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Physics VSQs for Class 11 with Answers Chapter 5 Laws of Motion

Q.1. State Newton’s first law of motion.

Answer Answer: Everybody continues to be in state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.

Q.2. Two objects having different masses have same momentum. Which one of them will move faster.

Answer Answer: Object with smaller mass as momentum p = mv.

Q.3. What is the effect on the acceleration of a particle if the net force on the particle is doubled?

Answer Answer: Since, a = E / m On doubling the force, the acceleration will also be doubled.

Q.4. Why does a heavy rifle not kick as strongly as a light rifle using the same cartridges?

Answer Answer: The recoil speed of rifle V = (m/M)v is inversely proportional to its mass. So far a heavy rifle the kick is less stronger.

Q.5. On what factors does the thrust on a rocket depend?

Answer Answer: The upward thrust on a rocket depends on exhaust speed of the gases w.r.t. the rocket and rate at which mass of the exhaust gases escapes.

Q.6. What do you mean by normal reaction?

Answer Answer: It is the reaction due to the surface on which the body moves. It acts perpendicular to the surface of contact.

Q.7. At which place on earth, the centripetal force is maximum?

Answer Answer: Since F=mv2/r , so at the pole, the value of F is maximum.

Q.8. What is the angle between frictional force and instantaneous velocity of the body moving over a rough surface?

Answer Answer: The angle is 180°, because force of friction always opposes the relative motion.

Q.9. Is earth an inertial frame of reference?

Answer Answer: Since earth rotates on its own axis and also revolve around the sun, there will be acceleration associated. So earth cannot be taken as inertial frame of reference.

Q.10. What provides the centripetal force in the following cases?
(i) Electron revolving around the nucleus.
(ii) Earth revolving around the sun.

Answer Answer: (i) Electrostatic force.
(ii) Gravitational force.

Q.11. Can a body in linear motion be in equilibrium?

Answer Answer: Yes, provided the vector sum of the forces acting upon the body is zero.

Q.12. Why are curved roads generally banked?

Answer Answer: Curved roads are generally banked so as to help in providing centripetal force needed for motion of vehicles on the curved road.

Q.13. A person sitting in the compartment of a train moving with a uniform speed throws a ball in the upward direction. What path of the ball will appear to him? What to a person standing outside?

Answer Answer: It will appear to fall in vertically downward direction to the person in the compartment and parabolic to a person standing outside.

Q.14. The length of an ideal spring increases by 0.1cm when a body of 1kg is suspended from it. If this spring is laid on a frictionless horizontal table and bodies of 1 kg each are suspended from its ends, then what will be the increase in its length?

Answer Answer: It would increase in length by 0.1cm only.

Q.15. The two ends of a spring-balance are pulled each by a force of 10 kg-wt. What will be the reading of the balance?

Answer Answer: 10 kg-wt.

Q.16. A retarding force is applied to stop a motorcar. If the speed of the motorcar is doubled, how much more distance will it cover before stopping under the same retarding force?

Answer Answer: Since distance α (velocity)2, motorcar will cover the distance four times longer than before.

Q.17. A lift is accelerated upward. Will the weight of person inside the lift increase, decrease or remain the same relative to its real weight? If the lift is going with uniform speed, then?

Answer Answer: Apparent weight will increase. When lift is going up with uniform speed, apparent weight will be same as real weight

Q.18. A ball of 0.5 kg mass moving with a speed of 10m/s rebounds after striking normally a perfectly elastic wall. Find the change in momentum of the ball.

Answer Answer: Change in linear momentum = – mv – mv = – 2mv = –2 × 0.5 × 10 = – 10 kgms–1.

Q.19. A body is suspended from the ceiling of a transparent cabin falling freely towards the earth. Describe the motion of the body as observed by an observer (a) sitting in the cabin, (b) standing on the earth.

Answer Answer: (a) The body will appear stationary in air, (b) The body will appear falling freely under the acceleration in air.

Q.20. Calculate the net force acting on a body of mass 10 kg, moving with a uniform velocity of 2 ms–1.

Answer Answer: In uniform velocity, acceleration = 0, F = ma = 0.

Q.21. (a) According to Newton’s third law of motion, every force is accompanied by an equal (in magnitude) and opposite (in direction) force called reaction, then how can a movement takes place?
(b) You can move a brick easily by pushing it with your foot on a smooth floor, but, if you kick it, then your foot is hurt. Why?

Answer Answer: (a) As the action and reaction never act on the same body, so the motion is possible.
(b) As Ft remains constant, so if t is reduced, then F will be increased and hence hurt our foot.

Q.22. “Friction is a self-adjusting force.” Is this statement correct.

Answer Answer: This statement is correct only so long as the friction is static friction. Upto the limiting friction, the force of friction is equal (in magnitude) and opposite to the applied force.

Q.23. What provides the centripetal force in the following cases?
(i) Electron revolving around the nucleus.
(ii) Earth revolving around the sun.
(iii) Car taking a turn on a banked road.

Answer Answer: It is provide by- (i) Electrostatic force of attraction between the electron and the nucleus. (ii) Gravitational force of attraction between Earth and Sun. (iii) A component of the reaction of the road

Q.24. (a) A body is moving with uniform velocity. Can it be said to be in equilibrium? Why?
(b) Why Newton’s second law of motion is not applicable
to the motion of a rocket?

Answer Answer: (a) Yes, it can be said to be in equilibrium when it moves with uniform velocity as no acceleration i.e. no net force acts on the body.
(b) Newton’s second law i.e. F = ma is applicable only if the mass (m) of the body remains constant. In case of the rocket, the mass continuously decreases and hence F = ma is not applicable.

Q.25. Name the forces which are in equilibrium in each of the following situations :
(a) a book resting on a table.
(b) a cork floating on water.
(c) a pendulum bob suspended from the ceiling with the help of a string.

Answer Answer: (a) Gravitational force on the book and a force of reaction of the table.
(b) Gravitational force on the cork and an upward thrust or buoyant force of water.
(c) Gravitational force on the bob and the tension in the string.

Q.26. A force of 5N changes the velocity of a body from 10 ms–1 to 20 ms–1 in 5s. How much force is required to bring about the same change in 2s.

Answer Answer: F1 = dp/dt1 and F2 = dp/dt2 ⇒F2 = F1 x dt1/dt2 = 5 × 5/2 = 12.5N

Q.27. Sand is thrown on the tracks covered with snow why?

Answer Answer: Sand increases the friction.

Q.28. Why is it easier to maintain the motion than to start it?

Answer Answer: As the dynamic friction is less than the force of limiting friction.

Q.29. Why are the wheels circular in shape?

Answer Answer: Wheels convert the sliding friction to rolling friction

Q.30. Rubber tyres are preferred to steel tyres, why?

Answer Answer: Coefficient of friction between the rubber and road is smaller than that between the steel and road.

Q.31. What is the angle of friction between two surfaces in contact, if coefficient of friction is 1/√3?

Answer Answer: Here, μ = 1/√3, tanθ=1/√3 ⇒ θ = 300

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