Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Forces and energy demonstration box

The box is made from a framework about 1.5 m high, 0.5 m wide and 10 cm deep. It has a projecting wooden bar at the back, which can be G-clamped to a laboratory bench. Three freely running pulleys are mounted at the top of the framework. Strings running over these pulleys link three curtain rings (outside) to masses of 1 lb if used, 1 kg and 102 g (inside). 
 
The front of the box carries the labels 'pull with a force of 1 lb weight', ‘pull with a force of 1 kg-wt',' pull with a force of 1 newton' depending on what weights you are using. 
 
Leave the back of the box open so that the mechanism can be seen. 

Forces and energy demonstration boxThe positions of the masses inside the box are arranged so that the strings will travel 1 foot, 1 metre and 1 metre respectively. This means that pulling the strings to the full extent will transfer 1 ft lb-wt (imperial unit of energy, no longer used), 1 kg-wt metre (10 joules) and 1 joule respectively.