# forces and motion

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A force acting on an object can cause it to accelerate. The relationship between force, mass and acceleration is given by the equation

force = mass × acceleration.

The stopping distance of a car depends on two factors - thinking distance and braking distance. Under normal driving conditions, the thinking and braking distance depend on the speed of the car.

# Force, mass and acceleration

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.

## Resultant force and acceleration

If the resultant force is zero, a moving object will stay at the same speed.

If the result 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.

Note that the object could also change direction, for example if the resultant force acts at an angle.

## Calculating acceleration

Here is an equation relating acceleration to force and mass:

Force = mass × acceleration

• Force is measured in newtons, N
• Mass is measured in kilograms, kg
• Acceleration is measured in metres per second squared, m/s2.

For example, the force needed to accelerate a 10 kg mass by 5 m/s2 is

10 × 5 = 50N

The same force…