KS3 · Physics

Balanced and unbalanced forces

A parked car and a car cruising steadily down a straight road have something surprising in common: the forces on both add up to exactly zero. How?

Physics · Forces

Parked, cruising, falling: what do the arrows say?

Tap each scene. Compare the arrows, then read the resultant force. The numbers are made-up examples in newtons (N).

Resultant force · F = ma

Net force0 N

A parked car. Its weight pulls down. The road pushes up just as hard. Equal and opposite arrows cancel, so the resultant force is 0 N.

Selected scenario

Parked car

Net force

0 N

Motion

Stays still

Stays still. The forces are balanced, so there is no resultant force and no change in speed. It was still, so it stays still.

Adding forces along a line

Where does the resultant land?

Right is + and left is −. For each object, work out the resultant force and drag its letter to that spot on the line. The forces are made-up examples in newtons.

A — Tug of war: 6 N pulling right, 4 N pulling leftB — A box pushed from both sides: 3 N to the right, 8 N to the leftC — Two friends push a door from opposite sides, 5 N eachD — Two ropes both pull a box to the right: 3 N and 4 N (no other forces)

Physics · What happens next?

Will the speed change? Follow the forces

Start at the top. Choose the branch that matches the object, and see where you end up.

Is there a resultant force? → What is the object doing?

Always start here: add the forces along the line to find the resultant, taking direction into account.

5 possible outcomes.

On Add up the forces on the object. 2 branches to choose from.

Every object, every time, goes through the same two questions.

Watch out: Slowing down needs a resultant force too. It just points backwards, against the motion.

Test your gut feeling

What's keeping the cyclist going?

A cyclist rides along a flat, straight road at a steady speed. She isn't speeding up and she isn't slowing down.

Which is closest to what you think about the forces on the cyclist right now?

The forward force must be bigger than the backward forces, or she wouldn't be moving.The forward and backward forces are balanced, so the resultant force is zero.The forces can't be balanced. Balanced forces would mean she's standing still.There are no forces on her at all, because nothing is changing.

How sure are you?

I'm sureI think soI'm guessing

The forward force must be bigger than the backward forces, or she wouldn't be moving.

This is a common misconception

Lots of people think this, so you're in good company. But a forward resultant force would make her speed up, and she isn't speeding up. Forces don't keep things moving. They make things change. Her pedalling force forwards is exactly matched by friction and air resistance backwards.

The forward and backward forces are balanced, so the resultant force is zero.

This matches the evidence

Yes. Her speed isn't changing, so there can't be a resultant force. The push from pedalling is exactly matched by the backward forces. Balanced forces mean no change in speed. They don't mean no movement.

The forces can't be balanced. Balanced forces would mean she's standing still.

This is a common misconception

Balanced forces mean the speed stays the same. Something standing still stays still, and something already moving keeps moving at the same speed. Remember the parked car and the cruising car: both had a resultant of 0 N.

There are no forces on her at all, because nothing is changing.

This is a common misconception

Nearly, but not quite. There are plenty of forces on her: the push from pedalling, friction and air resistance, her weight, and the road pushing up. They're all there. They just cancel out. A zero resultant means the forces add up to zero, not that there aren't any.

A steady speed in a straight line means the forces are balanced. Forces don't keep things moving. They make things change, like speeding up or slowing down.

How big a change?

Same trolley, four different floors

How quickly the trolley's speed changes. It's the same trolley each time, pushed forwards on different floors (made-up values).

1 · Speed changes fastest4 · Speed doesn't change at all

Sticky floor: push 20 N forwards, friction 16 N back
Smooth floor: push 15 N forwards, friction 3 N back
Carpet: push 9 N forwards, friction 9 N back
Tiled floor: push 12 N forwards, friction 4 N back

Watch out: The biggest push doesn't win. The biggest resultant force does. And this comparison only works for the same object.

