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Energy Transfer

Work and Energy Calculations.

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.      

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