Forces, motion, and magnets (grade 3) · lesson

Balanced forces

💡 When two forces are the same size and push in opposite ways, they cancel out and nothing changes.

A force is a push or a pull. Most things have more than one force on them at the same time.

When two forces are the same size but go opposite ways, they are balanced. They cancel each other out. A thing that is still stays still.

A book on a table has forces on it. Gravity pulls it down. The table pushes it up just as hard. So the book does not move.

🧠 Let's work it out

Ask: is the thing moving or still?
If it stays still, the forces on it must be balanced.
Two kids pull a rope the same amount.
Neither one wins, so the rope stays put.

What do you think comes next?

force: a push or a pull
balanced: equal forces in opposite ways that cancel out

🔗 Forces and Motion: Basics · PhET Interactive Simulations ↗

Unbalanced forces

💡 An unbalanced force makes a thing start, stop, speed up, or change direction.

When one force is bigger than the force against it, the forces are unbalanced.

Unbalanced forces change how a thing moves. A still cart starts to roll. A rolling ball stops.

If two people push a box from opposite sides, the bigger push wins. The box moves the way of the bigger push.

🧠 Let's work it out

Compare the pushes.
Leo pushes right hard.
Jo pushes left a little.
The pushes are not equal, so the box moves.
It goes right, the way of the bigger push.

What do you think comes next?

unbalanced: one force is bigger, so motion changes

Magnets

💡 Magnets pull on iron and steel, and their poles push or pull on each other even without touching.

A magnet pulls on things made of iron or steel. Steel has iron in it. A magnet does not pull on wood, plastic, or paper.

Each magnet has two ends called poles. The pull is strongest at the poles. One is north and one is south, just like the North and South Poles of Earth. Two of the same pole push apart. A north and a south pull together.

A magnet can pull from a short way off, without touching. The pull gets weaker as the magnet gets farther away.

🧠 Let's work it out

Look at which ends face each other.
North and north? Same poles, so they push apart.
Then think about distance.
Close up, the pull is strong.
Far away, it may be too weak to move a clip.

What do you think comes next?

pole: one end of a magnet, north or south
attract: pull together
repel: push apart

🔗 Magnet and Compass · PhET Interactive Simulations ↗

Patterns of motion

💡 When a motion repeats the same way, you can use the pattern to predict what comes next.

Some things move the same way again and again. A swing goes back and forth. A merry-go-round goes round and round.

When a motion repeats, it makes a pattern. You can use the pattern to say what will happen next.

Other motions do not repeat. A leaf in gusty wind goes a new way each time. Its path is hard to predict.

🧠 Let's work it out

Watch a few trips.
Each swing trip takes about 2 seconds.
The pattern stays the same, so the next trip will take about 2 seconds too.

What do you think comes next?

pattern: something that repeats the same way
predict: say what will happen before it happens

Static electricity

💡 Rubbing two things together can build up a charge that pulls on things or jumps as a tiny shock.

Rub a balloon on your hair. Tiny bits of charge move from one to the other. Now the balloon and your hair pull toward each other.

This is static electricity. It can pull without touching, like a magnet. But it is not a magnet. It works on hair and paper too.

Charge can build up on you when you shuffle across a rug in socks. When you touch metal, the charge jumps. You feel a small shock.

🧠 Let's work it out

Look for rubbing.
Was something rubbed first? Then a pull or a shock is most likely static.
Magnets only pull on iron, and hair has no iron.

What do you think comes next?

charge: a tiny bit of electricity that can build up when things rub
static electricity: charge that builds up on a thing and stays until it jumps away

🔗 Balloons and Static Electricity · PhET Interactive Simulations ↗