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Physics Department

Location: 180-204
Phone: (805) 756-2448
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Chair: Jennifer Klay

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Physics 121 Weeks 9 & 10

Circular Motion

Airplane on a String

A battery powered airplane provides enough thrust to maintain uniform circular motion which can be used to introduce the topic while discussing various defining attributes.

Arrows on a Wheel

Using the attached magnet the bike wheel can stick to your blackboard. Magnetic arrows can then be added in various directions to illustrate circular motion vector quantities.

Centripetal Force

Colored Water in a Bottle

Swing the bottle in a vertical circle and show how the water’s inertia causes it to appear to be pushed towards the end of bottle, or use this demo to show how an object's velocity or radial distance influences circular motion.

Measuring the Centripetal Force

Swing the ball around in a horizontal circle and have a student attempt to read the spring scale. This can also be used both qualitatively and quantitatively to show how varying velocity can affect the centripetal force.

Equilibrium and Circular Motion

This demonstration is a great qualitative way to show your students how the ratio between an object's tangential velocity and its radius of rotation can influence the centripetal force. Can you calculate what period is necessary to suspend the mass "X" cm below the tube?


Using a loop-the-loop you can discuss circular motion, energy conservation, and pose many engaging and intricate questions to your students.  For example, what happens to the normal force at the top of the circle if the ball barely makes it around?

Centrifugal Force Puzzle

Pose this question to your students, "If the centrifugal force is in fact a pseudo-force, then why do these balls drift towards the edge when the device is rotated?" Use this puzzle to engage your students in a critical thinking exercise  about inertia.

Centripetal Force Apparatus

Rotate the device to begin stretching the spring. Have your students calculate a centripetal force using it's radius and a calculated period. From here, use a spring scale to verify their theoretical predictions.

Ball on a String

Show that there must be a centripetal force (tension) to allow for circular motion, otherwise the rope would not remain taught. You can also swing it vertically to discuss what happens to the centripetal force at the top and bottom of the loop.

Water in a Bucket

Using a bucket full of water, you can engage students in a discussion about what is necessary to stay dry while swinging it over your head. You could even derive the minimum velocity (when normal force = 0N) necessary to keep the water in the bucket!


Torque Wrench

After clamping the metal to a table, you can show students how a torque wrench works as you attempt to tighten or loosen the bolt.

Walking the Spool

The spool will either “wind-up” or “unwind” depending on what angle you exert a torque by pulling on the string. You can also challenge your students to determine at what critical angle can you pull the string such that the spool just slides across the table?

Torque Feeler

This "T" shaped bar has an adjustable location to attach masses. Have students rotate the bar up while their arms are extended outwards. Then, move the same mass to a different position to allow students to feel the difference.

Moment of Inertia

Inertia Wands

Two identical wands have the same mass, but in one wand the mass is located at its center, and on the other the mass is located on the ends. Holding the wands at their centers, have students attempt to rotate the wands to feel how mass distribution affects an object’s moment of inertia.

Moment of Inertia Races

Using an inclined plane compare how an object’s moment of inertia influences its motion down the ramp. Discs, balls, and cans are provided to offer plenty of different objects. You could have the students predict the outcome of the race before releasing the object.

Rotational Energy

Rolling Spool

A large spool is rolled down a meter stick along its small axis, and allowed to roll onto a table. Even though angular velocity is constant, since tangential velocity is larger on the large axis the spool will speed up when it contacts the table.

Ball Transfers to Cup (Faster than "g" acceleration)

Use the wooden dowel to hold the hinged plank open. Place the ball in the depression at the top of the angled plank. Then, quickly remove the dowel by pulling it outwards from the bottom. The plank rotates faster than the ball falls, so the ball will fall straight down landing in the cup.

Angular Momentum

Angular Momentum with a Rotating Stool

A rotating stool or platform is used to have student volunteers show what happens in various situations...

  • CHANGING MASS DISTRIBUTION: Students hold weights in outstretched arms and are given a slight push, once spinning, ask students to bring their arms into their chest.
  • ELIMINATING EXTERNAL TORQUEStudent is given a baseball bat and asked to swing it while feet are not touching the ground. Compare what happens with when their feet are touching ground and they swing the bat.
  • BIKE WHEEL EXERTS INTERNAL TORQUE: Student is asked to hold a bike wheel from its pegs while sitting on the stool. Spin the wheel and then have the students move the wheel's axis.

Gyroscopic Precession

Rotate a bike wheel and then suspend it by the string. The wheel will remain relatively vertical but begin to precess due to the conservation of angular momentum.

Rotational Equilibrium


Balance a meter stick on a fulcrum at its center. You can use the adjustable hangers to change where masses are hung so that the meter stick balances. This is a great demonstration to allow your students to calculate a theoretical statics problem, and then verify their prediction!

Torque on a Lever Arm

Just like above, but the fulcrum is at the end of the meter stick. This demonstration quickly provides a qualitative result for a statics problem.

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