Using a brick to introduce energy
How the vertical position of a brick determines how much energy there is in a gravitational store
Apparatus and materials
Pulley, single, on clamp
Compression spring, large
Retort stand base
Mass, 0.5 kg
Hardboard sheet, approx 500 mm x 100 mm
Retort stand rod, long
Health & Safety and Technical notes
In all activities where bricks may fall on toes, precautions (such as using cardboard boxes full of waste material) should be taken.
a Start with a brick on the bench and ask students to describe it. They will probably give a lot of details but not mention its position. Now place the brick on the floor and ask for a description again. This should extract the relevance of the brick's position to its ability to do a job for you.
b Place some folded newspapers on the floor as protection and let the brick drop onto them. Make sure that the acceleration of the brick is obvious.
c Tie the brick securely with the cord. Pass the cord over the pulley and allow the falling brick to raise a load.
d Support the spring in a clamp from a retort stand and attach the brick, via the cord, to the spring.
Allow the brick to drop gently. Let students observe the oscillations that take place.
e Support the hardboard 'bridge' on two bricks and place the third brick on top. Ask students to describe the arrangement. Now lift the third brick sufficiently so that when it is released the hardboard 'bridge' cracks when it is hit.
1 These simple experiments are intended to introduce the idea that objects store differing amounts of energy, depending on their vertical position in relation to other objects and the Earth's gravitational field. Formally, this is described as their 'gravitational potential energy'. At an introductory level, you could call it 'up-hill energy'.
The other type of energy common to these demonstrations is the energy which any moving object has, formally 'kinetic energy'. At an introductory level, you could call this 'motion energy'.
2 What you might be saying, then, with each part of this demonstration is:
a, b 'When the brick is higher up it can do a job for you that it cannot do when it is on the floor. The brick has stored 'up-hill' energy when it is in a high position.'
c 'When the brick is attached to another load across a pulley and is allowed to fall, it transfers its 'uphill' energy to the other load which rises, so gaining 'uphill' energy.'
d 'This time the raised brick is attached to a fixed spring . When the brick is allowed to fall the 'uphill' energy stored in the brick is carried to elastic energy stored in the extended spring. In fact a vertically supported spring-brick system will continue to oscillate up and down, with the moving brick carrying energy between these two different energy stores.'
e 'When the brick is raised it is given 'uphill' energy. If the brick is dropped onto the 'bridge' its 'uphill' energy is carried by motion energy before breaking the strip of hardboard and coming to rest. All the motion energy, which the brick had, has been used to break the hardboard (by separating its atoms).'
This experiment was safety-checked in December 2005