On a level road, a scooterist applies brakes to slow down from a speed of 10 m/s to 5 m/s. If the mass of the scooterist and the scooter be 150 kg, calculate the work done by the brakes. (Neglect air resistance and friction)

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Solution

Given:
Mass, m = 150 kg
Final velocity, v = 5 m/s
Initial velocity, u = 10 m/s

To find:
Work done by the brakes, W

Solution:
Using work energy theorem,
Work done = Change in Kinetic energy
= 0.5mv² - 0.5mu²
= 0.5m (v² - u²)
= 0.5 × 150 kg × [(5 m/s)² - (10 m/s)²]
= -5625 J
= -5.625 kJ

Work done by brakes is -5.625 kJ
Here negative sign indicates that work is done against the motion of the scooter.

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