Question

How much energy is gained by a box of mass 20 kg when a man

(a) carrying the box waits for 5 minutes for a bus?

(b) runs carrying the box with a speed of 3 m $s^{-1}$ to catch the bus?

(c) raises the box by 0.5 m in order to place it inside the bus?(g=10 m $s^{-2}$).

Answer 1

Given, m = 20 kg

(a) P.E of box = mgh

Here, h is fixed for 5 min

So, gain in energy by box = 0

(b) Man is running with speed = 3 m/s

energy gained by box $=\frac{1}{2}m{v}^2=\frac{1}{2}\times 20\times (3{)}^2=90J$

(c) Box is raised by 0.5 m

so, gain in energy = mgh

= 20 $\times$ 10 $\times$ 0.5

= 100J (With respect to man is bus)