Work, Energy and Power : Physics

 

Q.1. A body of mass 0.5 kg travels in a straight line with velocity v = kx  where k = 5 m  s . What is the work done by the net force during its displacement from x = 0 to x = 2 m ?

A) 25 J

B) 50 J

C) 100 J

D) 150 J

Explanation-

 

Q.2.  Two particles of masses m and 4m have kinetic energies in the ratio of 2:1. What is the ratio of linear momenta ? 

 A)  

 B)  

 C)  

 D)  

Explanation-

 

Q.3. A running man has the same kinetic energy as that of a boy of half his mass . The man speeds up by 2 m s and the boy changes his speed by x m s so that kinetic energies of the boy and the man are again equal .Then x in m s is 

A) 1/2

B) 

C) 

D) 2

Explanation-

Let the mass of man be m . 

Therefore , mass of the boy is  .

 

Q.4. Which of the following statements is correct?

A) Kinetic energy of a system can be changed without changing its momentum

B) Kinetic energy of a system cannot be changed without changing its momentum

C) Momentum of a system cannot be changed without changing its kinetic energy

D) A system cannot have energy without having momentum

Explanation-

In the explosion of a bomb or inelastic collision between two bodies as force is internal, momentum is conserved while KE changes. Hence the KE of a system can be changed without changing its momentum Similarly, the reverse is also true, e.g., if a force acts perpendicular to motion, work done will be zero and so KE will remain constant. However, the force will change the direction of motion and so the momentum. Further body may have energy (i.e., potential energy) without having momentum.

Q.5. A bus can be stopped by applying a retarding force F when it is moving with speed v on a level road. The distance covered by it before coming to rest is s. IF the load of the bus increases by 50% because of passengers for the same speed and same retarding force. the distance covered by the bus to come to rest shall be

A) 1.5 s

B) 2 s

C) 1 s

D) 2.5 s

Explanation-

 

Q.6.  A body of mass m moving at a constant speed undergoes an elastic collision with a body of mass m2 initially at rest . The ratio of the kinetic energy of mass m after the collision to that before the collision is 

A) 

B) 

C) 

D)  

Explanation-

Let  be the speed of mass  before the collision. Here . Therefore, the speeds of masses   and  after the collision respectively are

 

Q.7.  A uniform chain of mass M and length L is held on a horizontal frictionless table with th of its length hanging over the edge of the table .The work done is pulling the chain up on the table is 

A) 

B) 

C) 

D)  

Explanation-

The mass per unit length of the chain .The mass of the hanging portion of the chain is  . This mass can be assumed to be concentrated at the centre of the hanging portion of the chain which is a distance of  from the edge of the table. Therefore, the work done in pulling the hanging portion of the chain on on to the table top is

 .

Q.8.  A body of mass 5 kg rests on a rough horizontal surface of coefficient of friction 0.2 . The body is pulled through a distance of 10 m by a horizontal force of 25 N .The kinetic energy acquired by it is  ( take g = 10 ms ) 

A) 200 J

B) 150 J

C) 100 J

D) 50 J

Explanation-

 

Q.9. One end of an upstretched vertical spring is attached to the ceiling and an object attached to the other end is slowly lowered to its equilibrium position . If S is the gain in spring energy and G is the loss in gravitational potential energy in the process , then 

A) S = G

B) S = 2 G

C) G = 2 S

D) None of these 

Explanation-

At equilibrium position , x = 

 

Q.10. A body is moving up an inclined plane of angle   with an initial kinetic energy E . The coefficient of friction between the plane and the body is .The work done against friction before the body comes to rest is : 

A) 

B) 

C) 

D)