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Chapters
Chapter 2: Units and Measurements
Chapter 3: Motion in a Straight Line
Chapter 4: Motion in a Plane
Chapter 5: Laws of Motion
Chapter 6: Work, Energy and Power
Chapter 7: System of Particles and Rotational Motion
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
Chapter 3: Motion in a Straight Line
NCERT solutions for Class 11 Physics Chapter 3 Motion in a Straight Line Exercises
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.
The positiontime (xt) graphs for two children A and B returning from their school O to their homes P and Q respectively are shown in Figure 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).
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 xt graph of her motion.
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 xtgraph of his motion. Determine graphically and otherwise how long the drunkard takes to fall in a pit 13 m away from the start.
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 ground?
A car moving along a straight highway with a 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
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?
On a twolane 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?
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?
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 xaxis, 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).
Read each statement below carefully and state with reasons and examples, if it is true or false;
A particle in onedimensional motion with zero speed at an instant may have nonzero acceleration at that instant
Read each statement below carefully and state with reasons and examples, if it is true or false;
A particle in onedimensional motion with zero speed may have nonzero velocity,
Read each statement below carefully and state with reasons and examples, if it is true or false;
A particle in onedimensional motion with constant speed must have zero acceleration
Read each statement below carefully and state with reasons and examples, if it is true or false;
A particle in onedimensional motion with positive value of acceleration mustbe speeding up.
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 speedtime graph of its motion between t = 0 to 12 s.
Explain clearly, with examples, the distinction between:
 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.

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 onedimensional motion only].
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!]
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?
Look at the graphs (a) to (d) Figure carefully and state, with reasons, which of these cannot possibly represent onedimensional motion of a particle.
Figure shows the xt plot of onedimensional 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.
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).
Suggest a suitable physical situation for each of the following graphs
Suggest a suitable physical situation for each of the following graphs:
Suggest a suitable physical situation for each of the following graphs:
Figure gives the xt plot of a particle executing onedimensional 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.
Figure gives the xt plot of a particle in onedimensional 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.
Figure gives a speedtime 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?
A threewheeler 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 n^{th }second (n = 1,2,3….) versus n. What do you expect this plot to be during accelerated motion: a straight line or a parabola?
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. 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 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?
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?
Two stones are thrown up simultaneously from the edge of a cliff 200 m high with initial speeds of 15 m/s and 30 m/s. Verify that the graph shown in Fig. 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.
The speedtime 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.
What is the average speed of the particle over the intervals in (a) and (b)
The velocitytime graph of a particle in onedimensional motion is shown in Figure
Which of the following formulae are correct for describing the motion of the particle over the timeinterval t_{2} to t_{1}?
(a) x(t_{2}) = x(t_{1}) + v(t_{1})(t_{2}–t_{1}) + (1/2)a(t_{2}–t_{1})^{2}
(b) v(t_{2})= v(t_{1}) + a(t_{2}–t_{1})
(c) v_{Average} = (x(t_{2}) – x(t_{1})) / (t_{2} – t_{1})
(d) a_{Average} = (v(t_{2}) – v(t_{1})) / (t_{2} – t_{1})
(e) x(t_{2}) = x(t_{1}) + v_{Average}(t_{2} – t_{1}) + (1/2)a_{Average}(t_{2} – t_{1})^{2}
(f) x(t_{2}) – x(t_{1}) = area under the v–t curve bounded by the taxis and the dotted line shown.
Fill in the blanks by suitable conversion of units
1 m =..... ly
State the number of significant figures in the following:
2.64 × 10^{24} kg
Chapter 3: Motion in a Straight Line
NCERT solutions for Class 11 Physics chapter 3  Motion in a Straight Line
NCERT solutions for Class 11 Physics chapter 3 (Motion in a Straight Line) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, stepbystep solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Class 11 Physics solutions in a manner that help students grasp basic concepts better and faster.
Further, we at Shaalaa.com provide such solutions so that students can prepare for written exams. NCERT textbook solutions can be a core help for selfstudy and acts as a perfect selfhelp guidance for students.
Concepts covered in Class 11 Physics chapter 3 Motion in a Straight Line are Acceleration (Average and Instantaneous), Relative Velocity, Kinematic Equations for Uniformly Accelerated Motion, Elementary Concept of Differentiation and Integration for Describing Motion, Uniform and Nonuniform Motion, Position, Path Length and Displacement, Average Velocity and Average Speed, Instantaneous Velocity and Speed, Uniformly Accelerated Motion, Positiontime, Velocitytime and Accelerationtime Graphs, Position  Time Graph, Relations for Uniformly Accelerated Motion (Graphical Treatment), Introduction of Motion in One Dimension.
Using NCERT Class 11 solutions Motion in a Straight Line exercise by students are an easy way to prepare for the exams, as they involve solutions arranged chapterwise also page wise. The questions involved in NCERT Solutions are important questions that can be asked in the final exam. Maximum students of CBSE Class 11 prefer NCERT Textbook Solutions to score more in exam.
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