Forces: pushes and pulls
Every time something starts moving, stops, or changes direction, a force is at work. See how forces balance.
Open a door. Kick a ball. Pick up a bag. Squeeze a sponge. What do all of these have in common? In each one, you used a force.
What is a force?
A force is a push or a pull. That is all it is.
- When you push a box, the force goes away from you.
- When you pull a wagon, the force goes toward you.
Forces are invisible, but we can draw them as arrows. The arrow points the way the force goes. A longer arrow means a stronger force.
What can a force do?
A force can make something:
- start moving, like a ball that you kick
- stop, like a goalkeeper catching it
- speed up or slow down, like pedalling or braking
- change direction, like a tennis ball hit by a racket
- change shape, like a squashed sponge or a stretched elastic band
If you ever see something change how it moves or what shape it is, look for the force.
How strong is a force?
Scientists measure force in newtons (N), named after Isaac Newton. A small apple pulls down on your hand with a force of about 1 newton. Pushing a shopping trolley might take 20 newtons.
Two kinds of forces
Contact forces need touching. A push, a pull, and friction are contact forces.
Forces at a distance work without touching. Gravity pulls a falling apple to the ground. Magnets pull on iron even when they are not touching.
Forces come in pairs, and they can balance
A force rarely works alone. Usually, there are at least two. What happens when they fight each other?
Imagine a tug of war. If both teams pull equally hard, the arrows are the same size, but they point in opposite directions:
The forces are balanced. They cancel each other out, so the rope does not go anywhere. Nothing changes.
Now imagine the right team gets stronger:
The forces are unbalanced. The bigger pull wins, and the rope starts to move that way.
This is a very important idea:
- Balanced forces: nothing changes. A resting thing stays at rest.
- Unbalanced forces: the movement changes. The thing speeds up, slows down or turns.
Balanced forces are everywhere
A book on a table is not moving. But there are forces on it! Gravity pulls it down. The table pushes it up. The two forces are balanced, so the book stays put. Take the table away, and the forces are unbalanced. The book falls.
Next time you are walking down the street, try to name every force you can find. Gravity pulls you down, the ground pushes you up, friction stops your shoes from slipping. In Marble Machine, gravity pulls the marble down the track, and friction slows it a little.
Words to know
- Force
- A push or a pull.
- Newton (N)
- The unit of force.
- Balanced forces
- Forces that are equal and opposite, so they cancel out.
- Unbalanced forces
- Forces that are not equal, so the movement changes.
- Contact force
- A force that needs two things to touch.
Check yourself
What is a force?
Show the answer
A push or a pull.
Two teams pull on a rope with the same force in opposite directions. What happens?
Show the answer
Nothing. The forces are balanced, so the rope does not move.
A book rests on a table. What two forces act on it?
Show the answer
Gravity pulls it down and the table pushes it up. They are balanced.
Name three things a force can do.
Show the answer
Start something moving, stop it, speed it up or slow it down, change its direction, or change its shape. Any three of these.