Friction
Friction is the force that opposes motion between surfaces in contact. It depends on how rough the surfaces are and how hard they press together.
What friction is
A ball rolling on the ground slows down and stops. A bicycle stops soon after you stop pedalling. Some force must act against the motion. This force is friction. It acts between all surfaces in contact and always opposes motion. To keep a body moving at a steady speed, you must push with a force equal to friction. The forces are then balanced, so the velocity does not change.
Static vs kinetic friction
Rolling friction, when a wheel or a ball rolls, is less than sliding friction.
Activity: friction on different surfaces
You need a plank and some books to make a slope, a pencil cell, a glass sheet, some sand and a ruler.
- 1Make a gentle slope on a table with a plank resting on a few books. Mark a start line near the top.
- 2Release the pencil cell from the line. Measure how far it rolls on the bare table.
- 3Repeat with a glass sheet on the table, then with a thin, even layer of sand.
- 4Record the three distances in a table.
- 5★Observe: the cell goes farthest on glass and the least distance on sand.
- 6Conclude: smoother surfaces offer less friction.
★ key step
What friction depends on
- Nature of the surfaces: rough surfaces give more friction than smooth ones. A bicycle moves more easily on a smooth road than on a rough one.
- Normal reaction: the push of the surface back on the body, against its weight. A heavier box presses harder, so it has more friction.
- Sliding or rolling: rolling friction is much less than sliding friction.
Friction: friend and foe
| Advantages (useful) | Disadvantages (harmful) |
|---|---|
| We can walk without slipping | It wastes energy as heat in machines |
| We can write with a pen or chalk | It wears out machine parts, shoe soles and stair steps |
| Brakes can stop vehicles | It lowers the efficiency of machines |
| Nails and screws stay fixed in walls | It makes heavy loads harder to move |
| Treaded tyres grip the road | It heats and damages moving parts that are not oiled |
Reducing and increasing friction
- Put a lubricant such as oil or grease between moving parts, as in a sewing machine or a bicycle chain.
- Use ball bearings, which change sliding into rolling, as in ceiling fans and cycle hubs.
- Make surfaces smooth and polished, and use wheels to move heavy loads, as in a trolley bag.
- To increase friction where it helps: treads on tyres, grooves on shoe soles, sand on a slippery path.
A cycle mechanic oils the chain and greases the fan's axle to cut friction. Shoe soles and tractor tyres have deep grooves to increase it.
2 marks: “A shopkeeper needs a bigger push to start a loaded crate sliding than to keep it sliding. Name the friction acting in each case and say which is larger.” Before the crate moves, static friction acts. Once it slides, kinetic (sliding) friction acts. Kinetic friction is slightly less than static friction, so a smaller push keeps it moving.
2 marks: “Why are ball bearings fitted between the moving parts of a ceiling fan? Give one other way of reducing friction.” Ball bearings change sliding friction into rolling friction, which is much smaller, so less energy is wasted. Another way: put oil or grease (a lubricant) between the moving parts.
3 marks: “Friction is called a necessary evil. Explain with two advantages and two disadvantages.” Advantages: it lets us walk without slipping, and it makes brakes work. Disadvantages: it wastes energy as heat, and it wears out machine parts and soles. So we increase it where it helps (treads) and reduce it where it harms (oil, ball bearings).
Friction fights motion: rough and heavy means more; rolling and oiled means less.