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

**Q.1.** What is projectile?
**Ans.** Any object projected into space or air touch that
it moves under the effect of gravity alone is called a
projectile.

**Q.2.** What do we mean by trajectory?
**Ans.** The path followed by the projectile is called its
trajectory.

**Q.3.** Define angular displacement.
**Ans.** The angular displacement of an object in a given
time interval be defined as the angle which its
position vectors sweeps out in that time interval.

**Q.4.** Define unit vector.
**Ans.** A vector divided by its magnitude is called a unit
vector along the direction of the vector.

**Q.5.** What is meant by resolution of a vector?
**Ans.** The process of splitting up a vector into two or
more vectors is known as the resolution of a vector.

**Q.6.** Can a vector be zero if one of its components is
not zero?
**Ans.** No. For example, consider a vector along Z-axis.
Its components along x-axis and y-axis will be zero
but it itself will not be equal to zero.

**Q.7.** Can the walking on a road be an example of
resolution of vectors?
**Ans.** Yes, when a man walks on the road he presses
the road along an oblique direction the horizontal
component of the reaction helps the man to walk on
the road.

**Q.8.** Can any of the components of a given vector have
a greater magnitude than that of the vector itself?
**Ans.** Yes, provided the components are not
rectangular.

**Q.9.** Give a few examples of motion in two
dimensions.
**Ans.** A bag dropped from an aircraft flying
horizontally, a gun shot fired at some angle with the
horizontal, etc.

**Read also:** Motion In a Plane Class 11 Physics notes Chapter 4

**Q.10.** What are horizontal and vertical components
of acceleration of a body thrown horizontally with
uniform speed?
**Ans.** When a body is thrown horizontally with
uniform speed, then

acceleration along the horizontal = 0

and acceleration along the vertical = g

**Q.11.** What is the angle of projection for a projectile
thrown parallel to horizontal?
**Ans.** It is 0°.

**Q.12.** Velocity of a projectile is 10 metre per second. At
what angle to the horizontal should it be projected so
that it covers maximum horizontal distance?
**Ans.** At an angle of 45° to the horizontal.

**Q.13.** Two balls are released simultaneously from
certain height, one is allowed to fall freely and other
thrown with some horizontal velocity. Will they hit
the ground together? Substantiate your answer with
proper reasoning.
**Ans.** Both the balls will hit the ground together. It is
because the two balls fall through the same height
and the initial velocity of both the balls along the
vertical is zero.

**Q.14.** A hunter aims his gun and fires directly at a
monkey sitting in a tree. The moment the bullet
leaves the barrel of the gun the monkey falls. Will the
bullet hit the monkey?
**Ans.** Yes, the bullet will hit the monkey. It is because
of the fact that both the monkey and the bullet fall
through the same height and the initial velocity of
both of them along the vertical is zero.

**Q.15.** Give two examples of circular motion with
constant speed.
**Ans.**

- (a) The motion of the hour or the minute hand of watch.
- (b) The motion of the blades on the fan when the electric fan has attained constant speed.

**Q.16.** What is the direction of velocity vector of a
particle in circular motion?
**Ans.** At every point, the velocity vector of a particle in
circular motion acts along the tangent to the circular
path.

**Read also:** Class 11 Physics Chapter 4 MCQs with Answer Motion in a Plane

**Q.17.** Can the velocity of a particle change even if its
speed is constant? Give example.
**Ans.** Yes. For example, when a body moves along a
circular path with constant speed, it possesses
centripetal acceleration due to continuous change in
its direction of motion.

**Q.18.** A body is dropped freely from the window of a
train. Will the time of the free fall be equal, if the
train is stationary, the train moves with a constant
velocity, the train moves with an acceleration?
**Ans.** The motion of the train only affects the
magnitude of the horizontal component of velocity
and the acceleration of the body along the horizontal,
if any. It does not affect the nature of the motion
along the vertical. Therefore, in all the three cases,
the time of free fall of the body will be equal.

**Q.19.** A person sitting in a moving train throws a ball
vertically upwards how does the ball appear to move
to an observer (a) inside the train (b)outside the
train?
**Ans.**

- (1) The observer inside the train the ball will appear to move straight vertically upwards and then downwards.
- (2) To the observer outside the train, the ball will appear to move along the parabolic path.

**Q.20.** While firing, one has to aim little above the
target and not exactly on the target. Why?
**Ans.** During the time interval, the bullet moves up to
the target, it falls through some vertical distance
under the effect of gravity. As a result, it will not hit
the target at the point, it was aimed at. It will hit a
little below. Therefore, so as to hit the target exactly
at that point, one has to am a little above that point
on the target.

**Q.21.** Which is greater -- the angular velocity of the
hour hand of a watch or angular velocity of earth
around its own axis?
**Ans.** The angular velocity of the hour hand is greater
than that of the earth.

**Q.22.** Justify the statement that a uniform circular
motion is an accelerated motion.
**Ans.** In uniform circular motion, the speed of the
body remains the same but the direction of motion of
the body changes continuously. Since velocity is a
vector quantity the continuous change in the
direction of motion of the body implies continuous
change in the velocity of the body. As the rate of
change of velocity acceleration, a uniform circular
motion is an accelerated motion.

**Q.23.** How can we say that the object is accelerating?
**Ans.** An object is said to be accelerating if its speed is
changing or its direction of motion is changing or
both.

**Q.24.** Write dimensional formula of angular velocity.
**Ans.** The dimensional formula of angular velocity is
[M^{0} L^{0} T^{-1}].

**Q.25.** Write dimensional formula of angular
acceleration.
**Ans.** The dimensional formula of angular acceleration
is [M^{0} L^{0} T^{-2}].

**Q.26.** What is the dimensional formula of angular
displacement?
**Ans.** Angular displacement is a dimensionless
quantity.

**Q.27.** Write the characteristics of uniform circular
motion.
**Ans.** Following are the characteristics of the uniform
circular motion:

- Angular speed of the object is constant.
- Angular acceleration of the object is zero.
- Linear velocity of the object has constant magnitude but its direction keeps on changing continuously.
- Uniform circular motion is an accelerated motion, in which acceleration is directed along the radius of the circle and towards its centre.
- It is a periodic motion having a definite time period and frequency.

**Q.28.** Define time period in uniform circular motion.
**Ans.** The time period of an object in uniform circular
motion is defined as the time taken to complete one
revolution. It is denoted by T.

**Q.29.** Give few examples of the projectile.
**Ans.** Following are a few examples of the projectile:

- A bullet fired from the gun.
- A shell fired from a Canon.
- At javelin or hammer thrown by an athlete.
- A stone thrown from the top of a hill or a tower, etc.

**Q.30.** What is resolution of a vector?
**Ans.** The process of splitting up a vector into two or
more vectors is known as the resolution of a vector
the vectors into which a given vector is plated are
called component vectors.