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NCERT Exercises and Solutions: Motion in a Straight Line

🎓 Class 11 Physics CBSE Theory Ch 2 – Motion in a Straight Line ⏱ ~8 min
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NCERT Exercises and Solutions: Motion in a Straight Line

Chapter Summary

Key Concepts

  • Kinematics describes motion without investigating its causes.
  • Path length (distance) is a scalar; displacement is a vector. |displacement| ≤ path length.
  • Average velocity = displacement/time; average speed = path length/time.
  • Instantaneous velocity \(v = dx/dt\) = slope of the x-t graph at a point.
  • Acceleration \(a = dv/dt = d^2x/dt^2\). The slope of v-t graph gives acceleration; area under v-t graph gives displacement.
  • An object can have zero velocity with non-zero acceleration (e.g., ball at highest point).
  • Free fall: \(a = g = 9.8\) m/s² downward, regardless of mass (ignoring air resistance).
  • Stopping distance \(d = u^2/(2|a|)\) — proportional to the square of speed.
  • Relative velocity: \(v_{AB} = v_A - v_B\).

The Three Kinematic Equations (Uniform Acceleration)

\[ \begin{aligned} &\text{1. } v = u + at \\[6pt] &\text{2. } s = ut + \tfrac{1}{2}at^2 \\[6pt] &\text{3. } v^2 = u^2 + 2as \end{aligned} \]

Sign Conventions

QuantityPositive (+)Negative (−)
DisplacementRight / UpwardLeft / Downward
VelocityMoving right / upwardMoving left / downward
AccelerationSpeeding up (if v > 0) or slowing down (if v < 0)Slowing down (if v > 0) or speeding up (if v < 0)

Keywords

Kinematics
Point Object
Position
Path Length
Displacement
Average Velocity
Average Speed
Instantaneous Velocity
Acceleration
Uniform Acceleration
Kinematic Equations
Free Fall
Stopping Distance
Reaction Time
Relative Velocity
x-t Graph
v-t Graph

NCERT Exercises

Exercise 2.1

Under what condition is the magnitude of displacement equal to the path length covered by an object moving along a straight line?

Solution:
The magnitude of displacement equals the path length when the object moves in a single direction without reversing. In other words, there is no change in the sign of velocity throughout the motion. The moment the object turns back, path length exceeds |displacement|.

Exercise 2.2

Can an object be accelerating even though its speed is zero? Explain with an example.

Solution:
Yes. A ball thrown vertically upward has zero velocity at its highest point, but its acceleration is \(g = 9.8\) m/s² downward at every instant — including at the top. The acceleration is non-zero because the velocity is about to change (from zero to downward). If acceleration were also zero at the top, the ball would remain suspended in the air forever.

Exercise 2.3

A ball is thrown vertically upward with a velocity of 20 m/s. Neglecting air resistance, what is the velocity and acceleration of the ball at the highest point?

Solution:
At the highest point:
Velocity = 0 (the ball momentarily stops before reversing direction).
Acceleration = \(-g = -9.8\) m/s² (directed downward). Gravity acts at all points of the trajectory, including the top. The acceleration does not become zero just because the velocity is zero.

Exercise 2.4

Can the average speed of a moving body ever be zero? Can the average velocity be zero? Under what conditions?

Solution:
Average speed: No, the average speed of a moving body can never be zero. Path length is always positive for a moving body, and time is positive, so their ratio is always positive.

Average velocity: Yes, average velocity can be zero. This happens when the object returns to its starting position, making the net displacement zero. Example: a ball thrown up and caught back at the same height — the displacement is zero, so average velocity = 0, even though average speed is non-zero.

Exercise 2.5

A man walks on a straight road from his home to a market 2.5 km away in 25 minutes. He then instantly turns around and walks back home in 20 minutes. Find (a) the average speed and (b) the average velocity for (i) the walk to the market, (ii) the walk back home, and (iii) the entire trip.

