What is a force?
A force is a push or a pull on an object. Forces arise from the interaction of at least two objects. You cannot push a door without touching it, and a magnet cannot attract a nail unless the nail is there.
Force has both a size (magnitude) and a direction. The SI unit of force is the newton (N).
Two forces in the same direction add up.
Two forces in opposite directions act against each other, and the net force is their difference. In a tug-of-war, if both teams pull equally hard, the rope does not move.
What can a force do?
A force can:
move an object that is at rest: kicking a football,
change the speed of a moving object: pedalling harder speeds up a bicycle, braking slows it down,
change the direction of motion: a batsman deflecting a ball,
change the shape of an object: squeezing a ball of dough, stretching a spring.
A force cannot always do all of these. Pushing a wall does not move it, but the force is still there.
Contact and non-contact forces
Contact forces act only when objects touch.
Muscular force: the force of our muscles, used in lifting, walking and pushing.
Friction: the force that opposes motion between surfaces in contact. It slowly stops a rolling ball.
Non-contact forces act from a distance.
Magnetic force: a magnet attracts iron without touching it.
Electrostatic force: a comb rubbed in dry hair attracts small bits of paper.
Gravitational force (gravity): every object in the universe attracts every other object. Earth's gravity pulls things down when they are dropped. The force of gravity on an object is its weight.
Pressure
Why is it easier to cut with a sharp knife than a blunt one? The force may be the same, but a sharp edge concentrates it on a much smaller area.
Pressure = Force ÷ Area on which it acts
The SI unit of pressure is the pascal (Pa): 1 Pa = 1 N/m².
Smaller area → more pressure: knife blades, needles and nails are sharp-pointed.
Larger area → less pressure: porters place a round cloth on their heads, school bags have broad straps, and heavy trucks and tractors have wide tyres.
Example. A 600 N box rests on a face of area 0.5 m². The pressure is 600 ÷ 0.5 = 1200 Pa. If it is turned onto a face of area 0.2 m², the pressure becomes 600 ÷ 0.2 = 3000 Pa.
Pressure in liquids
Liquids exert pressure on the bottom and the walls of their container.
Pressure in a liquid increases with depth. Make three holes at different heights in a bottle full of water: the water from the lowest hole spurts out farthest.
At the same depth, a liquid exerts equal pressure in all directions.
This is why the walls of a dam are made much thicker at the bottom, where the water pressure is greatest.
Atmospheric pressure
The air around the Earth also has weight, so it presses on everything. This is atmospheric pressure. At sea level, it is roughly equal to the weight of 10 tonnes on every square metre of surface.
We are not crushed because the pressure inside our bodies balances the air pressure outside.
You can see atmospheric pressure at work in a rubber sucker. When you press it against a smooth surface, the air between them is pushed out. The air pressure outside then holds the sucker firmly in place.