Friction, drag and changing motion

Friction and drag act against movement. Sometimes we want lots of them, sometimes as little as possible.

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Friction

Friction acts where two surfaces rub or try to slide past each other. It always acts against the motion.
Useful: grip for shoes and tyres, brakes, being able to walk.
A nuisance: it wears parts away and heats them up.
To reduce it: make surfaces smoother, or add a lubricant such as oil.

Drag

Drag is the force on an object moving through a fluid: air resistance in air, water resistance in water.
Drag happens because the object must push the air or water out of the way. The faster it moves, the bigger the drag.
Streamlined shapes (smooth and pointed, like a fish or a racing car) reduce drag. A parachute increases drag.

Forces change motion

A force is needed to start, stop, speed up, slow down or change the direction of an object.
A skydiver jumps: at first weight > air resistance, so she speeds up. As she gets faster, air resistance grows until it equals her weight. Now the forces are balanced and she falls at a steady speed.
When the parachute opens, air resistance becomes bigger than her weight, so she slows down until the forces balance again at a lower steady speed.
Worked example

A skydiver weighs 700 N. At one moment, air resistance on her is 300 N. Find the resultant force and say what happens.

  1. The forces act in opposite directions, so subtract.
  2. 700 − 300 = 400 N downwards
  3. The resultant is downwards, so she speeds up.

Answer: 400 N downwards; she speeds up

Key idea

Friction and drag act against motion. Drag increases with speed and is reduced by streamlining. Unbalanced forces change speed or direction; balanced forces mean a steady speed (or staying still).

Key words
Speed
How far something travels each second: speed = distance ÷ time, often in m/s.
Unbalanced forces
Forces with a resultant that is not zero, so the object speeds up, slows down or changes direction.
Balanced forces
Forces that cancel out, so the resultant force is zero. The object stays still or keeps moving at the same speed in the same direction.
Mean
An average: add up the readings and divide by how many there are.
Force
A push or a pull, measured in newtons (N). It is an interaction between two objects.
Resistance
How hard it is for current to flow through a component, measured in ohms (Ω). R = V ÷ I.

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Extra practice

2 more questions

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  1. Which is an example of unwanted friction?

    1. Shoes gripping the floor
    2. Parts in an engine wearing away
    3. Brakes slowing a car
    4. Tyres gripping the road
    Show the answer

    Answer: Parts in an engine wearing away

    Friction can cause wear and heating.

  2. Why is a playground slide made very smooth?

    1. To increase friction
    2. To make it heavier
    3. To reduce friction so you slide more easily
    4. To increase air resistance
    Show the answer

    Answer: To reduce friction so you slide more easily

    Smooth surfaces have less friction.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.