A force is a push or a pull on an object. Forces have both a size and a direction, and it is the combination of every force acting at once that decides what an object does.
Balanced and unbalanced
When the forces on an object cancel out, the forces are balanced and the motion does not change. A book on a table has gravity pulling it down and the table pushing it up with equal strength, so it stays put. When forces do not cancel, they are unbalanced, and the object speeds up, slows down, or changes direction.
Balanced does not mean still. A car travelling at a steady speed on a straight road has balanced forces too: the engine pushes forward exactly as hard as friction and air resistance push back. Balanced forces mean no change in motion, not no motion.
Gravity
Gravity is the pull between any two objects with mass. It is what keeps you on the ground and the moon in orbit. Its strength depends on how much mass the objects have and how far apart they are. The earth is enormous, so its pull dominates everything near its surface.
Weight is the force of gravity on an object, so it changes with location. Mass is the amount of matter in it and does not change. An astronaut has the same mass on the moon and weighs about one sixth as much, because the moon pulls more weakly.
Friction
Friction is a force that opposes motion between two surfaces in contact. It always acts against the direction of sliding. Rough surfaces produce more friction than smooth ones, and heavier objects press harder and so meet more friction.
| Situation | Friction is |
|---|---|
| Shoes gripping a floor | helpful, you could not walk without it |
| Brakes stopping a bicycle | helpful, it is the whole mechanism |
| Moving parts of an engine wearing down | a problem, reduced with oil |
| Dragging a heavy box across carpet | a problem, reduced with wheels |
Why a rolling ball stops
A ball rolled across a floor slows and stops, which makes it look as though motion naturally runs out. It does not. Friction with the floor and the air is what removes the motion. On a very smooth surface the ball travels much further, and with no friction at all it would keep going indefinitely. That is the idea behind an object in space continuing to drift for years without any engine.
Common mistakes
Thinking a moving object always has a force pushing it
A force is needed to change motion, not to continue it. A ball in flight has gravity and air resistance acting on it, and nothing pushing it forward.
Using weight and mass as the same word
Mass is how much matter. Weight is how hard gravity pulls on it.
Check yourself
A box sits still on the floor. Are the forces balanced?
Yes. Gravity pulls down and the floor pushes up equally, so the motion does not change.
Why do bicycle brakes work?
Friction. The pads press on the wheel and oppose its turning, turning the movement into heat.
An astronaut travels to the moon. What happens to mass and to weight?
Mass stays the same. Weight drops to about a sixth, because the moon's gravitational pull is weaker.
Why would a ball rolled in space keep moving?
Because there is essentially no friction and no air resistance to slow it, so nothing acts to change its motion.
See also
Sources
- Forces, gravity and friction as taught in United States elementary physical science. Examples on this page are our own.