An object can have one or more forces acting on it at the same time. Each of these can have different strengths and different directions. The sum of these forces is called the resultant force and it’s this single force which can cause an object to move or change in how it’s moving.

In GCSE Physics, forces can be represented in force diagrams. The longer an arrow is the larger its force on the object.

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Zero resultant force

A resultant force is zero when all the forces are balanced.

• – A stationary object stays stationary as long as the resultant force is zero.
• – An object that’s moving, on the other hand, will continue to move at the same speed and in the same direction.

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Non-zero resultant force

If the forces aren’t balanced then the resultant force is not zero. An object will then move as dictated by the force.

• – A stationary object will start to move in the direction of the resultant force.
• – An object that’s moving will either speed up, slow down, or change direction, depending how the resultant force is moving and in which direction.

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Stopping distances

In order to drive safely it’s important to understand about stopping distances. The stopping distance can be calculated by the following equation:

stopping distance = thinking distance + braking distance

It takes a certain amount of time for a driver to react and begin to apply the brakes. During this reaction time the car is still moving and the distance travelled is called the thinking distance.

The thinking distance increases with the speed at which the vehicle is travelling. It also increases as the reaction time increases. The reaction time can be affected by:-

• – tiredness
• – distractions
• – bad visibility
• – driving under the influence of alcohol or other drugs

The braking distance is the distance it takes a vehicle to stop once the brakes have been applied. As the speed at which the vehicle travelling increases so will the braking distance.

The braking distance will also increase if:

• – the weather conditions are poor which affects the road making them, for example, wet or icy
• – the brakes or tyres of the vehicle are in a bad condition