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.
Read also: Laws of Motion Class 11 Physics Notes Chapter 5
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.
Read also: Class 11 Physics Chapter 5 MCQs with Answer Laws of Motion
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.
- Friction helps us to walk.
- Brakes make use of friction to stop the vehicles.
- 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.