CBSE Class 11 Physics Work Energy Power Assignment

Comprehension-1

Work done is the scalar product of force and displacement. To calculate the work done by Time varying a force generally apply the work energy theorem. According to this sum of the work done by all forces is equal to change in kinetic energy. Potential energy depends separation between the interacting particles. For the conservative force negative potential energy gradient is equal to force.

1. The potential energy of a particle U = a/r²-b/r . The system is in
(a) unstable equilibrium
(b) stable equilibrium
(c) neutral equilibrium
(d) equilibrium stage is not obtained for a given potential energy field

2. A block of mass 5 kg is being raised vertically upwards by the help of string attached to it. It rises with an acceleration of 2 m/s². The work done by tension in the string if block rises by 2.5 m
(a) 125 J (b) 147.5 J (c) 193.4 J (d) –87.5 J

3. W = FS sin θ is applicable for
(a) only constant force (b) only time varying force
(c) any force (d) neither for constant nor for variable force

4. A pump is required to lift 1000 kg of water per minute from a well 20 m deep and eject it all the rate of 20 m/s. What horse power engine is required for the purpose of lifting water
(a) nearly 9 (b) nearly 2 (c) nearly 16 (d) nearly 25

Comprehension-2

Physics problem can be solved by using of conservation of energy. When no external force acts on the system then linear momentum is constant and conserved. The concept of momentum is a term used in describing objects in motion. For conservation of linear momentum only requirement is that the force must be internal to system. Impulse is the product of force and small time interval. It is equal to change of linear momentum. In many physical situations, we shall use the impulse. Conservation of linear momentum is valid for all types of collision, rocket propulsion etc.

5. A body is projected with velocity v and angle of projection is θ. At highest point it is split into two fragments, then
(a) linear momentum is conserved only at highest point
(b) linear momentum is conserved only in horizontal direction
(c) linear momentum is conserved in only vertical direction
(d) at any arbitrary point

6. A shell is projected with velocity v and angle of projection of θ. At highest point it is split into two fragments. Ratio of their masses is 1 : 2. Heavy part retraces its path. The speed of other fragment is
(a) 3v cos θ (b) 2v cos θ (c) 3/2 v cos θ (d) 5v cos θ

7. A lighter body and heavier body both are moving with same K.E. Then
(a) linear momentum of lighter body is lesser (b) both have same momentum
(c) both have unequal momentum (d) none of these

8. If momentum of a body increases by 50%. The kinetic energy increases by
(a) 100% (b) 125% (c) 150% (d) 225%

Each of the questions given below consists of two statements, an assertion (A) and reason (R).
Select the number corresponding to the appropriate alternative as follows
(a) If both A and R are true and R is the correct explanation of A
(b) If both A and R are true but R is not the correct explanation of A
(c) If A is true but R is false
(d) If A is false but R is true

9. A: In an elastic collision of two billiard balls, the total K.E. is conserved during the short time of collision of the balls (i.e., when they are in contact)
R: Energy spent against friction follows the law of conservation of mechanical energy.

10. A: Tennis ball bounce higher on hills than in plains.
R: Value of g on the hills is less than that at the plains.

11. A: A system is in stable equilibrium when its kinetic energy is minimum.
R: In stable equilibrium position, the body should have minimum energy or minimum kinetic energy

12. A: A system is in stable equilibrium if its potential energy is minimum.
R: For stable equilibrium, centre of gravity of the system should be lowest or its potential energy should be minimum.

13. A: Work done by an external force is always equal to gain in its K. E.
R: This statement is always true.

14. A: When a small body of mass m collides elastically head-on against a stationary heavy body of mass M such that the small body comes back, then the velocity of centre of mass of the system will also change the direction, after the collision.
R: Motion of a body means motion of its centre of mass.

15. A: When a body is raised from the surface of earth, through a heigh h, increase in its potential energy is mgh.
R: Work done = F. s, always.

16. A: No work is done by centripetal force, howsoever large force may act on the body to make it moving in a circular path.
R: The force acting on the body is always perpendicular to the direction of its velocity, at any instant.

17. A: If in a collision, the first body gets embedded into the second body and the two move together with same velocity, then there is no loss of kinetic energy.
R: In a perfectly inelastic collision, momentum is conserved; but kinetic energy is not conserved.

18. A: When a body of mass m moving with a velocity v strikes the surface making an angle of θ with the normal to the surface at the point of impact, then change in the momentum along the surface is zero, if the collision is elastic.
R: The component of the momentum along the surface after the collision is same as that before the collision.

19. A: When a body of mass m is thrown up, its momentum goes on decreasing; but sum of its kinetic energy and potential energy remains same (ignoring air resistance).
R: This is an exceptional case of non-conservation of momentum.

Miscellaneous Assignment
1. (b) 2. (b) 3. (a) 4. (a) 5. (b) 6. (d) 7. (a) 8. (b) 9. (d) 10. (a) 11. (d) 12. (a) 13. (a) 14. (d) 15. (a) 16. (a) 17. (d) 18. (a) 19. (c)