Friction and its effects on motion
Key idea: Friction opposes sliding, or the tendency to slide, between surfaces. It can slow an object or provide forward grip; air and water resistance oppose motion through the fluid.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Find friction’s direction from sliding or the tendency to slide.
- Find both friction forces and the range of the maximum.
- Explain why friction acts on the box at rest.
- Subtract resistance before calculating acceleration.
- Explain the acceleration during sliding and the forces after stopping.
- Find the driving force, then predict motion without it.
- Explain how grip helps the sprinter accelerate.
- Predict the box’s motion without friction.
- Compare friction during braking and acceleration.
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ContinueFriction slows a sliding box to a stop and wears out brake pads, but without it you could not walk a single step. It is one of the forces on almost every free-body diagram, so its direction and size feed straight into the resultant force and into resultant force = mass × acceleration. The key is to ask which way the surfaces would slide without it.
Friction and its effects on motion
Friction opposes sliding
Friction is a contact force between two surfaces. It acts along the surfaces and opposes the sliding of one surface over the other. When the surfaces grip and do not slide, friction opposes the tendency to slide.
To find the direction of friction on a body, ask: which way would the body slide over the surface if there were no friction? Friction on the body acts the opposite way.
- A box sliding to the right on a floor: friction on the box acts to the left.
- A box pushed gently to the right that does not move: it would slide right, so friction acts to the left. It is exactly as large as the push, so the resultant force is zero and the box stays at rest.
- Push harder and friction grows, up to a maximum. Once the push is larger than that maximum, the box starts to slide.
Air resistance and water resistance act in a similar way. They oppose a body’s motion through the air or water.
Check your understanding 1 A push that does not move it
A book stays at rest when pushed right with 3.0 N, and again with 5.0 N. It slides when the push reaches 6.0 N. Find the leftward friction magnitudes in the two stationary tests. What range contains the maximum static friction in this model?
Show answer
3.0 N to the left, then 5.0 N to the left: the book tends to slide right and stays at rest, so the friction matches the push and the resultant force is zero. The maximum static friction is at least 5.0 N, because that push was balanced, and no greater than 6.0 N, where sliding begins. These tests leave the exact maximum uncertain.
Check your understanding 2 At rest on a slope
A box rests without moving on a rough slope. State the direction of the friction on the box, and explain why there is friction even though nothing slides.
Show answer
Up the slope. Without friction the box would slide down the slope, so friction opposes that tendency to slide and holds the box at rest.
Friction in the resultant force
Friction is one of the forces on the free-body diagram, so it goes into the resultant force. That gives three effects:
| Situation | Effect of friction |
|---|---|
| a driving force is larger than friction | friction reduces the resultant force, so the acceleration is smaller |
| friction is the only force along the motion | friction is the resultant force, so the body slows down and stops |
| a driving force equals friction | the resultant force is zero, so the body stays at rest or keeps its constant velocity |
Worked example 1
Friction reduces acceleration
Problem
A 1200 kg car has a 900 N driving force. Friction and air resistance together are 300 N. Find its acceleration.
Show full solution
Find the resultant force
Method
Take forward as positive and subtract the resistive forces.Reason
Friction and air resistance act against the motion.Working
Answer
F = 900 - 300 = 600 N forward
Find the acceleration
Method
Divide the resultant force by the mass.Reason
F = ma uses the resultant force, not the driving force alone.Working
Answer
a = F/m = 600/1200 = 0.50 m/s² forward
Compare with no resistance
Working
Answer
Without the resistive forces the acceleration would be 900/1200 = 0.75 m/s². Friction and air resistance reduce it.
Worked example 2
Friction stops a sliding box
Problem
A 2.0 kg box slides to the right across a floor. The only horizontal force on it is 4.0 N of friction. Find its acceleration.
Show full solution
Give friction its direction
Method
Take right as positive; friction acts to the left.Reason
Friction opposes the sliding, which is to the right.Working
Answer
F = -4.0 N
Find the acceleration
Method
Divide the resultant force by the mass.Reason
Friction is the only horizontal force, so it is the resultant.Working
Answer
a = F/m = -4.0/2.0 = -2.0 m/s²
Interpret the sign
Working
Answer
The box slows down at 2.0 m/s² until it stops. Once it stops, it no longer slides, and the friction on it becomes zero.
Check your understanding 3
A cyclist travels steadily at 6.0 m/s on level ground against 40 N of resistance. Find the forward driving force. What happens immediately after the cyclist stops pedalling while resistance still acts?
Show answer
Constant velocity means zero resultant force, so the driving force is 40 N. Immediately after the cyclist stops pedalling, resistance is still about 40 N and acts opposite to the motion, so the cyclist slows down. Air resistance then decreases as the speed falls.
Useful and unwanted friction
| Useful friction | Unwanted friction |
|---|---|
| grip between shoes and the ground, so you can walk and run | it heats moving parts, such as an engine’s parts, and wastes energy |
| grip between tyres and the road, so a car can speed up, turn and stop | it wears surfaces away, such as tyres and brake pads |
| brakes slow down a bicycle or a car | a larger driving force is needed to keep a vehicle moving |
Lubricating oil and ball bearings reduce friction between moving parts. A streamlined shape reduces air resistance. Rough surfaces or spikes improve grip. Tyre tread keeps grip on wet roads by letting water escape from under the tyre.
Check your understanding 4 Running spikes
Explain why a sprinter wears shoes with spikes.
Show answer
The spikes increase the friction (grip) between the shoes and the track. The foot can push backward harder without slipping, so the forward friction on the sprinter is larger and the sprinter can accelerate faster.
Friction does not always oppose the body’s motion
When you walk, your foot pushes backward on the ground. Without friction, the foot would slip backward, as on ice. So the friction from the ground on your foot acts forward. That forward friction is the force that moves you forward.
So friction opposes the sliding between the two surfaces, not the motion of the whole body. For a walking person or a car’s driven wheels, friction on the body acts forward, in the direction of motion.
Check your understanding 5
A box sits on the flat back of a lorry. The lorry speeds up forward and the box moves with it without slipping. Which way does friction act on the box?
Show answer
With no friction, the lorry would speed up under the box and the box would slide backward on the lorry’s floor. Friction opposes that sliding, so friction on the box acts forward. It is the only horizontal force on the box, so it is the force that makes the box speed up with the lorry.
Check your understanding 6 The box still moves forward while friction points back
A lorry brakes on a level road. A box stays fixed on its floor without slipping. Which way does friction act on the box, and how does its speed change? Compare with the accelerating lorry: which way would the box slide relative to the floor without friction?
Show answer
Without friction the box would continue forward while the lorry slows beneath it, so the box would slide forward relative to the floor. Friction on the box acts backward and slows it with the lorry. In the accelerating-lorry case, the box would instead slide backward relative to the floor, so friction acts forward. In both cases friction opposes the tendency of the two surfaces to slide relative to each other.
Common mistakes
- Saying friction always acts opposite to the body’s motion. It opposes sliding between the surfaces; a walker’s foot gets forward friction.
- Saying a box that does not move has no friction on it. Friction balances the push until the box starts to slide.
- Saying friction always equals the driving force. They are equal only when the resultant force is zero.
- Saying friction is always a nuisance. Without it you could not walk, turn or brake.
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Syllabus and review details
- SEC G2 Science Physics component 2027 · 2027
Physics section, PDF pages 8–23
Last reviewed: