Facts At Your Fingertips: Energy and forces – Forces
Forces are pushes and pulls that change an object’s shape or movement.
Every time an object speeds up, slows down, or changes direction, this happens because a force is acting on it. Forces such as gravity can act over huge distances, keeping the planets in orbit around the Sun.
Gravity
The force of gravity pulls all objects toward each other. Objects with just a small mass only pull very weakly, but the gravity of our planet Earth is strong enough to hold us to the ground.
Space rockets use powerful engines to push them away from the ground and break free from Earth’s gravity.
Under pressure
A force acting on a particular area causes pressure. A force applied over a small area produces more pressure than the same force applied over a larger area. The tracks on a tank’s wheels spread its weight over a large area. This reduces the pressure and stops it from sinking in soft ground.
Balanced forces
More than one force can act on an object at the same time. Two forces pulling in opposite directions will stretch an object, but will not move it. The cables holding up a suspension bridge pull against the weight of the bridge to stop it from collapsing.
Air resistance
Also called drag, air resistance is a force that pushes against an object as it moves through the air. The faster the object moves, the greater the air resistance, which tries to slow the object down. Jet fighter planes have a smooth, streamlined shape to reduce air resistance.
Friction
When two objects rub against each other, the action produces a force called friction. Friction slows down movement and generates heat. The brakes in cars use friction to slow their wheels down by pressing a disk against them.
Reducing friction
Friction is caused when the rough surfaces of objects catch one another as the objects rub against each other.
Rolling movement reduces friction because a rolling object rubs much less against the flat surface it is rolling on.
A black hole is a very dense object in space. It has such strong gravity it pulls everything around it, even light from stars.