Pushes, pulls and the net force

What is a force?

You kick a ball, pull a door, squeeze a sponge. Each time you push or pull. A push or a pull is a force. A force can

• start something moving, or stop it;
• make it go faster or slower;
• change its direction;
• change its shape.

We measure force in newtons (N). A small apple weighs about $1\ \text{N}$. A spring balance measures force by how far its spring stretches.

Contact and non-contact forces

A push from your hand and friction need two things to touch: they are contact forces. Gravity and the pull of a magnet act across a gap: they are non-contact forces.

Adding forces

A force has a size and a direction. When several forces act along one line, we add them with signs: one direction is plus, the other minus. The total is the net force.

• If the net force is zero, the forces are balanced. The object does not change its motion.
• If it is not zero, the forces are unbalanced. The object speeds up, slows down or turns.

You should find that the net force is zero when the two pushes add up to the friction, and that it points left (minus) when friction is bigger than both pushes.

Watch out. Balanced forces do not mean 'not moving'. A car at a steady speed has balanced forces too.

A worked example

Asha pushes a box to the right with $60\ \text{N}$ and Ben with $45\ \text{N}$. Friction pulls it back with $30\ \text{N}$. Find the net force.

1. Signs. Right is plus: $+60\ \text{N}$, $+45\ \text{N}$, $-30\ \text{N}$.
2. Add. $60\ \text{N} + 45\ \text{N} - 30\ \text{N} = 75\ \text{N}$.
3. So the forces are unbalanced, and the box speeds up to the right.

Practise with new numbers

Try it in Python

The program adds the forces of the example.

Remember these

• A force is a push or a pull, measured in newtons.
• The net force adds the forces with their directions.
• Balanced forces (net force zero) do not change the motion; unbalanced forces do.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name one contact force and one non-contact force.

2. Forces of $25\ \text{N}$ right and $25\ \text{N}$ left act on a block. What is the net force?

3. A box has $80\ \text{N}$ pushing it right and $50\ \text{N}$ pushing it left. Which way does the net force point, and how big is it?

4. A car moves at a steady speed on a straight road. Are the forces on it balanced?