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What is the connection between forces and motion?

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The momentum of an object is its mass multiplied by its velocity. The larger the mass and velocity the larger the momentum.

Forces change momentum - the larger the force the more quickly the momentum changes.

The resultant force is the overall result of all forces acting on an object.

# Resultant force

Sometimes several forces act on the same object. Look at this diagram of a moving car:

There are several forces acting on the car, shown by the arrows.

• Gravity pulls down on the car.
• The reaction force from the road pushes up on the wheels.
• The driving force from the engine pushes the car along.
• There is friction between the road and the tyres.
• Air resistance acts on the front of the car.

## Resultant force

The resultant force is the sum of all the different forces acting on the car.

You have to take account of the directions – the reaction forces on the wheels (blue arrows) add up to the same as the weight (green arrow), so these cancel out.

The driving force from the engine (yellow arrow) is in the opposite direction to the counter forces of friction (red arrows) and air resistance (purple arrow).

When the car is increasing its speed then all these forces add to give a single resultant force forwards.

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