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Center of Percussion

The motion (or lack of motion) of the suspension point of an object is observed when the object is struck a blow.

What it shows

The center of percussion (COP) is the place on a bat or racket where it may be struck without causing reaction at the point of support. When a ball is hit at this spot, the contact feels good and the ball seems to spring away with its greatest speed and therefore this is often referred to as the sweet spot. At points other than this spot, the bat or racket may vibrate or even sting your hands. This experiment shows the effect by demonstrating what...

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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 Icm 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 first one is given by I = Icm +...

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Loop-the-loop

A toy car rolling down a loop-the-loop track demonstrates the minimum height it must start at to successfully negotiate the loop.

What it shows:

For an object to move in a vertical circle, its velocity must exceed a critical value vc=(Rg)1/2, where R is the radius of the circle and g the acceleration due to gravity. This ensures that, at the top of the loop, the centripetal force balances the body's weight. This can be shown using a toy car on a looped track.

How it works:

The car is released from the top of a ramp and runs down a slope towards...

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Pendulum and Nose

Faith in the conservation of energy is tested by taking the demonstrator's nose to task.

What it shows:

The principle of conservation of energy ensures that a pendulum released at a particular amplitude will not exceed that amplitude on the return swing. A lecturer's faith in their subject is put to the test using a 50lb (22.7kg) iron ball.

How it works:

Technique is very important here. The best method to employ is to stand with your back against the blackboard with your head also touching the board. This ensures that you don't lean forward after release....

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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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Gravi Car

A falling weight propels a car forward.

What it shows:

Gravitational potential energy can be converted into mechanical kinetic energy.

gravi car

How it works:

A Gravicar is a vehicle powered by gravitational potential energy that it stores in a 2.5kg mass on a thread which is coupled...

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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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Rocket Car

Single seat CO2 powered rocket cars.

rocket car
Photo by Rose Lincoln

Reactionary Roadbed

Radio controlled car moves one way while road moves the other.

What it shows:

We tell our students that, when a car drives down the road, the road and the Earth move in the opposite direction, albeit imperceptibly. This demonstration is a realization of that concept, made possible (and perceptible) by the fact that the road is not attached to the Earth.

...

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Reaction on Conveyor Track

What it shows:

A straightforward demonstration of Newton's 3rd law, that forces are interactions and thus come in pairs.

How it works:

Two people, each sitting (cross-legged) on their own board, position themselves in the center of the track facing each other. Upon pushing against each other with their hands, they glide apart down the length of the track. Repeat this with one person turned around — the other person pushes on his/her back instead of pushing against each other with their hands. The ensuing motion down the track is exactly the same as before.

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