Physics Demo Number: 076

Approximate Run Time: 5 min

Oscillations with the help of A Wilberforce Pendulum and A Small Newton's Cradle

Demo Description

 One may tune a mass spring system so that its kinetic energy switches between a purely rotational state and a purely translational state.

One may also demonstrate oscillations with the small Newton's Cradle as outlined in the description of the giant bowling ball version.

Scientific Principles

• Coupled oscillations (rotational and longitudinal)

• Exchange of energy between two coupled modes of an oscillating mechanical system.

Equipment

• Wilberforce Pendulum

• Small Newton's Cradle

Equipment Location

• The Wilberforce Pendulum lives in Kit (076) on [E-3-5].

• The Small Newton's Cradle lives on [E-3-5].

Instructions

The shop-made Wilberforce Pendulum consists of a torsionally stiff stand and a helical spring that has a heavy six-sided metal bob attached at its lower end.

The pendulum can execute longitudinal and torsional oscillations.

The metal bob has three threaded bolts inserted into three of its six vertical faces.

One may vary the radial distribution of the bob's mass, and thus its rotational inertia, by screwing the bolts in or out in the radial direction.

The normal setting for the bolts results in coupling between the rotational and linear modes of oscillation of the pendulum.

Indeed, if one starts the bob moving up and down with no rotational motion, it will soon stop the up-down motion and rotate in rotational harmonic motion at a constant vertical elevation.

Soon, however one sees the rotational motion dwindle down to zero as the bob regains all its vertical simple harmonic motion.

Writeup created by David A. Burba

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