# Shift a joule

##### Demonstration

Students ‘experience’ forces and reinforce their understanding of force and energy units.

#### Health & Safety and Technical notes

If you do not have a forces demonstration box, simply use a 100 g bag of beans (weight about 1 N) on a string over a pulley. Students pull the string out by 1 m to experience 1 joule of work.

#### Procedure

Let students pull each of the masses in turn and ‘feel’ the pull of the Earth on masses of one pound, one kilogram and 102 grams. (1 lb-wt, 1 kg-wt and 1 newton.)

If you leave the box available in the lab over a period of weeks, students can return to it to reinforce their knowledge. It also avoids a queue, which would be wasteful if all the class had to try the forces in one session.

#### Teaching notes

1 This piece of equipment is very useful to have around the laboratory so that students get a feel for force and energy units. The Imperial units are included as a few students may be more familiar with a pound mass, but you can omit if it is more likely to confuse.

2 The pull of the Earth on any body is a force. This is known as the weight of that body. The pull of the Earth on a mass of 102 g is 1 newton and on a mass of 1 kg is about 10 N (9.81 N). The weight of an apple is about 1 N.

3 The joule is a secondary unit. It depends on the units of force and distance, and so it is a difficult concept for beginners to grasp. Joulemeters, which read in joules directly, have been developed for measuring the transfer of energy from an electrical supply to a heater, just like the electricity meters in the home. But there is nothing so convenient for measuring transfers from mechanical work.

This experiment was safety-checked in November 2005.