Your turn to explain

Explain it like a physicist

A car is driving along a straight road. The driver eases off the accelerator. Now the drive force is 1500 N forwards, and friction and air resistance add up to 2000 N backwards. Explain what happens to the car's speed. [4 marks]

Your answer

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

The mark scheme

Works out the resultant force: 2000 − 1500 = 500 N. (1 mark)
Says the resultant acts backwards, the direction of the bigger force. (1 mark)
Says the forces are unbalanced (the resultant isn't zero), so the speed changes. (1 mark)
Concludes the car slows down, because the resultant points against the way it is moving. (1 mark)

Model answer

The forces are unbalanced. The resultant force is 2000 − 1500 = 500 N, and it acts backwards because that's the direction of the bigger force. A resultant force changes the speed. This one points against the way the car is moving, so the car slows down, even though there is still a forward force on it.

A strong explanation runs in order: the forces → the resultant (size and direction) → balanced or unbalanced → what happens to the speed, and which way.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Add up the arrows, find the resultant, and you can predict what happens to the speed.

What you need to know

A force is a push or a pull. Forces come from two objects interacting, and force arrows show the forces acting on an object.
Forces along one line add up to one resultant force. Take their directions into account.
Equal forces in opposite directions are balanced: the resultant is zero and the speed doesn't change.
A resultant force that isn't zero (unbalanced forces) changes the speed, in the direction of the resultant.

The big picture

Forces acting along one line add up to a single resultant force, as long as you take their directions into account. If the resultant is zero, the forces are balanced and the speed doesn't change, whether the object is still or already moving. If it isn't zero, the forces are unbalanced and the speed changes in the direction of the resultant: faster if it points along the motion, slower if it points against it.

Key points

1Resultant force = all the forces added together, taking direction into account. Same direction: add. Opposite directions: subtract, and the resultant points the way of the bigger force.

2Balanced forces → zero resultant → no change in speed. A still object stays still. A moving object keeps moving at the same speed.

3Unbalanced forces → a resultant force → the speed changes. Resultant along the motion: speeds up. Resultant against the motion: slows down.

4The resultant shows the direction of the change, not the direction of motion. Forces make things change.

5For the same object, a bigger resultant force gives a bigger, faster change in speed.

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Worked example

Problem

A rowing boat is floating still on a calm lake. Two people pull on ropes tied to it: one pulls with 50 N to the right, the other with 35 N to the left. Will the boat's speed change? If so, how?

⚠ Watch out

Thinking a moving object must have a bigger force pushing it forwards. If something is moving at a steady speed, the forces on it are balanced. A forward resultant force would make it speed up.

🧠

Memory hook

Balanced means 'same speed', not 'stopped'. Unbalanced means 'speed changes', and the resultant arrow points the way of the change.

✓

Check yourself

A lift is going up at a steady speed. The cable pulls it up with 8000 N. Without looking back, how big is the total downward force on the lift, and how do you know?

Flashcards

(13)

What is a force?

A push or a pull. Forces come from an interaction between two objects.

What do force arrows on a diagram show?

The forces acting on an object: which way each one acts, with a longer arrow for a bigger force.

What unit are forces measured in?

Newtons (N).

What is the resultant force?

The single force you get by adding up all the forces on an object, taking their directions into account.

Two forces act on an object in the same direction. How do you find the resultant?

Add their sizes. The resultant acts in that same direction.

Two forces act in opposite directions. Which way does the resultant act?

In the direction of the bigger force.

When are two forces balanced, and what is the resultant then?

When they are equal in size and act in opposite directions. The resultant is zero.

The forces on an object are balanced. What happens if it is still? What if it is moving?

Still: it stays still. Moving: it keeps moving at the same speed.

What does an unbalanced force (a non-zero resultant) do to an object?

It changes the object's speed.

The resultant force points the same way an object is moving. What happens?

It speeds up.

The resultant force points the opposite way to an object's motion. What happens?

It slows down.

What does the direction of the resultant force tell you?

The direction of the change in speed, not the direction of motion.

For the same object, what does a bigger resultant force do?

Gives a bigger, faster change in speed.

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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Parked, cruising, falling: what do the arrows say?

Where does the resultant land?

Will the speed change? Follow the forces

What's keeping the cyclist going?

Same trolley, four different floors

Explain it like a physicist

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