NCERT Solutions for Class 11 Physics Chapter 2 Motion in a Straight Line are designed to help students understand the fundamentals of motion using simple concepts, clear definitions, and standard equations. This chapter lays the groundwork for kinematics and is essential for solving numericals in higher Physics chapters.
These solutions strictly follow the latest NCERT textbook and are written in a clear, exam-oriented, and student-friendly manner, making them ideal for school exams, CBSE board preparation, and competitive exams like JEE & NEET.
NCERT Solutions for Class 11 Physics Chapter 2: Motion in a Straight Line
Q.
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.
Q.
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.
Q.
A police van moving on a highway with a speed of 30 kmh-1 fires a bullet at a thief’s car speeding away in the same direction with a speed of 192 kmh-1. If the muzzle speed of the bullet is 150 ms-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).
Q.
Figure 3.21 shows 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.
Q.
Look at the graphs (a) to (d) carefully and state, with reasons, which of these graphs cannot possibly represent one dimensional motion of a particle.
Q.
Figure 3.23 gives the x-t plot of a particle executing one dimensional simple harmonic motion. Give the signs of position, velocity and acceleration variables of the particle at t=0.3 s, 1.2 s, -1.2 s.
Q.
In the above questions 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 the magnitude of velocity. The instantaneous speed is always equal to the magnitude of instantaneous velocity. Why?
Q.
A man walks on a straight road from his home to a market 2.5 km away with a speed of 5 kmh–1. Finding the market closed, he instantly turns and walks back home with a speed of 7.5 kmh–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 itis 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!]
Q.
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 time true? [For simplicity, consider one dimensional motion only].
Q.
Read each statement carefully and state with reasons and examples if it is true or false;
(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 may have zero acceleration
(d) With positive value of acceleration must be speeding up.
Q.
The position-time (x-t) graphs for two children A and B returning from their school to their homes P and Q respectively are shown in Fig. 3.19. Choose the correct entries in the brackets below;
(a) (A/B) lives closer to 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).
Q.
A player throws a ball upwards with an initial speed of 29.4 ms-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 and t = 0 be the location and time 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 returns to the player’s hands. (Take g = 9.8 ms-2 and neglect air resistance)
Q.
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 kmh–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?
Q.
On a two lane road, car A is travelling with a speed of 36 kmh-1. Two cars B and C approach car A in opposite directions with a speed of 54 kmh-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?
Q.
Two trains A and B of length 400 m each are moving on two parallel tracks with a uniform speed of 72 kmh-1 in the same direction with A ahead of B. The driver of B decides to overtake A and accelerate by 1 ms-2. If after 50 s, the guard of B just brushes past the driver of A, what was the original distance between them?
Q.
A car moving along a straight highway with speed of 126 kmh-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?
Q.
A jet airplane travelling at the speed of 500 km h–1 ejects its products of combustion at the speed of 1500 kmh–1 relative to the jet plane. What is the speed of the latter with respect to an observer on the ground?
Q.
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 1m 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.
Q.
A woman starts from her home at 9.00 am, walks with a speed of 5 kmh–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 kmh–1. Choose suitable scales and plot the x-t graph of her motion.
Q.
A three wheeler starts from rest, accelerates uniformly with 1 ms-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 the plot to be during accelerated motion: a straight line or a parabola?
NCERT Solutions for Class 11 Physics Chapter 2: Motion in a Straight Line
NCERT Solutions for Class 11 Physics Chapter 2 – Exercise Questions
NCERT Solutions – Chapter 2: Motion in a Straight Line
Q.1) In which of the following examples of motion, can the body be considered approximately a point object?
- a railway carriage moving without jerks between two stations
- a monkey sitting on top of a man cycling smoothly on a circular track
- a spinning cricket ball that turns sharply on hitting the ground
- a tumbling beaker that has slipped off the edge of a table
Answer:
- The railway carriage can be considered a point object because its size is negligible compared to the distance between stations.
- The monkey can be considered a point object as its size is very small compared to the size of the circular track.
- The spinning cricket ball cannot be considered a point object because its size is comparable to the distance it turns after hitting the ground.
- The tumbling beaker cannot be considered a point object as its size is comparable to the height of the table.
Q.2) The position-time (x–t) graphs for two children A and B are shown. Choose the correct options:
- (A/B) lives closer to school than (B/A)
- (A/B) starts from school earlier than (B/A)
- (A/B) walks faster than (B/A)
- A and B reach home at the (same/different) time
- (A/B) overtakes (B/A) on the road (once/twice)
Answer:
- A lives closer to school than B.
- A starts earlier than B.
- B walks faster than A.
- A and B reach home at the same time.
- B overtakes A once.
Q.3) A drunkard walks 5 steps forward and 3 steps backward repeatedly. Each step is 1 m and takes 1 s. Find the time taken to fall into a pit 13 m away.
Answer:
Distance covered in one cycle = 5 − 3 = 2 m
Time taken in one cycle = 5 + 3 = 8 s
Distance covered in 4 cycles = 8 m in 32 s.
In the next 5 s, the drunkard moves 5 m forward and reaches 13 m.
Total time taken = 32 + 5 = 37 seconds
Q.4) State whether the following statements are true or false with reasons:
- With zero speed at an instant, a body may have non-zero acceleration.
- With zero speed, a body may have non-zero velocity.
- With constant speed, a body may have zero acceleration.
- With positive acceleration, a body must be speeding up.
Answer:
- True. At the highest point of motion, speed becomes zero but acceleration exists.
- False. Speed is the magnitude of velocity and cannot be zero if velocity exists.
- True. Constant velocity implies zero acceleration.
- True (conditionally). If velocity and acceleration are in the same direction.
Q.5) Which graphs cannot represent one-dimensional motion of a particle?
Answer:
- (a) A particle cannot have two positions at the same instant of time.
- (b) A particle cannot have two velocities at the same instant in one dimension.
- (c) Speed cannot be negative.
- (d) Total path length cannot decrease with time.
Q.6) Does the given x–t graph show straight-line motion for t ≥ 0?
Answer:
No. The graph does not represent a straight-line path. It can represent motion with
constant acceleration.
Q.7) A police van fires a bullet at a thief’s car. Find the speed with which the bullet hits the car.
Answer:
Speed of police van = 30 km/h = 8.33 m/s
Muzzle speed of bullet = 150 m/s
Speed of thief’s car = 192 km/h = 53.33 m/s
Effective speed of bullet = 150 + 8.33 = 158.33 m/s
Relative speed w.r.t thief’s car = 158.33 − 53.33 = 105 m/s
Note: Q&A containing MathML/LaTeX may not render inside PDF.
Frequently Asked Questions (FAQs)
Q1. Is Chapter 2 of Class 11 Physics important for exams?
Yes, this chapter is very important as it introduces kinematics and forms the base for solving numerical problems in later chapters.
Q2. Are equations of motion important for competitive exams?
Yes, equations of motion are frequently used in JEE and NEET numerical problems.
Q3. What is the most scoring part of this chapter?
Numericals based on equations of motion, graphs, and motion under gravity are highly scoring.
Q4. Are these NCERT Solutions enough for board exams?
Yes, these solutions cover all NCERT exercise questions and are sufficient for CBSE school exams.
Q5. Can I download these NCERT Solutions as a PDF?
Yes, the chapter-wise PDF is available for easy offline access and revision.