A force is a push or a pull. You cannot see one, but you can always see what it does — start, stop, speed up, slow down, turn or change shape.
Study Zone is free for our students — you just need to log in with your WhatsApp number first.
Log in with WhatsApp →The first two chapters of every book are free once you're logged in. To open this one, request an unlock and we'll get back to you on WhatsApp.
A force can change an object’s speed, its direction or its shape — often more than one at a time.
💪 Contact and non-contact forces
| CONTACT — must touch | NON-CONTACT — acts at a distance |
|---|---|
| muscular force | gravity |
| friction | magnetic force |
| normal (push of a surface) | electrostatic force |
| tension in a rope | — |
Force is measured in newtons (N) with a spring balance. Two forces in the same direction add up; two in opposite directions subtract. If they cancel exactly, the object does not change what it was doing.
A force does not always cause movement. Push hard against a wall and nothing moves, because the wall pushes back just as hard. Movement happens only when the forces do not balance.
Friction is the force that opposes motion between two surfaces in contact. It always acts against the direction you are trying to move.
🛞 More friction or less?
| More friction when… | Less friction when… |
|---|---|
| surfaces are rough | surfaces are smooth or polished |
| the object is heavier | the object is lighter |
| surfaces are dry | oil or grease is applied |
| sliding rather than rolling | rolling instead of sliding — ball bearings, wheels |
Friction is a friend and an enemy at once. Without it you could not walk, write, or stop a cycle — but it also wears out shoe soles and machine parts and wastes energy as heat. That is why some places need more of it and others less.
Rolling friction is much less than sliding friction. That is the whole reason for the wheel, and for ball bearings inside a fan or a cycle hub.
Every object pulls every other object. For everyday life only the earth’s pull is big enough to notice.
Mass or weight? Mass is the amount of matter, measured in kilograms, and it never changes. Weight is the force of gravity on that mass, measured in newtons, and it changes with location. On the moon your weight is about one-sixth of what it is here — your mass is exactly the same.
Why a feather falls slower than a stone. Not because it is lighter — because air resistance affects it more. With the air removed, a feather and a coin fall together, which has been demonstrated on the moon.
12 questions on forces and friction. 🍀
