Shift a joule
Students ‘experience’ forces and reinforce their understanding of force and energy units.
Apparatus and materials
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.
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.
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.