# Class 11 Physics Chapter 5 Important Questions Laws of Motion

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 5 Laws of Motion. In this lesson, students will learn about Laws of Motion. This will not only help the students to know the important questions but will also help them during revision.

Q.1. Is force needed to keep a body moving with uniform velocity?
Ans. No.

Q.2. What is inertia? What gives the measure of inertia?
Ans. The inherent property of the bodies that they do not change their state unless acted upon by an external force is called inertia. Mass of the body give the measure of its inertia.

Q.3. How is inertia related to mass of a body?
Ans. More is the mass of a body, more is its inertia.

Q.4. If you jerk a piece of paper under a book quick enough the book will not move. Why?
Ans. The book remains there as much due to the inertia of rest.

Q.5. Define the term momentum. Give its SI unit.
Ans. Momentum of a body is defined as the total quantity of motion contained in a body and is measured as the product of the mass of body and its velocity. The SI unit of momentum is kg ms-1.

Q.6. A bullet fired from a rifle is more dangerous than an air molecule hitting a person, though both of them have almost the same speed. Explain.
Ans. It is because mass of the bullet is about 1025 times that of the air molecule. As such the momentum of bullet is very large.

Q.7. From which Newton's law of motion the definition of force comes? Ans. First law of motion.

Q.8. Is Newton's second law (F = m.a) always valid. Give an example in support of your answer.
Ans. It is valid in an inertial frame of reference. In a non inertial frame of reference (such as a car moving along a circular path) Newton's second law does not hold apparently.

Q.9. the rate of change of momentum of a body is 5 kg m s-1. What is the force acting on the body?
Ans. 5 N.

Q.10. Give and state SI unit of force.
Ans. The SI unit of force is Newton(N). Force is said to be 1 N, if it produces an acceleration of 1m s-2 in a body of mass 1 kg.

Q.11. What is the difference between mN and nm?
Ans. The symbol mN stands for millinewton (10-3 N) and nm stands for nanometre (10-9 m).

Q.12. Action and reaction forces do not balance each other. Why?
Ans. When one body exerts a force on another body, it gets an equal and opposite reaction. The action and reaction forces do not cancel each other, as they do not act on the same body.

Q.13. What is an impulsive force?
Ans. A force which acts for a small time and also varies with time is called an impulsive force.

Q.14. What is the net force on a cork floating on water?
Ans. Zero.

Q.15. Which is greater -- the attraction of 10 kg mass for earth or the earth's attraction for 10 kg mass?

Ans. Both are equal.

Q.16. A body is moving along a circular path such that its speed always remains constant. Should there be a force acting on the body?
Ans. Yes, the force is needed to change the direction of motion of the body i.e. to deflect the body from its straight path to circular path.

Q.17. State principle of conservation of momentum.
Ans. The principle of conservation of momentum states that if no external force acts on a system the momentum of the system remains constant.

Q.18. State Newton's first law of motion.
Ans. Newton's first law of motion states that every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled by some external force to change that state.

Q.19. State Newton's second law of motion.
Ans. Newton's second law of motion states that the time rate of change of momentum of a body is directly proportional to the external force applied on it and the change in momentum takes place in the direction of force.

Q.20. State Newton's third law of motion.
Ans. Newton's third law of motion states that to every action there is an equal and opposite reaction.

Q.21. What is an impulse?
Ans. Impulse received during an impact is defined as the product of the average force and the time for which the force acts.

Q.22. Can a body remain in state of rest when external forces acting on it. Explain your answer.
Ans. Yes, if the external forces acting on the body can be represented in magnitude and direction by the sides of a closed polygon taken in the same order.

Q.23. If the net force acting on a body be zero, will the body remain necessarily in rest position? Explain your answer.
Ans. In case the body is in uniform motion along a straight line, it will continue to do so, if the net force acting on a body be zero.

Q.24. Name physical situation, where the mass of a body changes with time.
Ans. When the body moves with a speed comparable to the speed of light. For example, the mass of a charged particle increases, when it is accelerated in a cyclotron.

Q.25. When are the two bodies said to possess equal masses?
Ans. If the same force acting on the two bodies produces the same acceleration, then the two bodies must be of equal masses.

Q.26. Is a bus moving along a circular track, an inertial frame of reference?
Ans. No. It is a non-inertial frame of reference.

