Question

A person brings a mass of 1kg from infinity to a point A. Initially the mass was at rest but it moves at a speed of 2m/s as it reaches A. The work done by the person on the mass is —3J. The potential at A is
(a) -3J/kg 

(b) -2J/kg 

(c) -5JAg 

(d) none of these.

Answer 1

The correct answer is (c) -5 J/kg

EXPLANATION:   

Work done by the person = -3 J  (Given) Work done in increasing the velocity from rest to 2 km/s = K.E. of the mass at A =½mv² = ½*1*2² = 2 J Let the potential at A be X J/kg, 

So, the change in P.E. of the mass = the work done in slowly bringing the mass from infinity to A = mX 

= 1*X = X J 

So the total work done = Change in P.E + Change in K.E. = X+2 J, but it is given -3 J so, X+2 = -3

 →X = -3-2 = -5 J