Coaching Centre
What forces are
Forces as pushes and pulls between two objects, the agent and the receiver, the six things a force can do, measuring forces in newtons with a spring balance, and contact and non-contact forces.
Right now, while you sit still reading this, at least four forces are acting on you. The Earth is pulling you down. The chair is pushing you up. The air is pressing on every part of your skin. And the floor is pushing up on your feet. You don't notice any of them, because they cancel each other out.
Pushes and pulls
In everyday English, "force" can mean lots of things: being forced to do homework, a police force, forcing a door. In science it has one exact meaning. A force is a push or a pull on an object, caused by its interaction with another object.
Every force involves two objects. The object that exerts the force is the agent. The object that feels the force is the receiver. When you kick a football, your foot is the agent and the ball is the receiver. When the ball falls back down, the Earth is the agent pulling it. A ball can't throw itself: nothing changes its motion all by itself.
Words like shove, press, tug, drag, lift, squeeze and twist all describe pushes or pulls, so they are all forces.
What a force can do
You can't see a force, but you can see what it does. A force acting on an object can make it:
| Effect | Example |
|---|---|
| Start moving | A golf club hits a ball sitting on the tee |
| Speed up | Wind pushes on a yacht's sails |
| Slow down or stop | A parachute catches the air |
| Change direction | A tennis racquet sends the ball back over the net |
| Change shape | Your hands squash a lump of plasticine |
| Stay still | A pot plant sits on a shelf: gravity pulls down, the shelf pushes up |
Scientists group the first four together and say a force can change an object's motion: its speed, its direction, or both.
Measuring forces
Forces are measured in newtons (symbol N), named after Isaac Newton, who first described the rules that forces follow. To get a feel for one newton, hold a small apple or a 100 g block of chocolate in your hand. The Earth pulls it down with about 1 N.
In the lab, forces are measured with a spring balance (also called a newton meter or force meter). The harder you pull on the hook, the more the spring stretches, and a pointer shows the force on a scale. A stretchy spring can measure small forces accurately; a stiff spring can measure big forces but can't show small differences. Bathroom scales and kitchen scales measure pushes.
Before any measuring instrument can be trusted, it must be calibrated: checked against known forces and the scale marked to match. You can make your own force meter by hanging known masses from a rubber band and marking how far it stretches for 1 N, 2 N, 3 N and so on.
- Hang each object on the stretchy spring. Which ones are too heavy for it?
- Switch to the stiff spring. Why is the apple hard to measure accurately on it?
- Press "Hide the number" and read the scale yourself, then check.
Contact and non-contact forces
Some forces only act when two objects touch. These are contact forces: a hand pushing a door, a bat hitting a ball, friction between your shoes and the ground, the air pushing on a parachute.
Other forces act across a gap, without touching. These are non-contact forces, and there are three you'll meet in this chapter:
- Gravity: every object with mass pulls on every other object. The Earth pulls you down; the Moon's gravity pulls the oceans and makes the tides.
- Magnetic force: a magnet pulls on iron or pushes another magnet away, even through your hand.
- Electrostatic force: a charged balloon pulls your hair towards it before it touches.
Non-contact forces act through a field, a region of space around an object where it can push or pull on other things. You can't see a field, but you can detect it: drop a ball and you see the Earth's gravitational field at work.
Test yourself
The unit of force is the
Symbol N, after Isaac Newton.
When a hockey stick hits a ball, the receiver of the force is
The ball experiences the force; the stick is the agent.
Roughly how much does a 300 g can of drink weigh?
100 g weighs about 1 N.
Which of these is a non-contact force?
A magnet works across a gap.
A stiff spring is better than a stretchy spring for measuring
It doesn't overstretch, but small changes are hard to see.
Label the spring balance.
Contact or non-contact?
A force is a push or a . The object that exerts a force is called the . Forces are measured with a spring in units called . Gravity acts at a distance through a .
A student says, "A book lying on a table has no forces acting on it, because it isn't moving." Explain why the student is wrong, naming the agent and receiver of each force.
Two forces act on the book. The Earth (agent) pulls the book (receiver) downwards with gravity, and the table (agent) pushes the book (receiver) upwards. These forces are the same size but act in opposite directions, so they are balanced and cancel out. That is why the book doesn't move, even though forces are acting on it.