment of inertia) changes.

    To show this in a different way, a single user with a single weight can move themself in a circle by swinging their arm wide holding the weight from front to back, then drawing it inwards before extending their arm forwards again and repeating the motion. This is essentially a rotational analogue of pumping a swing.
    FS0

  • D3-05 ROTATING CHAIR AND BICYCLE WHEEL

    D3-05
    Illustrates conservation of angular momentum

    Sit on the chair (chair not rotating) with the wheel spinning and its axis oriented vertically. Reverse the angular momentum vector of the wheel by inverting the wheel, thus causing the entire chair to rotate in the original direction of the wheel rotation. Returning the wheel to its initial orientation causes the chair to cease its rotation.

    Because the friction in the bearing of the rotating chair is very low, several cycles of this procedure can usually be completed before the system loses its energy and stops.

    Engagement Suggestions
    • Consider inviting a participant from the class.
    • Note that this demonstration can lead to sudden changes in motion. Be careful not to collide with your volunteer.
    FS0

    Bicycle Wheel Gyro v2

  • F1-01 FLUID PRESSURE VS. DEPTH

    F1-01
    Demonstrates that fluid pressure increases linearly with depth and is isotropic.
    An L-shaped glass tube connected to a liquid manometer is inserted into a tank of water. The pressure in the water tank can be measured at any depth. Holding the tube at a particular depth and rotating it about the end will show no change in pressure, demonstrating that pressure is isotropic.
    F1, FS2

  • F1-03: PASCAL'S VASES

    F1-03
    Demonstrate that pressure is dependent only on depth, and not on the shape of the container.
    Three "vases" with different shapes can be connected to a water reservoir and pressure gauge assembly. Using the wire as a depth indicator, it is shown that the pressure in the vase depends only on the depth, not on the shape of the vase.

  • F1-11: HYDRAULIC PRESS

    F1-11
    Demonstrate dramatically Pascal's Law and the large forces attainable using hydraulic systems.
    Place the provided 2x4 board between the jaws of the press as shown in the photograph. Tighten the pressure release valve and pump the handle to increase the force and crush the 2x4. Pressure is read directly in tons. DO NOT exceed 5 tons.

  • F2-01 ARCHIMEDES' PRINCIPLE

    F2-01
    Demonstrates the buoyant force on a body submerged in a fluid to be equal to the weight of the displaced fluid.
    Hanging from the balance are a hollow can and a solid cylindrical metal block of the same volume V. Lowering the metal block into a beaker of water results in a buoyant force equal to the weight of a volume V of water. Pouring the volume V of water into the can restores the original weight as read on the spring scale.
    FS2

  • F2-03: CARTESIAN DIVER - EXPLICIT VERSION

    F2-03
    Demonstrate explicitly how a cartesian diver works by showing how the water enters the diver when the pressure in the cylinder is increased.
    When no additional pressure (above normal atmospheric pressure) is applied to the membrane on top of the cylinder, the diver floats at the surface of the water. The location of the water surface inside the diver is indicated by the orange bob floating in the diver tube. When additional force is applied to the membrane the pressure in the tube increases, forcing more water into the diver tube and compressing the air in the tube, as indicated by the bob. Because the average density of the diver becomes greater than that of water, the diver sinks to the bottom. When the force is released, the diver again rises.

    f2-03a

  • F2-05 BUOYANCY - BOAT AND ROCK

    F2-05
    Illustrates buoyancy
    Boat and rock float in a closed pond. removing rock from boat and dropping it in pond will cause the water level of the pond to go down
    F2

  • F2-07: BUOYANCY - PEPSI AND DIET PEPSI

    F2-07
    Show the difference in density between soft drinks with and without sugar.
    Unopened cans of Pepsi and Diet Pepsi are floated in water. The Pepsi sinks, while the Diet Pepsi floats. The density of the Pepsi is increased by the dissolved sugar, which occupies space between the water molecules. Diet Pepsi has no additional sugar, and is therefore less dense.

  • G1-01 EXAMPLES OF SIMPLE HARMONIC MOTION

    G1-01
    Illustrates simple harmonic motion

    This demonstration lets you compare three typical pendula: a simple pendulum (mass on string), a physical pendulum (swinging rod), and a mass on a spring. Any of these produce simple harmonic motion, with a variety of periods. Useful for showing that the same equation describes the motion of any type of oscillating body.
    You can also compare these real-world pendula with some simulated ones:
    1. Erik Neumann's Single Spring simulation
    2. Erik Neumann's Pendulum simulation
    3. PhET Masses on Springs
    4. PhET Pendulum Lab
    FS2