Question

The work to be done to stop a car of mass 1200 kg and moving at a speed of 36 km/h is

Answer 1

Given that

Mass = 1200 kg

Speed = 36 km/h

Initial velocity = 36 km/h = 15 m/s

Final velocity = 0 m/s

To find

Work done to stop the car

By using

Work energy theorem

As we know

Work done = change in Kinetic Energy

$\mathsf{\bold{\therefore\:work\:done = \frac{1}{2}mv^2 - \frac{1}{2}mu^2}}$

$= \mathsf{\frac{1}{2}m\:(v^2 - u^2)}$

$= \mathsf{\frac{1}{2} \times 1200\:(0^2 - 15^2)}$

$= \mathsf{\frac{1}{2} \times 1200\:(0-225)}$

$= \mathsf{\frac{1}{2} \times 1200 - 225}$

$= \mathsf{\frac{1}{\cancel{2}} \times \cancel{1200} \times - 225}$

$= \mathsf{600 \times - 225}$

$= \mathsf{\bold{-135,000\:J\:or\:-135\:kJ}}$

Thus the work done to stop the car is - 135,000 J or - 135 kJ.

The negative sign (-) implies that the work done is in negative direction.