# Class 11 Physics Chapter 3 Important Questions Motion in a Straight Line

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 3 Motion in a Straight Line. In this lesson, students will learn about **Motion in a Straight Line**. This will not only help the students to know the important questions but will also help them during revision.

**Q.1**. Are rest and motion absolute or relative terms?
**Ans.** Both rest and motion are relative terms.

**Q.2.** Under what condition will the distance and
displacement of a moving object will have the same
magnitude?
**Ans.** When the object moves along a straight line and
always in the same direction.

**Q.3.** Can the speed of a body change if its velocity is
constant?
**Ans.** No, it is because speed of the body is equal to
magnitude of its velocity.

**Q.4.** Can a particle in one dimensional motion with
zero speed may have non zero velocity?
**Ans.** No, it is because speed of a particle is equal to
magnitude of its velocity.

**Q.5.** Is it possible for a body to be accelerated without
speeding up or slowing down. If it is so give an
example.
**Ans.** Yes, When a body is rotated along a circular path
with uniform angular speed, its speed is constant but
it has a varying velocity. The change in velocity
occurs due to a continuous change in its direction of
motion.

**Q.6.** What will be the nature of position-time graph
for uniform motion?
**Ans.** It will be a straight line, inclined with time axis.

**Q.7.** Can position-time graph be a straight line parallel
to time-axis?
**Ans.** No, It is because, the position-time graph parallel
to time-axis indicates that the object is at rest.

**Q.8.** What is the nature of position time graph for a
stationary object?
**Ans.** The position-time graph for a stationary object is
parallel to time-axis.

**Q.9.** Can position-time graph be a straight line parallel
to position-axis?
**Ans.** No, It is because the position-time graph parallel
to position-axis indicates that the position of the
object is changing at a given instant of time.

**Read also:** Motion in a Straight Line Class 11 Physics notes Chapter 3

**Q.10.** What does slope of position-time graph
represent?
**Ans.** Slope of position-time graph represents velocity
of the object.

**Q.11.** Can position-time graph have a negative slope?
**Ans.** Yes, It happens, when the velocity of the object is
negative.

**Q.12.** What does slope of velocity-time graph
represents?
**Ans.**The slope of velocity-time graph represents
acceleration.

**Q.13.** What does area under velocity-time graph
represents?
**Ans.** The area under velocity-time graph represents
displacement.

**Q.14.** Can a particle in one dimensional motion with
zero speed at any instant may have a non-zero
acceleration at that instant?
**Ans.** Yes for example consider that a body is thrown
vertically up at the highest point the body possesses
zero velocity and finite acceleration (g = 9.8 ms^{-2}).

**Q.15.** If the acceleration of a particle is constant in
magnitude but not in direction, what type of path the
particle follow?
**Ans.** If the acceleration of a particle is constant in
magnitude but not in direction, then the particle will
follow a non-linear path.

**Q.16.** What does the speedometer of a car measure?
**Ans.** The speedometer of a car measures
instantaneous speed.

**Read also:** Class 11 Physics Chapter 3 MCQs with Answer Motion in a straight Line

**Q.17.** How can the distance travelled be calculated
from velocity-time graph?
**Ans.** The area under velocity-time graph for a given
time interval is numerically equal to the distance
covered by the body in that time interval.

**Q.18.** Consider that the acceleration of a moving body
varies with time. What does the area under
acceleration-time graph for any time interval
represent?
**Ans.** The area under acceleration-time graph for any
time interval represents the change of speed of the
body during that time interval.

**Q.19.** A ball is thrown straight up. What is its velocity
and acceleration at the top?
**Ans.** Velocity at the top = 0
and acceleration at the top = 9.8 ms^{-2} (downwards)

**Q.20.** Two balls of different masses (one lighter and
other heavier) are thrown vertically upward with
same initial speed. Which one will rise to the greater
height?
**Ans.** Both the ball will rise to the same height. It is
because, for a body moving with given initial velocity
and acceleration, the distance covered by the body
does not depend on the mass of the body.

**Q.21.** When will the relative velocity of two moving
bodies be zero?
**Ans.** When the two bodies are moving with equal
velocities (i.e. same speed and in the same direction).

**Q.22.** Rest and motion are relative terms. Explain
**Ans.** Absolute rest and absolute motion have got no
meaning as such. For example, two persons sitting in
a moving bus at rest w.r.t each other but are in
motion w.r.t. a person standing on the roadside.
Further, trees, buildings, etc. on the surface of the
earth appears to be at rest. In fact they are in motion
as the Earth revolves around the sun. Thus, there is
no object which can be considered to be at absolute
rest. Hence, rest and motion are relative terms.

