🤝 Contact forces
The two bodies must be touching.
Friction, air resistance (drag), normal contact force, tension, upthrust and thrust are all contact forces.
Forces, Movement, Shape and Momentum » Forces and Their Effects
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 1.11, 1.12, 1.16
- The three things a force can do to a body: change its speed, its direction or its shape
- How to identify the different types of force, and which need contact and which do not
- How forces are measured in newtons and shown with force arrows
- Why friction always opposes motion, and when it helps or gets in the way
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Unlock This CourseWhat is a force?
A force is a push or a pull. A force always acts between two bodies. "Body" is just the physics word for an object. Your foot pushes a ball. The Earth pulls an apple. A magnet pulls a paper clip. In each case one body exerts the force and another body feels it.
You cannot see a force. You can only see what it does. Forces are measured in newtons (N). One newton is a small force: it is roughly the force needed to hold up a small apple. You can measure a force with a newton meter, which has a spring inside. The bigger the force, the more the spring stretches and the higher the reading.
Key terms:
- Force: a push or a pull that one body exerts on another.
- Newton (N): the unit of force.
- Newton meter: an instrument that measures the size of a force.
The effects of forces
A force can change a body in three ways. A football match shows all of them.
⚽ Change in speed
A kick makes a still ball start moving and speed up. The goalkeeper's gloves push against a moving ball, so it slows down and stops.
🔄 Change in direction
A player heads a ball that is flying across the goal. The force from her head sends the ball off in a new direction.
💥 Change in shape
For a moment, while the boot touches it, the ball is squashed. Forces can also stretch, bend or twist a body.
One force can have more than one effect at the same time. A header can change the speed and the direction of the ball, and squash it a little too.
Some changes of shape last. If you press your thumb into modelling clay, the dent stays. Others do not: a squashed football springs back as soon as the force is removed. How springs and other materials stretch is covered in the lesson Hooke's Law and Elastic Behaviour.
The reverse is also true. If a body is speeding up, slowing down, turning or changing shape, a force must be acting on it.
Types of force
The exam can ask you to identify the forces in a situation, so learn the names in this table.
| Force | What it is | Example |
|---|---|---|
| Gravitational force | The pull between bodies that have mass. The pull of the Earth on a body is called its weight, and it acts downwards. | An apple falling from a tree |
| Electrostatic force | The force between bodies that have an electric charge. It can attract or repel. | A rubbed balloon picking up scraps of paper |
| Magnetic force | The force between magnets, or between a magnet and a magnetic material such as iron. It can attract or repel. | A magnet holding a note on a fridge door |
| Normal contact force | The push from a surface on a body that presses against it. It acts at right angles to the surface. | A table pushing up on a book |
| Friction | The force between two surfaces that opposes motion. | Brake pads gripping a wheel |
| Drag (air resistance) | Friction from a liquid or a gas on a body moving through it. In air it is called air resistance. | The air pushing back on a parachute |
| Tension | The pull in a rope, string or cable that is being stretched. | A rope holding a climber |
| Upthrust | The upward push of a liquid or a gas on a body in it. | Water holding up a floating boat |
| Thrust | The push from an engine that drives something forwards. | A rocket lifting off |
Contact and non-contact forces
🧲 Non-contact forces
The two bodies do not need to touch. The force acts across the gap between them.
There are three: gravitational, electrostatic and magnetic.
Weight is the one that catches people out. The Earth pulls on a bird in the air just as it pulls on a bird on the ground. The pull does not need contact, so weight is a non-contact force.
Showing forces with arrows
You cannot draw a force itself, so you draw a force arrow. Follow these rules:
- Draw the body as a simple box or dot.
- Draw a straight arrow that starts on the body and points in the direction of the force.
- Make the length of the arrow show the size of the force. A bigger force gets a longer arrow.
- Label the arrow with the name of the force, and its size in newtons if you know it.
Worked example: a motor boat
A motor boat moves to the right across a lake. Describe the force arrows you would draw on it.
Draw a box for the boat. Then add four arrows:
1. An arrow pointing straight down, labelled weight. The Earth pulls the boat down.
2. An arrow pointing straight up, labelled upthrust. The water pushes the boat up.
3. An arrow pointing to the right, labelled thrust. The engine drives the boat forwards.
4. An arrow pointing to the left, labelled drag. The water and the air resist the motion.
What happens when several forces act together is covered in the lesson Scalars, Vectors and Resultant Forces.
Friction: the force that opposes motion
You need to know this sentence: friction is a force that opposes motion. If a box slides to the right across a floor, the friction on the box acts to the left. Friction never helps a sliding body along. It always acts in the direction that would slow it down.
Friction acts in two places:
- Between solid surfaces that slide, or try to slide, over each other. Rough surfaces usually give more friction than smooth ones.
- In liquids and gases. A body moving through water or air has to push the liquid or gas out of the way. This friction is called drag. In air it is also called air resistance.
Friction also makes surfaces warm. Rub your hands together quickly and you can feel it.
✅ When friction is useful
The soles of your shoes grip the ground, so you can walk without slipping.
Tyres grip the road, and brakes use friction to slow a wheel.
Friction holds a knot tight and keeps a nail in a wall.
❌ When friction is a problem
Moving parts in an engine rub together. They heat up and wear away.
Drag slows down cars, boats and swimmers, so more fuel or effort is needed.
It is reduced with oil (lubrication), wheels and smooth, streamlined shapes.
Optional investigation: sliding blocks
You do not have to do this, but it shows friction clearly. Hook a newton meter to a wooden block. Pull the block slowly and steadily along a table top and read the force needed to keep it sliding. Repeat with the same block on other surfaces, such as sandpaper, carpet and a sheet of glass. Rougher surfaces need a larger force, because the friction is larger. Use the same block each time so the test is fair.
Common mistakes
- Calling weight a contact force. Gravity acts across a gap, so weight is non-contact.
- Drawing friction in the direction of motion. Friction opposes motion, so its arrow points the opposite way.
- Writing just "gravity" as the label. The more exact name for the force on the body is its weight.
- Forgetting change of shape. There are three effects of a force, not two: speed, direction and shape.
- Arrows that float in space. A force arrow must touch the body it acts on, and it needs a label.
- Mixing up thrust and upthrust. Thrust comes from an engine. Upthrust comes from a liquid or a gas and always acts upwards.
Exam-style question
A worker uses a horizontal rope to drag a wooden crate to the right across a rough, level warehouse floor.
(a) Name the force that the rope exerts on the crate. [1 mark]
(b) State the direction of the friction force on the crate. Give a reason for your answer. [2 marks]
(c) Name the two forces that act vertically on the crate, and state the direction of each. [2 marks]
(d) State which one of the forces acting on the crate is a non-contact force. [1 mark]
(e) The worker lifts the crate onto a trolley with wheels. Suggest why the crate is now easier to move. [1 mark]
Model answer
(a) Tension. (1)
(b) To the left. (1) Friction opposes motion, and the crate is moving to the right. (1)
(c) Weight, acting downwards. (1) Normal contact force, acting upwards. (1)
(d) Weight. (1)
(e) There is less friction when the wheels roll than when the crate slides along the floor. (1)
Exam tip
When a question asks you to name forces, work through a checklist. Is the body near the Earth? Then it has weight. Is it touching a surface? Then there is a normal contact force, and friction if it is sliding. Is a rope pulling it? That is tension. In part (c), write "weight" rather than "gravity": it is the more exact name for the force.