Elastic and Inelastic Collision Model

What it shows:

Two cars have the same mass and same spring bumper. When given a push and allowed to collide with a wall, one car bounces off with only a small reduction in speed ("elastic" collison) whereas the other car comes nearly to a complere stop ("inelastic" collision).

How it works:

There are two impulse cars made of identical materials and have the same mass. The car that models an elastic collision has all its lead sinkers securely attached to the frame so that they can't move. In contrast, the car that models an inelastic collision has the lead sinkers...

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Toilet Paper

What it shows:

The speed with which you tug on a toilet roll determines whether a sheet breaks off, or the roll simply unravels.

How it works:

The force applied to the junction between the sheets of a toilet roll is proportional to the rate of change of momentum of your hand as you tug at the end. Thus a sharp tug (large ∆p) is sufficient to surpass the breaking stress of the perforated junction. A lesser tug however, below the breaking stress, will apply a torque to the roll itself; the ensuing rotation unravels the roll.

figure 1. Toilet roll...

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Karate Blow

The instructor breaks several boards with a swift blow of the hand.

What it shows:

The impulse momentum theorem is demonstrated in a most dramatic way by breaking several boards with the blow of your fist. You need not be a karate expert to show how the force of a well executed hammer-fist strike will easily break a stack of five to eight boards. The impulse is given by

impulse = F∆t = ∆mv

The point of the demonstration is: the greater the speed, the smaller ∆t will be and thus the greater the force.


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Potential Well

Orbital motion simulated by ball rolling on wooden potential well.

What it shows:

Motion in a central potential is demonstrated by a ball rolling on a circular 1/r curved surface.

How it works:

The 1/r potential well simulates the gravitational potential surrounding a point mass; a ball bearing moving in this potential follows a parabolic or elliptical orbit depending upon its initial trajectory and velocity. As it loses energy due to friction, the orbit decays and the ball spirals towards the centre of the well. You could...

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Reversible (Kater's) Pendulum

A physical pendulum with two adjustable knife edges for an accurate determination of "g".

What It Shows

An important application of the pendulum is the determination of the value of the acceleration due to gravity. By adding a second knife-edge pivot and two adjustable masses to the physical pendulum described in the Physical Pendulum demo, the value of g can be determined to 0.2% precision.

How It Works

Using a simple pendulum, the value of g can be determined by...

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Falling Faster than 'g'

What it shows:

Allow a board to rotate under the force of gravity and the free end will accelerate at a rate greater than g. Relation between angular acceleration and linear acceleration seems to give free-fall paradox.

How it works:

If a board, held in a vertical position with one end resting on the table, is allowed to...

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Double Atwood's Machine

Prediction of motion of masses in a more complex pulley/mass assembly.

double atwoods machine

What it shows:  This compund Atwood's Machine demonstrates an old and interesting problem. The two small weights on the right side are not of equal mass — one is 100 g and the other...

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Atwood's Machine

Combinations of weights suspended over pulley to show that asymmetry causes acceleration.

atwood's machine

Image on the left, of a lightweight plastic pulley with balanced 50 g brass weights, and on the right, the pulley in motion as the unbalanced weights accelerate. 

Inertia of Rest

Concrete block smashed on lecturer's chest with sledge hammer.

What it shows:

The lecturer (or someone else) lies on a bed-of-nails without discomfort, thus demonstrating the concept of pressure, which is the force per unit area. For added drama the person is sandwiched between two beds of nails with the added weight of a cinder block on top. The cinder block can be broken with a sledge hammer.

How it works:

The forces (weight of the body, cinder block, etc.) are distributed over the total area of all the nail...

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