Q.27. What can be said about the motion of the vehicle, if a plumb line hanging from its roof drops vertically?
Ans. Either the vehicle is at rest or it is moving with constant velocity.

Q.28. The assertion made by the Newton's first law of motion that everybody continues in a state of uniform motion in the absence of external force appears to be contradicted in everyday experience. Why?
Ans. When we roll a ball on the floor, it ultimately stops because of the force of friction of the ground. Thus, the state of uniform motion of the object changes due to the external force (friction). On the earth, every change in uniform motion of a body can be connected with an external force acting on it. However, in free space, when no external force acts, the state of motion described by the Newton's first law of motion can be obtained and experienced.

Q.29. According to Newton's first law of motion, a body moving with a uniform speed along a straight line should continue moving. In practice a body in motion stops after some time. Explain the reason.
Ans. The force of friction acts as the external retarding force. It brings the body to rest.

Q.30. An astronaut accidentally gets thrown out of his small space ship accelerating in interstellar space at a constant rate of 100 ms-2. What is the acceleration of the astronaut, the instant after he is outside the spaceship? Assume that there are no nearby stars to exert gravitational force on him.
Ans. In the interstellar space, moment the astronaut is out of the spaceship, no external force acts on him. It is because, there are no nearby stars to exert gravitational force on him. The spaceship too can not exert any appreciable gravitational force on him due to its small size. Therefore, once the astronaut is out of the spaceship, his acceleration will be zero.

Q.31. According to Newton's third law, every force is accompanied by an equal and opposite force. How can a movement ever take place?
Ans. Since action and reaction do not act on the same body, they do not cancel each other. Therefore, a body may move either under the effect of the action of force or the other body may move under the effect of reaction on it.

Q.32. It is easier to pull than to push a body. Explain.
Ans. When we pull a body, the vertical component of the applied pull acts opposite to the weight of the body and it reduces its effective weight. On the other hand, when a body is pushed, the vertical component of the applied push adds to the weight of the body and hence its effective weight increases. Thus, the effective weight is lesser, when the body is pulled. Hence, it is easier to pull than to push a body.

Q.33. A stone when thrown on a glass window smashes the window pane into pieces, but a bullet from the gun passes through making a clean hole. Why?
Ans. The velocity of the stone is much less than that of the bullet fired from a gun. Due to its low speed, the stone remains in contact contact with the window pane for a longer time and its motion is shared by whole of the window pane. As a result, it smashes into pieces. On the other hand, the bullet fired from the gun remains in contact with the window pane for such a small time that it can share its motion only with the portion of the window pane, it comes in contact with. As such, it makes a clean hole in the window pane.

Q.34. What is friction?
Ans. The opposing force that is set up between the surfaces of contact, when one body slides or rolls or tends to do so on the surface of another body is called friction.

Q.35. Define limiting friction.
Ans. The maximum value of the force of friction which comes into play before a body just begins to slide over the surface of another body is called limiting value of static friction.

Q.36. Why do we call friction a self adjusting force?
Ans. When applied force is zero, friction is zero. As the applied force is increased, friction also increases and becomes equal to the applied force. It happens so, till the body does not start moving. That is why, friction is called a self adjusting force.

Q.37. What is the unit of coefficient of limiting friction?
Ans. It has no unit.

Q.38. What are the factors on which the coefficient of friction between two surfaces depend?
Ans. The coefficient of friction between two surfaces depends upon the nature of the two surfaces and their state of roughness.

Q.39. What is the relation between angle of repose and angle of friction?
Ans. Angle of friction and angle of repose are equal.

Q.40. Why is friction a non-conservative force?
Ans. It is because work done against friction along a closed path is non zero.

Q.41. What happens to limiting friction, when a wooden block is moved with increasing speed on a horizontal surface?
Ans. The limiting friction decreases as the wooden block is moved with increasing speed on the horizontal surface.

Q.42. Why are tyres made of rubber and not iron?
Ans. It is because, the coefficient of friction between rubber and concrete (material of the road) is less than that between iron and the road.

Q.43. Give any three advantages of friction.
Ans.

1. Friction helps us to walk.
2. Brakes make use of friction to stop the vehicles.
3. Friction helps to transmit power from the motors and engines to other machines by making use of belts and clutches.

Q.44. Why are wheels made circular?
Ans. The rolling friction is less than the sliding friction. The wheels are made circular so as to convert the sliding friction into the rolling friction.

