Jobs needing food or fuel

Class practical

A series of simple experiments to develop the idea of doing useful work.

Apparatus and Materials

• Balloon
• Steel spring
• Long nails or doweling to hold spring
• Brick or wood block
• Pulley, single, on clamp
• Cord

Health & Safety and Technical Notes

Precautions are needed to prevent a spring flying into someone's face or a brick falling onto toes. If the class is positioned such that any member is at risk, eye protection should be worn or a box placed below the brick.

Read our standard health & safety guidance

Procedure

1. Blow up the balloon and tie the neck.
2. Put the blown-up balloon on a table and sit and watch it.
3. Hold the spring between your hands, stretch it and then release it.

4. Keep the spring stretched permanently between two nails.
5. Raise the brick (or block of wood) from the floor to a table by pulling on a string tied to the brick.

6. Use the brick on top of a pile of loose papers as a paper-weight.

7. Fix the pulley to the edge of the table and run the string over it and raise the brick by pulling the string horizontally along the table with the other end attached to the brick.

Teaching Notes

• These are all simple examples of ‘jobs' to be done, for quick discussion as to whether food or fuel is necessary for each, directly or indirectly:
• You might start by asking:
‘What does your food enable you to do besides keeping warm, breathing and generally living? Could an engine which uses petrol or a motor connected to the electricity supply do that job for you instead?’
• You can rule out some jobs that people have to do as not requiring food or fuel. You might say:
‘Yes I know that you get tired if you hold up four bricks above your head but you could just as easily put them on a shelf and you wouldn’t have to give the shelf any food or petrol or pay for an electric motor to hold up the shelf.’ ‘Yes you can use some of your food to clench your fist and grip your mobile phone, and you can go on gripping it for as long as you like. Food is not really needed to keep up the job of gripping. You could do the gripping by tightening up a gripping device with an electric motor and then stopping the motor while it, the gripping device, continued to grip. That would go on holding the phone, free of charge, as long as it stayed there, just like the ‘hands free’ device in a car.’
• In these examples, fuel-using jobs in which forces move are jobs in which energy is transferred. Invite students to give their own suggestions (e.g. cycling uphill, climbing a mountain, stirring custard or sweeping the floor). Note that energy is stored chemically in food and oxygen.
• The stretched spring in 3 and 4 has something like the energy that fuel and oxygen stores. Instead of using fuel to drive an engine or using food to keep a person going you could use a stretched spring to pull on a string and haul up a load or do some other useful job. So a stretched spring or a wound up clock spring stores energy elastically. You do work on the spring when you stretch it, but not while it is attached to the supports and is stationary.
• There is a problem when discussing the holding up of bricks, that may arise from 5 or 6. This is because muscles are not stationary objects and they keep ‘firing’ in and out of tension and in so doing become warm. You would need feeding in order to keep holding some bricks up. However the bricks could be put on a shelf and that would not need to be fed.
• A brick that has been raised up stores energy gravitationally because of work done by a moving arm or motor.

This experiment was safety-tested in March 2006.