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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 velocity^{2}
= 0·5 x
800 x 25^{2}
= 0·5 x 800 x 625

= 250,000J.

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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