Q.45. It is easier to roll a barrel than to slide it along the road. Why?
Ans. The rolling friction is lesser as compared to the sliding friction.

Q.46. What happens to the fluid friction, as speed of the object moving through it is increased?
Ans. The fluid friction increases, as the speed of the object moving through it, is increased.

Q.47. smoother the surface lesser is friction. Comment.
Ans. When the surfaces are made smoother, the size of the irregularities in the surfaces decreases. As a result, the area of actual contact decreases. As the number of atoms in contact will also decrease due to decrease in area of contact, the force of molecular attraction and hence the force of friction decreases.

Q.48. Polishing a surface beyond a certain limit may increase friction. Why?
Ans. When the surfaces are polished beyond a certain limit, the area at each point of contact becomes very small. However, the actual area of contact between the two surfaces increases appreciably. It is because, the number of points of contact becomes very large on making the surface is highly polished. Since number of atoms (or molecules) of the two surfaces in contact is proportional to their area in contact, the force of friction increases due to the greater force of molecular attraction between the two surfaces.

Q.49. A horse has to apply more force to start a cart than to keep it moving. Why?
Ans. When the cart is at rest, the friction between the wheels of the cart and the road is static in nature and once the cart starts moving, it is kinetic in nature. The kinetic friction is always less than the static friction. Due to this reason, a horse has to apply more force to start a cart than to keep it moving.

Q.50. Why do we slip on a rainy day?
Ans. On a rainy day, the wet ground becomes very smooth. As a result, the coefficient of friction between our feet and the wet ground gets much reduced. Consequently, the friction between the feet and the ground becomes very small. It may cause us to slip.

Q.51. Explain how friction helps in walking?
Ans. When we walk, with push the ground in backward direction. Our foot will get equal and opposite reaction from the ground only, if the food does not slip i.e. there is adequate friction between the food and the ground. The forward horizontal component of the reaction helps us to move forward.

Q.52. How can proper inflation of tyres save fuel?
Ans. Under the weight of the vehicle, the road surface gets depressed and a small hump is created just ahead of the wheel. In case the tyres are not properly inflated, the tyres require a large effort to climb the hump. In case the tires are properly inflated, the compression of the tyres as much smaller and hence they experience less resistance (rolling friction). In other words, it will make the vehicle to cover greater distance for the same quantity of fuel spent.

Q.53. What furnishes centripetal force for earth to go round the sun?
Ans. The gravitational pull of the sun on the earth.

Q.54. What provides the centripetal force to satellite revolving round the earth?
Ans. Gravitational force of attraction on the satellite due to the earth.

Q.55. What furnishes the centripetal force for the electrons to go round the nucleus?
Ans. The electrostatic force of attraction on the electrons due to the nucleus.

Q.56. For uniform circular motion, does the direction of the centripetal force depends upon the sense of rotation?
Ans. The direction of the centripetal force does not depend, whether the body is moving in clockwise or anticlockwise direction. It is always directed along the radius and towards the centre of the circle.

Q.57. Can centripetal force produce rotation?
Ans. No. A centripetal force can move a body along a circular path but it can not produce rotational motion.

Q.58. What provides the centripetal force to a car taking a turn on a level road?
Ans. The force of friction between the tyres and the road provides the necessary centripetal force to the car to take a turn on a level road.

Q.59. The speed of the stone is increased beyond the maximum permissible value (the value of speed at which the tension in the string becomes equal to the maximum tension the string can stand) the string breaks suddenly. What will be the trajectory of the stone after the string breaks?
Ans. At the instant, the string breaks, the stone will fly off tangentially from its position at that instant. It is because, the speed of the stone at any instant is directed along tangent to the circular path at that point.

Q.60. A train moves on an unbanked circular band of rails. Which rail will wear out faster?
Ans. Inner rail. It is because, the inward pressure on the inner rail is more than that on the outer rail.

Q.61. What are the advantages of banking?
Ans. The vehicle can be moved along a circular track at a reasonable speed without the fear of skidding. Further, while taking a turn, one may not need to decrease the speed of the vehicle.

Q.62. A stone is moved along a vertical circle so as to just loop the loop. What is the tension in the string, when the stone is at the highest point?
Ans. The tension is equal to six times the weight of the stone.