Total Energy of a System
Energy and Thermal Physics

Using a brick to introduce energy

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

How the vertical position of a brick determines how much energy is stored gravitationally.

Apparatus and Materials

  • Bricks, 3
  • Pulley, single, on clamp
  • G-clamp
  • Compression spring, large
  • Cord
  • 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.

Read our standard health & safety guidance


Procedure

  1. 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.
  2. 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.
  3. Tie the brick securely with the cord. Pass the cord over the pulley and allow the falling brick to raise a load.
  4. 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.
  5. 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.

Teaching Notes

  • These simple experiments are intended to introduce the idea that energy can be stored in different amounts gravitationally, 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 energy stored gravitationally.
  • Also common to these demonstrations is the energy of a moving object, formally kinetic energy. At an introductory level, you could call this energy stored kinetically.
  • What you might be saying, then, with each part of this demonstration is:
  • 1, 2: "When the brick is higher up it can do a job for you that it cannot do when it is on the floor. There is more energy stored gravitationally when it is in a higher position.'
  • 'When the brick is attached to another load across a pulley and is allowed to fall, it exerts a force on the other load, which rises. There is now more energy stored gravitationally of the load, and less stored gravitationally in the brick.
  • "This time the raised brick is attached to a fixed spring . When the brick is allowed to fall does work on the spring, which stretches. Afterwards, there is energy stored elastically in the extended spring. In fact, a vertically supported spring-brick system will continue to oscillate up and down, with energy shifting between two ways that it is stored."
  • "When the brick is raised it stores energy gravitationally. If the brick is dropped onto the bridge it does work on the bridge, which may be enough to break it. There may also be some sound produced, and some heating. Ultimately the extra energy stored gravitationally is stored thermally in the surroundings. This is dissipated energy, which cannot be used to do another job."

This experiment was safety-tested in December 2005

Total Energy of a System
appears in the relation dU=dQ+dW
is used in analyses relating to Thermal Equilibrium
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