Unit 2, lesson 11. About 12 minutes.

Balanced and unbalanced forces

Why can a tug of war stay perfectly still when both teams pull hard?

Play

Getting the experiment ready…

Explain

Same pull on both sides means no change.

A bigger pull wins.

Balanced forces are equal and opposite. They cancel out, so motion stays the same.

Unbalanced forces do not cancel out. There is a net force, so motion changes.

A book on a table is balanced: gravity pulls down and the table pushes up.

Add all the forces, using a sign for direction, to find the net force. Zero net force means zero acceleration.

Balanced does not always mean still. A plane in level flight at a steady speed has balanced forces too.

Worked example: 80 N pulls right and 60 N pulls left. The net force is 20 N to the right, so the rope accelerates to the right.

Add up all forces
Balanced: motion stays the same
Unbalanced: motion changes

Want the full story?

The sim treats the rope and teams as one moving mass, and ignores how feet grip the ground. In real tug of war, the winning team is the one that gets more friction from the ground. Forces at angles also need resolving into parts before adding.

The Formula

net force = force one way − force the other way

F_{\text{net}} = F_1 - F_2

$F_{\text{net}}$: the total force left over, measured in newtons (N)
$F_1$: the force pushing right, measured in newtons (N)
$F_2$: the force pushing left, measured in newtons (N)

Try the numbers

Force right: 60 NForce left: 60 N

Net force (positive means right): 0 N

Try it at home

Ask a grown-up to help

You need a piece of string or a towel, a friend.

You need: a piece of string or a towel, a friend.

Pull a towel with a friend. Who moves?

Hold a towel at each end, standing on a safe floor.
Pull gently with equal force.
Now one person pulls a little harder.

What's happening: equal pulls balance. A bigger pull leaves a net force, so the towel moves.

Physics is life

Your body

Your bones hold up your body.

When you stand still, your bones and muscles balance gravity.

Daily life

A book stays on a table.

Bridges and buildings are designed so that all their forces balance.

Nature

A duck floats on a pond.

A floating duck is balanced: water pushes up as hard as gravity pulls down.

Space

Stars stay the same size for a long time.

The Sun is balanced: gravity pulls in and hot gas pushes out.

Quick check

Question 1 of 3

What happens with balanced forces on a still object?

Words to know

Balanced forces: Forces that cancel out, so the net force is zero.

Balanced forces: Forces that cancel out, so the net force is zero.
Unbalanced forces: Forces that do not cancel out, so something changes its motion.
Net force: The total force left over after adding up all the pushes and pulls.

Next up

Gravity: why things fall