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PHYS121

  • D1-51 BANKED CURVE MODEL

    D1-51
    Aid in explaining banked turns
    The model of the curved road is banked such that at the suggested maximum rate of speed the horizontal component of the normal force provides the centripetal force required to keep the car moving in its circular path, independent of the friction of the car wheels with the road.
    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

  • 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.

  • D2-02: Miscellaneous Rolling Bodies On Inclined Plane

    D2-02
    Demonstrates effect of rotational inertia on acceleration of an object
    Different objects are rolled from rest down an incline, and their accelerations compared. The acceleration is less for those bodies with the smaller radius of gyration (square root of the moment of inertia per unit mass). Available rollers include rings, discs, and solid spheres of different masses and radii.
    D2, FS1

  • D2-05: DUMBBELL - VARIABLE MOMENT OF INERTIA

    D2-05
    Demonstrate the effect of moment of inertia.
    Hold the dumbbell at its center and rotate it rapidly in alternating directions. Then change the moment of inertia by sliding the weights along the rod. See how moment of inertia affects the speed and effort with which you can change rotation.
    D2

    d2-05a

  • D2-31 OBERBECK CROSS

    D2-31
    Illustrates rotational analog of Newton's second law of motion
    Various masses M can be hung on a string wound around an axle of either of two radii R to provide a torque T = MgR. Four brass masses m can be positioned along the arms at one of four distances l from the axis of rotation to provide a moment of inertia I = 4ml^2. The angular acceleration a = T/I = MgR/4ml^2 can then be calculated. The angular acceleration can be determined experimentally by measuring the time required for the system to rotate one complete revolution starting from rest: a = 2 d/t^2, where t is the time required for the device to rotate through the angle d in radians.
    FS1

  • 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

  • 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

  • F1-02: FLUID PRESSURE VS DEPTH - ANEROID GAUGE

    F1-02
    Show water pressure versus depth with an aneroid gauge.
    An L-shaped glass tube, connected to an aneroid gauge, is immersed in water. The pressure at any depth is indicated directly on the gauge. This enables students to see the pressure at any level.

    Invite students to make predictions about the relationship between depth and pressure, and perhaps even sketch what they expect the graph of this relationship to look like. Then take a few data points and see what happens.
    F1

  • F1-04: EQUILIBRIUM TUBES

    F1-04
    Demonstrate that pressure is transmitted equally throughout a fluid.
    By raising and lowering the reservoir, one can show that the water level in all three vases will rise and fall together. From this one concludes that pressure is transmitted uniformly throughout the water.
    F1

  • F1-05: DOES WATER SEEK ITS OWN LEVEL?

    F1-05
    A trick to challenge the students.

    The liquid level in the left side of the U-tube is higher than that in the right side of the U-tube. How does one explain this?

    Two immiscible fluids of different density which are identical in physical appearance are in the two ends of the U-tube. The point where they meet (which could be easily seen) is covered by the clamp which holds the U-tube.

  • F1-06 WATER SEEKS ITS OWN LEVEL

    F1-06
    Shows that pressure is dependent on depth, not shape of container

    This set of conjoined glass tubes is filled with green-dyed water. The water level in the four different tubes is the same even though the volumes and shapes of the tubes are very different.

    Engagement Suggestion
    • For advanced students, consider tilting the tubes slightly, then plugging them with corks so that the different amounts of trapped air cause the water to be at different levels. Challenge students to analyze why this changes the results, then remove the corks to show what happens.
    Background

    This illustrates that the pressure in an open container of liquid is dependent only on the depth, not the shape or area.

  • 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

  • F2-04: BUOYANCY - SPHERE AND WATER

    F2-04
    Challenge the students' thinking about the buoyant force by considering the question: "Will a round object, without a flat top and bottom surface, experience a buoyant force, as does a cylinder?"
    A round steel ball and a hollow metal can hang from the scale. The pressure is always normal to the surface of a body and increases linearly as the depth increases. When the steel ball is immersed it too experiences a buoyant force, because the upward vertical component of pressure applied to the lower hemisphere is greater in magnitude than the downward vertical component of the pressure exerted on the upper hemisphere.

    f2-04a

  • 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.

  • F2-13: BOUYANCY - EXPANDING BALLOON CONUNDRUM

    F2-13
    Conundrum involving weight of an expanding balloon.

    A flask containing vinegar is connected to a balloon containing baking soda. This system is balanced with a collection of weights on a pan balance as shown in the photograph above. The balloon is raised up so the baking soda falls into the vinegar, commencing a reaction that produces carbon dioxide and inflates the balloon. When the balloon becomes inflated, what happens to the balance? Will the balloon side go down, will the weight side go down, or will it remain balanced?

    Included here are before and after pictures for this experiment using an electronic balance, showing the weight in grams and tenths (of force), eg. 261.7g down to 258.4g for the balloon expanding to a diameter of about 18cm.

    f2-13af2-13b

  • F2-21 REACTION TO BUOYANT FORCE

    F2-21
    Demonstrates the reaction force using a liquid.
    A beaker of water is balanced by two brass weights. Stick your finger into the water about up to the first knuckle, the water side will go down. The water exerts a buoyant force on your finger, so your finger exerts a reaction to the buoyant force on the water, thereby causing the water side to go down.
    F2, ME1