Friction is the force that acts between two objects when they are next to each other. Even when objects are stationary they experience friction.
When the objects are moving, friction resists the movement.
To explore how friction affects motion, your students can make a friction machine. Take a small box lid (about 6cm by 6cm, although the exact dimensions do not matter) or make one from card. Make a hole in the middle of one side, and attach a piece of thin string, about 30cm long. Attach the other end of the string to a paper cup.
Place this friction machine on a table with the lid at the end farthest from you and the cup hanging over the end of the table nearest to you.
Place a pound;1 coin in the lid, and then add 1p pieces to the cup, until the lid moves across the table towards you. You can use anything for the weights, but coins work quite well. Note how many coins are needed.
Create a new surface to see how that affects the friction. Tape a plastic bag, aluminium foil, carpet or sandpaper over the table and repeat the experiment.
You could also try changing the surface of the lid by taping pieces of these materials to it. This enables your students to test different combinations of surfaces, for example sandpaper on sandpaper. Finally, experiment with adding additional coins to the lid.
These experiments will help your students draw conclusions about which surfaces experience greater friction.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn