KS3 · Physics

The different kinds of force

Kick a ball, lean on a wall, drop a pencil: three different forces, and every one hides the same secret. It takes two objects to make a force.

Contact or non-contact?

Sort the forces

Pick a force, then choose its group. The question to ask every time: do the two objects have to be touching for this force to act?

Still to sort

Contact forces (0)

Only act while the two objects are touching.

Where the line is: A contact force stops the instant the objects stop touching. Lift a cup off a table and the table's push on the cup is gone.

Non-contact forces (0)

Can act across a gap, touching or not.

Where the line is: A non-contact force doesn't switch off when things touch. The Earth still pulls on you while you stand on the ground.

9 of 9 still to sort.

Which two objects?

A sledge is pulled across flat snow by a horizontal rope. Tap each arrow and read its full name: the [type] force of [A] on [B]. (Air resistance is left off to keep it clear; it would point backwards too.)

Tap a force to see what it does.

What do you really think?

Where did the kick go?

A footballer kicks a ball. It leaves the boot and sails through the air across the pitch.

While the ball is flying through the air, which of these is closest to what you think?

The force of the kick is still in the ball, pushing it along until it runs out.The ball has its own force now. That's what keeps it moving.The kick stopped when the ball left the boot. The forces on it now come from other objects.

How sure are you?

I'm sureI think soI'm guessing

The force of the kick is still in the ball, pushing it along until it runs out.

This is a common misconception

Lots of people picture a force as something you can pour into an object. But the kick is the contact force of the boot on the ball, and a contact force stops the instant contact is lost. Once the ball leaves the boot, the kick is over. The ball keeps moving, but no kick is pushing it.

The ball has its own force now. That's what keeps it moving.

This is a common misconception

A ball can't 'have' a force, any more than it can 'have' a handshake. A force is always between two objects. Try naming the ball's force as 'the ___ force of ___ on ___' and you'll find there's no second object to put in the gap.

The kick stopped when the ball left the boot. The forces on it now come from other objects.

This matches the evidence

Exactly right. The boot's push ended the moment they stopped touching. In the air, the ball still has forces on it, but each comes from a second object: the gravitational force of the Earth on the ball, and the drag force of the air on the ball.

A force is never something an object owns. It is always between two objects, and a contact force ends the moment they stop touching.

Friction vs drag

FrictionvsDrag

The two forces that slow things down, and easy to mix up.

Focus

What pushes on what

Friction

One surface gripping or rubbing on another

Drag

A liquid or gas pushing on an object moving through it

The insight

Both are contact forces. For drag, the 'surface' doing the touching is air or water.

Which way it acts

Friction

Opposite to the motion

Drag

Opposite to the motion

When it acts

Friction

Only when there is movement, or something is trying to move

Drag

When an object moves through a liquid or gas

Other names

Friction

Just friction

Drag

Air resistance in air; water resistance in water

How we draw it

Friction

One arrow along the surface, against the motion

Drag

One arrow against the motion, though it pushes on every part of the surface

Predict, then check

So what do forces actually do? Three quick scenes.

A boot kicks a still football. A sledge slides across snow and slowly stops. An apple drops from a tree. In all three, what is the force doing to the object?

Your turn

Describe the forces yourself

A small boat is pulled slowly across a calm lake by a rope. Describe the four forces acting on the boat. Name each one as 'the ___ force of ___ on ___' and say whether it is a contact or a non-contact force. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The weight (gravitational force) of the Earth on the boat, not just 'gravity' (1 mark)
The upthrust force of the water on the boat (1 mark)
The tension force of the rope on the boat (1 mark)
The drag (water resistance) force of the water on the boat (1 mark)
Weight is non-contact, and upthrust, tension and drag are all contact (1 mark)

Model answer

The weight of the boat is the gravitational force of the Earth on the boat, and it is a non-contact force. The upthrust force of the water on the boat is a contact force. The tension force of the rope on the boat is a contact force. The drag force of the water on the boat, called water resistance, is a contact force too.

Start with 'on the boat', then hunt for the second object: the Earth, the water, the rope. Once you have both objects, the type of force and contact or non-contact follow.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Every force is an interaction between two objects.
Name a force as 'the [type] force of A on B'.
Contact forces need touching; non-contact forces don't.
Weight is the gravitational force on an object.
Forces can change speed, direction or shape.

The big picture

A force is a push or a pull between two objects, so every force can be named 'the [type] force of A on B'. Contact forces only act while the objects touch, and that includes upthrust and drag. Non-contact forces (magnetic, electrostatic and gravitational) can act across a gap. The gravitational force on an object is its weight, and forces can change an object's speed, direction or shape.

Key points

1A force is a push or a pull between two objects: one object provides it, and it acts on the other.

2Describe any force as 'the [type] force of object A on object B', for example the friction force of the ground on a sledge.

3Contact forces only act while objects touch and stop as soon as contact is lost. Upthrust and drag count, because the liquid or gas is touching the object.

4Non-contact forces (magnetic, electrostatic and gravitational) can act with or without the objects touching.

5Friction and drag both oppose motion. Drag in air is air resistance; drag in water is water resistance.

6The gravitational force on an object is its weight. It points towards the centre of the nearest planet, moon or star, and is labelled 'weight', never just 'gravity'.

7Forces can change an object's speed, direction or shape.

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Worked example

Problem

A skydiver is falling through the air. Name the two forces acting on the skydiver, in full, and say whether each is a contact or a non-contact force.

⚠ Watch out

Calling upthrust or air resistance non-contact because water and air don't feel like they are 'touching'. They are contact forces: the liquid or gas presses on the object's surface, and once the object is out of it, the force stops.

🧠

Memory hook

Two objects, one sentence: 'the ___ force of ___ on ___'. Then ask one question: do they have to touch?

✓

Check yourself

A fridge magnet holds a note on a steel fridge door. Name one contact force and one non-contact force on the magnet, each as 'the ___ force of ___ on ___'.

Flashcards

(14)

What is a force?

A push or a pull that comes from an interaction between two objects: one object provides it, and it acts on the other.

What sentence can you use to name any force?

'The [type] force of object A on object B', e.g. the friction force of the ground on a sledge.

What makes a force a contact force?

It only acts while the objects are touching, and it stops as soon as contact is lost.

What makes a force a non-contact force?

It can act with or without the objects touching, even across a gap.

Name the three kinds of non-contact force.

Magnetic, electrostatic and gravitational.

What is tension?

The pulling force from a string or rope.

What is upthrust, and is it contact or non-contact?

The upward force from a surrounding liquid or gas that makes things float. It is a contact force.

What is a normal contact force, and which way does it point?

The push of a surface back on an object that presses on it. It acts at 90° to the surface.

When does friction act, and in which direction?

Only when there is movement or attempted movement, and always opposite to the motion.

What are air resistance and water resistance?

Both are drag: the force opposing an object moving through a gas (air) or a liquid (water).

What is weight, and what should you never label it?

Weight is the gravitational force on an object. Label it 'weight' or 'gravitational force', never just 'gravity'.

Which way do gravitational forces point?

Towards the centre of the nearest planet, moon or star. Near Earth's surface, draw them straight down.

What is a driving force? What does 'thrust' strictly mean?

A driving force is any force that pushes something forwards. Thrust is a driving force made by pushing material away in the opposite direction, as a firework does.

What three things can a force change about an object?

Its speed, its direction or its shape.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Sort the forces

Where did the kick go?

Describe the forces yourself

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