The purpose of this practical is for students to understand how materials can exert friction forces when an object is dragged over its surface.
A metal sled or wooden block can be dragged using a forcemeter across the surface of different materials. Masses may be added to the sled to increase the weight so that the friction forces are made greater. Pupils read the scale on the forcemeter at the point the sled moves smoothly across the surface.
Equipment required (per set):
A selection of different materials may be put out with this equipment for pupils to use in groups. Because the material needs to be fairly taut, it may be easier to make up some friction boards that can be reused many times. These boards consist of a sheet of plywood approximately 50cm long and 20cm wide with one material fixed to the surface using drawing pins. Useful surfaces can include:
- Sled or wooden block with hook
- Slotted masses
- Selection of friction boards or materials
- Forcemeter (0-15N if masses are added)
- Sugar paper
- Rubber / latex
- OHP sheet
Some teachers may ask for trays of sand or earth which are fairly easy to make up. They may also ask for lubricants to show how friction forces can be reduced. Lubricants can include oil or silicone beads. Obviously pouring oil onto your hand crafted friction boards is not recommended, however an empty tray with a small amount of oil or silicone beads may be used to good effect.
Sleds can be simply made from a piece of metal sheet of approx. 10 x 5cm. Put a 45° bend about 1cm from one end and drill a small hole in the middle, big enough to put a hook from a forcemeter through. Remember to file down any sharp edges.
Alternatively cut a block of wood approx 10 x 5 x 5cm and screw a small threaded eye or hook onto the middle of one end. Either type will produce results although the wooden types may give greater variation in results because of the nature of the surface of the wood.
More complex equipment is available to investigate how an inclined plane affects friction forces (see Fig 2). With this equipment, pupils are able to adjust the angle of the slope until the block starts to move. Different surfaces can be attached to the sloping surface so pupils can investigate how much friction they produce.