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**Question 3.1** In which of the following examples of motion, can the body be considered
approximately a point object:

(a) a railway carriage moving without jerks between two stations.

(b) a monkey sitting on top of a man cycling smoothly on a circular track.

(c) a spinning cricket ball that turns sharply on hitting the ground.

(d) a tumbling beaker that has slipped off the edge of a table.

**Question 3.2** The position-time (x-t) graphs for two children A and B returning from their school
O to their homes P and Q respectively are shown in Fig.

**Question 3.19** Choose the correct
entries in the brackets below ;

(a) (A/B) lives closer to the school than (B/A)

(b) (A/B) starts from the school earlier than (B/A)

(c) (A/B) walks faster than (B/A)

(d) A and B reach home at the (same/different) time

(e) (A/B) overtakes (B/A) on the road (once/twice).

**Question 3.3 **A woman starts from her home at 9.00 am, walks with a speed of 5 km h–1 on a
straight road up to her office 2.5 km away, stays at the office up to 5.00 pm, and
returns home by an auto with a speed of 25 km h–1. Choose suitable scales and
plot the x-t graph of her motion.

**Question 3.4** A drunkard walking in a narrow lane takes 5 steps forward and 3 steps backward,
followed again by 5 steps forward and 3 steps backward, and so on. Each step is 1
m long and requires 1 s. Plot the x-t graph of his motion. Determine graphically
and otherwise how long the drunkard takes to fall in a pit 13 m away from the
start.

**Question 3.5 **A jet airplane travelling at the speed of 500 km h–1 ejects its products of combustion
at the speed of 1500 km h–1 relative to the jet plane. What is the speed of the
latter with respect to an observer on the ground ?

**Question 3.6** A car moving along a straight highway with speed of 126 km h–1 is brought to a
stop within a distance of 200 m. What is the retardation of the car (assumed
uniform), and how long does it take for the car to stop ?

**Question 3.7** Two trains A and B of length 400 m each are moving on two parallel tracks with a
uniform speed of 72 km h–1 in the same direction, with A ahead of B. The driver of
B decides to overtake A and accelerates by 1 m s–2. If after 50 s, the guard of B just
brushes past the driver of A, what was the original distance between them ?

**Question 3.8** On a two-lane road, car A is travelling with a speed of 36 km h–1. Two cars B and
C approach car A in opposite directions with a speed of 54 km h–1 each. At a
certain instant, when the distance AB is equal to AC, both being 1 km, B decides
to overtake A before C does. What minimum acceleration of car B is required to
avoid an accident ?

**Question 3.9** Two towns A and B are connected by a regular bus service with a bus leaving in
either direction every T minutes. A man cycling with a speed of 20 km h–1 in the
direction A to B notices that a bus goes past him every 18 min in the direction of
his motion, and every 6 min in the opposite direction. What is the period T of the
bus service and with what speed (assumed constant) do the buses ply on the
road?

**Question 3.10** A player throws a ball upwards with an initial speed of 29.4 m s–1.

(a) What is the direction of acceleration during the upward motion of the ball ?

(b) What are the velocity and acceleration of the ball at the highest point of its
motion ?

(c) Choose the x = 0 m and t = 0 s to be the location and time of the ball at its
highest point, vertically downward direction to be the positive direction of
x-axis, and give the signs of position, velocity and acceleration of the ball
during its upward, and downward motion.

(d) To what height does the ball rise and after how long does the ball return to the
player’s hands ? (Take g = 9.8 m s–2 and neglect air resistance).

**Question 3.11** Read each statement below carefully and state with reasons and examples, if it is
true or false ;
A particle in one-dimensional motion

(a) with zero speed at an instant may have non-zero acceleration at that instant

(b) with zero speed may have non-zero velocity,

(c) with constant speed must have zero acceleration,

(d) with positive value of acceleration must be speeding up.

**Question 3.12** A ball is dropped from a height of 90 m on a floor. At each collision with the floor,
the ball loses one tenth of its speed. Plot the speed-time graph of its motion
between t = 0 to 12 s.

**Question 3.13** Explain clearly, with examples, the distinction between :

(a) magnitude of displacement (sometimes called distance) over an interval of time,
and the total length of path covered by a particle over the same interval;
(b) magnitude of average velocity over an interval of time, and the average speed
over the same interval. [Average speed of a particle over an interval of time is
defined as the total path length divided by the time interval]. Show in both
(a)
and (b) that the second quantity is either greater than or equal to the first.
When is the equality sign true ? [For simplicity, consider one-dimensional
motion only].

**Question 3.14 **A man walks on a straight road from his home to a market 2.5 km away with a
speed of 5 km h–1. Finding the market closed, he instantly turns and walks back
home with a speed of 7.5 km h–1. What is the (a) magnitude of average velocity, and
(b) average speed of the man over the interval of time

(i) 0 to 30 min,

(ii) 0 to
50 min,

(iii) 0 to 40 min ?

[Note: You will appreciate from this exercise why it
is better to define average speed as total path length divided by time, and not
as magnitude of average velocity. You would not like to tell the tired man on
his return home that his average speed was zero !]

**Question 3.15** In Exercises 3.13 and 3.14, we have carefully distinguished between average speed
and magnitude of average velocity. No such distinction is necessary when we
consider instantaneous speed and magnitude of velocity. The instantaneous speed
is always equal to the magnitude of instantaneous velocity. Why ?