**Q.23.** Given below are some examples of motion. State
in each case, if the motion is one, two or three
dimensional: (a) a kite flying on a windy day (b) a
speeding car on a long straight highway (c) an insect
crawling on a globe (d) a carrom coin rebounding
from the side of the board (e) a planet revolving
around its star.
**Ans.**

- (a) three dimensional
- (b) one dimensional
- (c) three dimensional
- (d) two dimensional
- (e) two dimensional

**Q.24.** What are the characteristics of displacement?
**Ans.** Following are the main characteristics of
displacement :

- (a) The displacement has units of length.
- (b) The displacement of an object in a given time interval can be positive, zero or negative.
- (c) The actual distance travelled by an object in a given time interval is either equal to or greater than the magnitude of the displacement. Thus, the displacement of an object between two points tells the shortest distance between these two points.
- (d) The displacement of an object between two points does not tell exactly how the object actually moved between those two points.

**Q.25.** Distinguish between distance and displacement.
**Ans.** Following are the points of difference between
the displacement and distance :

- (a) The displacement is a vector quantity, whereas the distance covered is a scalar quantity.
- (b) The displacement of an object in a given time interval can be positive, zero or negative whereas the distance covered is always positive.
- (c) The actual distance travelled by an object in a given time interval is either equal to or greater than the magnitude of displacement.
- (d) The displacement of an object between two points does not tell exactly how the object actually moved between those two points.

**Q.26.** What are the characteristics of uniform motion?
**Ans.** Following are the main characteristics of
uniform motion :

- (a) Generally, the displacement may or may not be equal to the actual distance covered by an object. However, when the uniform motion takes place along a straight line in a given direction the magnitude of the displacement is equal to the actual distance covered by the object.
- (b) For an object to be in uniform motion, no cause or effort i.e. no force is required.
- (c) Since the velocity during uniform motion is same at each point of the path or at each instant, the average and instantaneous velocity is in a uniform motion always equal.
- (d) The velocity-time graph for an object having uniform motion is a straight line parallel to time-axis.

**Q.27.** Distinguish between speed and velocity.
**Ans.** Following are the points of difference between
speed and velocity :

- (a) Speed is a scalar quantity, whereas the velocity is vector quantity.
- (b) The speed of an object in a particular direction is called velocity of the object.
- (c) Whereas the velocity of an object can be positive, zero or negative, the speed of an object can only be positive or zero.
- (d) Speed of an object gives only quantitative knowledge of 'how fast object is moving'. It tells nothing about the direction of motion of the object. On the other hand, the velocity tells both about how fast the object is moving and its direction of motion.

**Q.28.** A particle in one dimensional motion with
positive value of acceleration must be speeding up. Is
it true? Explain.
**Ans.** The answer depends on the choice of the positive
direction of position axis.

To explain, let us consider the motion of a body falling freely under gravity and the positive direction of position to be a vertical line in downward direction. It follows that when a positive direction of position-axis is along the direction of motion, the body speeds up.

On the other hand, when a body is projected vertically upwards and the positive direction of position-axis is regarded as to be opposite the direction of motion, the body slows down, instead of speeding up.

**Q.29.** " The direction in which an object moves is
given by the direction of velocity of the object and not
by the direction of acceleration". Explain the above
statement with some suitable example.
**Ans.** When object is thrown up, the direction of
motion of the object and its velocity are along vertical
in upward direction. As it moves up, it is always
attracted in downward direction i.e. acceleration is
along vertical in downward direction. Therefore, the
direction of motion of the object is that of the velocity
and not that of acceleration.

**Q.30.** Can a body be at rest as well as in motion at the
same time? Explain.
**Ans.** Rest and motion are relative terms. A body at
rest with respect to another body may be in motion
with respect to some other body. For example, an
electric pole is at rest with respect to a tree on the
surface of the earth. But the same electric pole is in
motion with respect to an observer in a running
train.

**Q.31.** For a particle in one dimensional motion, the
instantaneous speed is always equal to the magnitude
of instantaneous velocity. Why?
**Ans.** In an accelerated motion, the velocity of an
object always keeps on changing. Therefore, one has
to measure the instantaneous velocity. However,
when accelerated motion takes place along a straight
line, the velocity of the body changes only due to the
change in magnitude of velocity. Therefore, the
instantaneous speed is always equal to the magnitude
of instantaneous velocity of the particle in one
dimensional motion.

