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Introduction to Forces

A force can be a push or a pull.                                                     

You cannot see a force but often you can see what it does. When a force is exerted on an object, it can change the object’s:

  • speed
  • direction of movement
  • shape

Forces can be contact forces, where objects must touch each other to exert a force. Other forces are non-contact forces, where objects do not have to touch each other. These include:

  • gravity
  • magnetism
  • forces due to static electricity

The unit of force is called the newton, and it has the symbol N. The greater the force, the bigger the number, so 100 N is a greater force than 5 N.

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Vectors and Scalers

A vector is something that has both size and direction. On the other hand a scaler is something that only has size.

When 2 or more forces are applied to a body, they combine to produce a single resultant force. The resultant force is the overall effect of all the forces acting together on a body.

Newton's First Law:

  • An object with UNBALANCED forces will accelereate or decelerate.
  • An object with BALANCED forces will either not move or keep moving at a constant speed.
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Calculating Unbalanced Forces

An object may have several different forces acting on it, which can have different strengths and directions. But they can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together.

If the resultant force is zero, a moving object will stay at the same speed. If there is no resultant force then a system is said to be in equilibrium.

If the resultant force is not zero, a moving object will speed up or slow down - depending on the direction of the resultant force:

  • it will speed up if the resultant force is in the same direction as the object is moving
  • it will slow down if the resultant force is in the opposite direction


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A moving object has momentum. This is the tendency of the object to keep moving in the same direction. It is difficult to change the direction of movement of an object with a lot of momentum.

Notice that momentum does not just depend on the object’s mass and speed. Velocity is speed in a particular direction, so the momentum of an object also depends on the direction of travel. This means that the momentum of an object can change if:

  • the object speeds up or slows down
  • the object changes direction


As long as no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions. We say that momentum is conserved. You can use this idea to work out the mass, velocity or momentum of an object in an explosion or collision.

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Force and Momentum


To change the momentum of an object you can apply a small force over a long time, or a larger force over a shorter time. Changing the direction of an oil tanker at sea is difficult because a large change in momentum is needed, but the force from the propeller is only relatively small, so it takes a long time.

Triangle showing change in momentum equals force multiplied by time (

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Car Safety Features

When there is a car crash, the car, its contents and the passengers decelerate rapidly. They experience great forces because of the change in momentum which can cause injuries. Modern cars have safety features that absorb kinetic energy in collisions. These typically include:

  • seat belts
  • air bags
  • crumple zones

These features reduce injuries to the people in the car by absorbing energy from the impact. They increase the time taken for the change in momentum on the occupants' bodies, and so reduce the forces involved and any subsequent injuries. These features absorb energy when they change shape. This reduces injuries to the people in the car. They increase the time taken for the change in momentum on the occupants' bodies, and so reduce the forces involved and any subsequent injuries.

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