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Energy Transfer

Work and Energy Calculations.

What Force is Needed to Stop a Moving Car?

If a car has a mass of 800 kg and moves with a
velocity of 25 m/s, what force is needed to stop the car
in 50 metres? (See also braking distance).

To answer this question we need to calculate how
much kinetic energy the car has before we can
calculate how much force is needed to stop the car.

Kinetic Energy = 0·5 x mass x velocity2
                = 0·5 x 800 x 252
                = 0·5 x 800 x 625
        = 250,000J.

Energy = Force x Distance

The equation can be rearranged to give

Force = Energy ÷ Distance
 = 250,000 ÷ 50
= 5,000 N.      

The force needed to stop the car in 50 metres is 5,000 N.

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