A force is a push or a pull. It can make something start to move, slow down or speed up, change direction or change shape or size. The greater a force, the more effect it has.
The wind is a force. Biting, twisting, stretching, lifting and many other actions are also forces. Every time something happens, a force is involved.
Force is measured in newtons (N). One newton is the force needed to speed up a mass of one kilogram by one meter per second every second.
When something moves there are usually several forces involved. When you throw a ball, the force of your throw hurls it forwards, the force of gravity pulls it down and the force of air resistance slows it down.
The direction and speed of movement depend on the combined effect of all the forces involved — this is called the resultant.
A force has magnitude (size) and works in a particular direction.
A force can be drawn on a diagram as an arrow called a vector (see vectors). The arrow’s direction shows the force’s direction. Its length shows the force’s strength.
Four fundamental forces operate throughout the Universe: gravity, electric and magnetic forces (together called electromagnetic force), and strong and weak nuclear forces (see nuclear energy).
A force field is the area over which a force has an effect. The field is strongest closest to the source and gets weaker farther away.
Every force acts in a straight line. Things move round because of a ‘turning effect.
A turning effect is a force applied to an object that is fixed or pivots in another place, called the fulcrum.
In a door the fulcrum is the hinge.
The size of a turning force is known as the moment. Load
The farther from the fulcrum that a force is applied, the bigger the moment is.
A lever makes it much easier to move a load by making use of the moment (size of turning force).
A first-class lever, such as pliers or scissors, has the fulcrum between the effort and the load; a second-class lever, such as a screwdriver or a wheelbarrow, has the load between the effort and the fulcrum; a third-class lever, such as your lower arm or tweezers, has the effort between the load and the fulcrum.
Gears are sets of wheels of different sizes that turn together. They make it easier to cycle uphill, or for a car to accelerate from a standstill, by spreading the effort over a greater distance.
The gear ratio is the number of times that the wheel doing the driving turns the wheel being driven.
The larger the gear ratio the more the turning force is increased, but the slower the driven wheel turns.