Hula Hoop Rotational Inertia

What it shows:

A suspended hula hoop has the same period of oscillation as a pendulum whose length is equal to the diameter of the hoop.

How it works:

The parallel-axis theorem allows us to readily deduce the rotational inertia of a hoop about an axis that passes through its circumference and is given by

\(I = I_{cm} + MR^2 = 2MR^2\)

where M is the mass of the hoop and R is its radius. The period of oscillation thus becomes


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Parallel-Axis Theorem

What it shows:

One can show that the period of oscillation of an object doesn't change for different suspension points, as long as they're the same distance from the COM. This is consistent with what the parallel-axis theorem tells us about the moment of inertia of the object.

How it works:

The parallel-axis theorm states that if \(I_{cm}\) is the moment-of-inertia of an object about an axis through its center-of-mass, then \(I\), the moment of inertia about any axis parallel to that...

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Tail Wags Dog

Lecturer tries to swing baseball bat while standing on turntable.


Special Bouncing Collisions

Same as previous except that mass ratio of balls is 1:3 (softball:basketball) leaving basketball dead and softball four times the height.

tennis and basketball

Rocket Car

Single seat CO2 powered rocket cars.

rocket car
Photo by Rose Lincoln