**Question 3.16 **Look at the graphs (a) to (d) (Fig. 3.20) carefully and state, with reasons, which of
these cannot possibly represent one-dimensional motion of a particle.

**Question 3.17 **Figure3.21 shows the x-t plot of one-dimensional
motion of a particle. Is it correct to say from the
graph that the particle moves in a straight line for
t < 0 and on a parabolic path for t >0 ? If not, suggest
a suitable physical context for this graph.

**Question 3.18** A police van moving on a highway with a speed of
30 km h–1 fires a bullet at a thief’s car speeding away
in the same direction with a speed of 192 km h–1. If
the muzzle speed of the bullet is 150 m s–1, with
what speed does the bullet hit the thief’s car ?

(Note:
Obtain that speed which is relevant for damaging
the thief’s car).

**Question 3.19** Suggest a suitable physical situation for each of the
following graphs (Fig 3.22):

**Question 3.20 **Figure3.23 gives the x-t plot of a particle executing one-dimensional simple
harmonic motion. (You will learn about this motion in more detail in Chapter14).
Give the signs of position, velocity and acceleration variables of the particle at
t = 0.3 s, 1.2 s, – 1.2 s.
Fig.

**Question 3.21** Figure 3.24 gives the x-t plot of a
particle in one-dimensional motion.
Three different equal intervals of time
are shown. In which interval is the
average speed greatest, and in which
is it the least ? Give the sign of average
velocity for each interval.

**Question 3.22** Figure 3.25 gives a speed-time graph of
a particle in motion along a constant
direction. Three equal intervals of time
are shown. In which interval is the
average acceleration greatest in
magnitude ? In which interval is the
average speed greatest ? Choosing the
positive direction as the constant
direction of motion, give the signs of v
and a in the three intervals. What are
the accelerations at the points A, B, C
and D ?

**Question 3.23** A three-wheeler starts from rest, accelerates uniformly with 1 m s–2 on a straight
road for 10 s, and then moves with uniform velocity. Plot the distance covered by
the vehicle during the nth second (n = 1,2,3….) versus n. What do you expect this
plot to be during accelerated motion : a straight line or a parabola ?

**Question 3.24** A boy standing on a stationary lift (open from above) throws a ball upwards with
the maximum initial speed he can, equal to 49 m s–1. How much time does the ball
take to return to his hands? If the lift starts moving up with a uniform speed of
5 m s-1 and the boy again throws the ball up with the maximum speed he can, how
long does the ball take to return to his hands ?

**Question 3.25** On a long horizontally moving belt (Fig.3.26), a child runs to and fro with a speed
9 km h–1 (with respect to the belt) between his father and mother located 50 m apart
on the moving belt. The belt moves with a speed of 4 km h–1. For an observer on a
stationary platform outside, what is the

(a) speed of the child running in the direction of motion of the belt ?.

(b) speed of the child running opposite to the direction of motion of the belt ?

(c) time taken by the child in (a) and (b) ?
Which of the answers alter if motion is viewed by one of the parents ?

**Question 3.26 **Two stones are thrown up simultaneously from the edge of a cliff 200 m high with
initial speeds of 15 m s–1 and 30 m s–1. Verify that the graph shown in Fig.

**Question 3.27** correctly represents the time variation of the relative position of the second stone
with respect to the first. Neglect air resistance and assume that the stones do not
rebound after hitting the ground. Take g = 10 m s–2. Give the equations for the
linear and curved parts of the plot.3.27 The speed-time graph of a particle moving along a fixed direction is shown in
Fig. 3.28. Obtain the distance traversed by the particle between
(a) t = 0 s to 10 s,
(b) t = 2 s to 6 s.
Fig.3.28
What is the average speed of the particle over the intervals in (a) and (b) ?

**Question 3.28** The velocity-time graph of a particle in one-dimensional motion is shown in
Fig.3.**.**29 :
Fig

**Question 3.29** Which of the following formulae are correct for describing the motion of the particle
over the time-interval t
1
to t
2
:

(a) x(t2 ) = x(t1) + v (t1) (t2 – t1) +(½) a (t2 – t1)2

(b) v(t2 ) = v(t1) + a (t2 – t1)

(c) vaverage = (x(t2) – x(t1))/(t2 – t1)

(d) aaverage = (v(t2) – v(t1))/(t2 – t1)

(e) x(t2 ) = x(t1) + vaverage (t2 – t1) + (½) aaverage (t2 – t1)2
(f) x(t2 ) – x(t1) = area under the v-t curve bounded by the t-axis and the dotted line
shown.

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- Chapter 3: Motion in a Straight Line
**[Ques wise Ans]** - Chapter 1 Physical World
- Chapter 2 Units and Measurement
- Chapter 3 Motion in a Straight Line
- Chapter 4 Motion in a Plane
- Chapter 5 Laws of Motion
- Chapter 7 Systems of Particles and Rotational Motion
- Chapter 6 Work Energy and Power
- Chapter 8 Gravitation
- Chapter 9 Mechanical Properties of Solids
- Chapter 10 Mechanical Properties of Fluids
- Chapter 11 Thermal Properties of Matter
- Chapter 12 Thermodynamics
- Chapter 13 Kinetic Theory
- Chapter 14 Oscillations
- Chapter 15 Waves

- NCERT Solutions for Class 9
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- NCERT Solutions for Class 11
- NCERT Solutions for Class 12

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