**Q.32.** Is the rate of change of acceleration with time
important to mechanics? Comment.
**Ans.** The rate of change of acceleration with time can
be defined but it is not important to mechanics. It is
because the basic laws of motion involve only
acceleration and the quantity -- the rate of change of
acceleration is not required at all. Galileo and Newton
discovered that to understand and explain motion, it
is enough to define velocity (rate of change of
position) and acceleration (rate of change of velocity).

**Q.33.** A man standing on the edge of a cliff throws a
stone straight up with initial speed u and then throws
another stone straight down with same initial speed u
and from the same position. Find the ratio of speeds,
the two stones would attain, when they hit the
ground at the base of the cliff.
**Ans.** When the stone is thrown up with speed u, it will
rise up, till its speed becomes zero and then will start
falling down. It can be proved that when the stone
reaches the thrower, it will acquire the same speed u,
with which it was thrown up.

Since both the stones are falling under gravity with the same initial speed u, the two stones will head the ground with the same speed and hence the ratio of their speed is 1.

**Q.34.** What condition should be satisfied for vectors to
be equal?
**Ans.** Two vectors are said to be equal, if they have (i)
the same magnitude and (ii) the same direction.

**Q.35.** Does vector addition hold for any two vectors?
**Ans.** No. The vectors addition holds for two vectors of
same kind i.e. vectors representing two physical
quantities of same dimensions.

**Q.36.** Can we add velocity vector to the displacement
vector?
**Ans.** No, The vectors having the same nature only can
be added.

**Q.37.** Is it possible to add vectors representing force of
5 newton and 200 dyne?
**Ans.** Yes. It is because both the vectors represent
force. However, the given two vectors representing
force have to be expressed in the same system of
units i e. either in SI or CGS system.

**Q.38.** What is a resultant vector?
**Ans.** The resultant of two (or more) vectors is a single
vector, which produces the same effect as the
individual vectors together produce.

**Q.39.** Can the flight of a bird be an example of
composition of vectors?
**Ans.** Yes. When a bird has to fly, it presses air with its
two wings along the oblique directions. The resultant
of the reaction on the two wings of the bird enables it
to fly up in the sky.

**Q.40.** Is A quantity which has a magnitude and
direction always a vector? Give examples.
**Ans.** No. A quantity which possesses both magnitude
and direction may not be a vector. For example,
electric current, finite angular displacement, etc. For
a quantity having magnitude and direction to be a
vector, it must be the commutative law for addition.

**Q.41.** Can a vector be multiplied with both
dimensional and non dimensional scalars?
**Ans.** Yes. When a vector is multiplied with the
dimensionless scalar, it will be a vector having
dimensions as that of the given vector. On the other
hand, If a vector is multiplied with a dimensional
scalar, the resultant vector will have different
dimensions. For example, when a velocity vector is
multiplied with mass (a dimensional scalar), the
resultant vector has the dimensions of momentum.

**Q.42.** Vector addition is different from scalar addition.
Explain.
**Ans.** In scalar addition, one has to add the
magnitudes of scalars. Since the vectors possess
direction in addition to their magnitudes, the scalar
addition cannot be used to add two vectors. For this
reason, the vector addition is different from the
scalar addition.

**Q.43.** What is the essential condition for adding the
vectors?
**Ans.** The essential condition for adding the vectors is
that the quantity is represented by them should be of
the same physical nature. For example, all the vectors
to be added should be displacements or velocities or
forces. Forces cannot be added to velocities.

**Q.44.** Can three vectors not in one plane give a zero
resultant? Can four vectors do?
**Ans.** Three vectors not in one place cannot give zero
resultant. It is because, the resultant of any two
vectors lies in their own plane and therefore it cannot
cancel the effect of a third vector; which does not lie
in the plane of the other two.

The resultant of two vectors not in one plane may be zero.

**Q.45.** Can two vectors of different magnitudes be
combined to give zero resultant? Can three vectors
do?
**Ans.** Two vectors of different magnitudes cannot give
zero resultant.

Three vectors lying in a plane can give zero resultant.

**Q.46.** A man moving in rain holds his umbrella
inclined to the vertical even though the rain drops
are falling vertically downwards. Why?
**Ans.** When the man moves in the rain falling
vertically downwards, the rain drops appear to fall in
a direction inclined to the vertical.

So as to protect himself from the rain, he holds the umbrella inclined to the vertical in the direction of apparent velocity of the rain with respect to himself.