Directions : Each of these questions contain two statements, Assertion and Reason. Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.
Q.1. Assertion : A work done by friction is always negative.
Reason : If frictional force acts on a body its K.E. may decrease.
AnswerAnswer: (d) When frictional force is opposite to velocity, kinetic energy will decrease.
Q.2. Assertion : A spring has potential energy, both when it is compressed or stretched.
Reason : In compressing or stretching, work is done on the spring against the restoring force.
Q.3. Assertion : A force applied on the body always does work on the body.
Reason : If a force applied on a body displaces the body along the direction of force work done will be maximum.
Q.4. Assertion : A man rowing a boat upstream is at rest with respect to the bank. He is doing no external work.
Reason : Work done by constant force, W = F s cosθ.
AnswerAnswer: (a) In this case, s = 0, and so W = 0.
Q.5. Assertion : The work done in moving a body over a closed loop is zero for every force in nature.
Reason : Work done depends on nature of force.
AnswerAnswer: (b) In close loop, s = 0, and so W = Fs = 0.
Q.6. Assertion : The rate of change of total momentum of a many particle system is proportional to the sum of the internal forces of the system.
Reason : Internal forces can change the kinetic energy but not the momentum of the system.
Q.7. Assertion : The change in kinetic energy of a particle is equal to the work done on it by the net force.
Reason : Change in kinetic energy of particle is equal to work done only in case of a system of one particle.
AnswerAnswer: (c) Change in kinetic energy = work done by net force. This relationship is valid for particle as well as system of particles.
Q.8. Assertion: Kinetic energy of a system can be increased or decreased without applying any external force on the system.
Reason: This is because K.E. =mV2/2, so it independent of any external forces.
AnswerAnswer: (c) K.E. can be increased or decreased without applying any external force, as internal forces can do work e.g., explosion of a bomb.
Q.9. Assertion: Kinetic energy of a body is quadrupled, when its velocity is doubled.
Reason : Kinetic energy is proportional to square of velocity. If velocity is doubled the K.E. will be quadrupled
Q.10. Assertion: If the velocity of a body is tripled, then K.E. becomes 9 times.
Reason : Kinetic energy , K.E.=mv2/2