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Forces and motion

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Acceleration is caused by a force, eg gravity or friction, acting on a moving object. The relationship between force, mass and acceleration is expressed in the equation 'force = mass x 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.

# Forces, mass and acceleration

When an object accelerates it does so because an extra force is acting on it. For example:

• Gravity causes a falling apple to speed up as it falls.
• Air resistance causes a parachutist to slow down just after opening the parachute.
• The brake pads on a bike cause extra friction, which slows the bike down.

## The relationship between force, mass and acceleration

For the same mass:

• more force causes more acceleration
• less force causes less acceleration

For the same force:

• more mass causes less acceleration
• less mass causes more acceleration

For the same acceleration:

• more mass needs more force
• less mass needs less force

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