Question

A body is projected up vertically with a velocity of 60 m/s. What percentage of its initial kinetic energy is converted into potential energy after 3 seconds? (Take g = 10 m/s^2)

(a) 25 %

(b) 50%

(c) 75%

(d) 100%

I had been asked this question by my school teacher while I was bunking the class.

My query is from Kinetic Energy in section Work, Energy and Power of Physics – Class 11

Answer 1

Correct answer is (c) 75%

Best explanation: Initial KE = (1/2) x M x (60 x 60)

 = 1800 x M

From Newton’s laws;

s = ut + (1/2)gt^2

 = (60 x 3) – (10 x 3 x 3)/2

 = 180 – 45

 = 135 m

Potential energy at 135m (PE) = M x g x s

 = 1350 x M

So KE at 135m = Initial KE – PE

 = 1800M – 1350M

 = 450M J; This is 25% of the initial kinetic energy. Hence, 75% is converted into potential energy.