# Illustrations

Example-1

A force of 30 N acts on a body and displaces it through 10 m. Calculate the work done, if the displacement is

- along the force and
- â€‹at 60Â° to the direction of the force.

Solution

Given: *F* = 30 N and *s* = 10 m.

- Displacement is along the force, i.e.,
*Î¸*= 0.*W*=*Fs*= 30 Ã— 10 J = 300 J - Displacement is at 60Â° to the force, i.e.,
*Î¸*= 60Â°.*W*=*Fs*cos*Î¸*

Example-2

A boy weighing 350 N runs up a flight of 30 steps each 20 cm high, in 5 s. Calculate the power expended.

Solution

*h* = Total height of 30 steps

= 30 Ã— 20 cm = 6 m

*W* = Work done by the boy

= Weight Ã— Height

= 350 Ã— 6 = 2100 J

Example-3

A bullet of mass 50 g strikes a wooden plank with a velocity of 200 m s^{â€“1} and emerges out with a velocity of 50 m s^{â€“1}. Calculate the work done by the bullet against the resistive force offered by the plank.

Solution

*m* = 50, *g* = 50 Ã— 10^{â€“3} kg, *u* = 200 m s^{â€“1}, *v* = 50 m s^{â€“1}

Work done = Loss of kinetic energy

= 25 Ã— 10^{â€“3} Ã— (3.75)10^{4}

= 93.75 Ã— 10 J = 937.5 J