Q.1. A man raises a mass ‘m’ to a height ‘h’ and then shifts it horizontally by a length ‘x’. What is the work done against the force of gravity?
AnswerAnswer: Work done against gravity is mgh, as no work is done in the horizontal displacement.
Q.2. Name and define SI unit of work.
AnswerAnswer: The SI unit of work is joule (J). One joule of work is said to be done when a force of one newton displaces a body through a distance of one metre in the direction of applied force..
Q.3. Give an example in which a force does work on a body but fails to change its kinetic energy.
AnswerAnswer: When a body is pulled on a rough horizontal surface with constant velocity, work is done by the applied force on the body but K.E. of the body remains unchanged.
Q.4. An artificial satellite is at a height of 36,500 km above earth’s surface. What is the work done by earth’s gravitational force in keeping it in its orbit ?
Q.5. Name the process in which (i) momentum is conserved but K.E. is not conserved and (ii) momentum changes but K.E. does not change.
AnswerAnswer: (i) Inelastic collision.
(ii) Uniform circular motion.
Q.6. What is Einstein’s mass-energy equivalence relationship?
AnswerAnswer: E = mc2.
Q.7. A mass m collides with another mass 2m and sticks to it. What is the nature of collision?
AnswerAnswer: Inelastic, as the two bodies are moving together after collision.
Q.8. Two equal masses, one at rest and another moving, undergo elastic oblique collision. If one mass goes at an angle p/3 with its original direction of motion, what is the direction of motion of the other?
Q.9. Two bodies m1 and m2 (m1 > m2) have equal kinetic energies. Which will have more momentum?
Q.10. Ten identical balls are placed in contact on a smooth surface. If an eleventh identical ball moving with a speed ‘u’ collides on the first, what will be the resultant motion of the system?
AnswerAnswer: When the eleventh ball hits the first ball, the tenth ball starts moving with the same speed due to transfer of momentum in equal proportion.
Q.11. Can a constant velocity be maintained in a body moving on a rough surface without doing any work on it?
AnswerAnswer: No, work has to be done to compensate the energy loss due to friction.
Q.12. Does the work done in raising a box on to a platform depends upon how fast it is raised up? If not, why?
AnswerAnswer: Since, the gravitational force is conservative in nature, so work done against it depends only on initial and final points, not on time.
Q.13. A man rowing a boat upstream is at rest with respect to the shore, is he doing work?
AnswerAnswer: Yes, he is doing work to oppose the river current.
Q.14. A light body and a heavy body have the same momentum. Which one will have greater kinetic energy?
Q.15. A spring is cut into two equal halves. How is spring constant of each half affected?
Q.16. What is one electron-volt ?
AnswerAnswer: 1 eV = 1.6 x 10–19 joule. It is the energy required to transfer electron (charge = 1.6 x 10–19 C) through a potential difference of 1 volt.
Q.17. Draw the variation of potential energy stored in a spring as a function of extension.
Q.18. The earth moving round the sun in circular orbit is acted upon by a force and hence work must be done on the earth by the force. Explain.
AnswerAnswer: It is wrong. Since force and displacement are perpendicular to each other work done is zero.
Q.19. If one of the two colliding particles is initially at rest, is it possible for both of the particles to be at rest after collision ?