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• Multiple forces act on an object at the same time.

• The size and direction of these forces determines the movement of the object.

Show the forces acting on an object in a free body diagram. The arrows represent the size and direction of the forces acting.

When drawing a force diagram:

• represent the object with a small box or dot
• draw the arrows with a pencil and ruler
• draw the arrows from the centre of the box or dot
• label the arrow with the name of the force and the size of the force

Examples of force diagrams include:

A book resting on a table

Skiing down a hill

A boat floating

Multiple forces act on an object at once and the resultant force that is acting on the object can be calculated. A resultant force is the overall force that acts on the object.

Remember that forces are vectors.

When you calculate the resultant force you need to also say the direction it is acting in. You can calculate this in two ways.

1. Forces acting in the same direction

In this diagram there are two forces, which, when added together give a resultant force of 30 newtons (N).

Resultant force = 20 N + 10 N

Resultant force = 30 N to the right

2. Forces acting in opposite directions

In this diagram there are two forces, acting in opposite directions:

Resultant force = 20 N – 10 N

Resultant force = 10 N to the left

The movement of the object and the resultant force acting on it are linked.

If the forces acting on the object are balanced there is no resultant force. This means the object could be travelling at a constant velocity. It could also be stationary or not moving at all.

Have a look at these examples.

1. A car is travelling down a motorway. The thrust from the engine is 500 N and the air resistance acting against the car is also 500 N.

Describe the motion of the car.

Resultant force = 500 N - 500 N

Resultant force = 0 N

There is no resultant force and the car is travelling on a motorway, this means that the car is travelling at a constant velocity.

2. A box is placed on a table. The weight of the force on the box acting downwards is 100 N. The table applies a 100 N normal reaction force on the box upwards.

Describe the movement of the box.

Resultant force = 100 N – 100 N

Resultant force = 0 N

There is no resultant force acting on the box and the box is at rest on the table, meaning that the box is stationary.

If the forces acting on the object are not balanced then there is a resultant force acting on the object this means that the object is either accelerating or decelerating.

Have a look at these examples.

1. A rocket has just launched. The thrust from the engine is 50 000 N and the weight of the rocket is 10 000 N.

Describe the motion of the rocket.

Resultant force = 50 000 N – 10 000 NResultant force = 40 000 N upwards

The rocket has just launched. The resultant force is acting upwards, which in the same direction as the rocket is moving, which means the rocket is accelerating upwards.

2. A sky diver has just opened her parachute. The weight of the sky diver is 500 N and the air resistance acting on her is 700 N

Resultant force = 700 N – 500 NResultant force = 200 N upwards

The parachutist is travelling downwards, the resultant force is acting against the movement. This means that the parachutist is decelerating.

Looking for something new to help your students get to grips with physics? In this short series of explainer films, Professor Brian Cox addresses popular misconceptions and talks viewers through the laws of forces, speed and motion.

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