Solution:
Taking direction from home to market as positive.

(i) Walk to market:
Distance = Displacement = 2.5 km, Time = 25 min = 25/60 h
Average speed = 2.5/(25/60) = 2.5 × 60/25 = 6 km/h
Average velocity = +2.5/(25/60) = +6 km/h (towards market)

(ii) Walk back home:
Distance = 2.5 km, Displacement = −2.5 km, Time = 20 min = 20/60 h
Average speed = 2.5/(20/60) = 2.5 × 60/20 = 7.5 km/h
Average velocity = −2.5/(20/60) = −7.5 km/h (towards home)

(iii) Entire trip:
Total distance = 2.5 + 2.5 = 5 km, Total displacement = 0 (returns to start)
Total time = 25 + 20 = 45 min = 45/60 h
Average speed = 5/(45/60) = 5 × 60/45 = \(\boxed{\frac{20}{3} \approx 6.67 \text{ km/h}}\)
Average velocity = 0/(45/60) = \(\boxed{0}\)

Exercise 2.6

A ball is dropped from a height of 90 m. Taking \(g = 10\) m/s², find the position and velocity of the ball at (i) \(t = 1\) s, (ii) \(t = 2\) s, (iii) \(t = 3\) s. Plot x-t and v-t graphs.

Solution:
Taking downward as positive, origin at drop point: \(u = 0\), \(g = 10\) m/s²
Position: \(x = \frac{1}{2}gt^2 = 5t^2\)
Velocity: \(v = gt = 10t\)

t (s)x = 5t² (m)v = 10t (m/s)Height from ground (m)
00090
151085
2202070
3453045

Time to reach ground: \(90 = 5t^2 \Rightarrow t = \sqrt{18} \approx 4.24\) s
Velocity at ground: \(v = 10 \times 4.24 \approx 42.4\) m/s

x-t graph: Parabola opening upward (x = 5t²).
v-t graph: Straight line through origin with slope 10 (v = 10t).

Exercise 2.7

Explain with an example how an object can have zero velocity but non-zero acceleration.

Solution:
Example: Consider a ball thrown vertically upward with velocity \(u\).

At the topmost point, the ball's velocity is momentarily zero (\(v = 0\)). However, gravity continues to act on it, providing a constant downward acceleration \(a = -g = -9.8\) m/s². This acceleration is what causes the ball to reverse direction and fall back down.

If both velocity and acceleration were zero at the top, the ball would remain stationary in mid-air, which does not happen. Therefore, \(v = 0\) does not imply \(a = 0\).

Exercise 2.8

A car moving along a straight highway at a constant speed of 40 km/h. Determine the position of the car at (i) \(t = 0\), 1 h, 2 h, 3 h relative to its starting point. Plot the x-t graph.

Solution:
Speed = 40 km/h (constant), so position \(x = 40t\) (in km, with \(t\) in hours).

t (h)0123
x (km)04080120

The x-t graph is a straight line passing through the origin with slope = 40 km/h. This constant positive slope confirms uniform velocity.

Exercise 2.9

From the x-t graphs below, identify which represent (a) constant velocity, (b) acceleration, (c) deceleration, (d) an object at rest.

Solution:
(a) Constant velocity: A straight line with constant (non-zero) slope on the x-t graph. The steeper the line, the higher the speed.

(b) Acceleration (speeding up): A curve that bends upward — the slope (velocity) increases with time. The x-t curve becomes steeper.

(c) Deceleration (slowing down): A curve that bends toward the horizontal — the slope (velocity) decreases with time. The x-t curve becomes flatter.

(d) Object at rest: A horizontal straight line (zero slope). The position does not change with time.

Exercise 2.10

A car moving at 30 m/s decelerates uniformly to rest. The braking deceleration is 6 m/s². Find (a) the time taken to stop, and (b) the stopping distance.

Solution:
Given: \(u = 30\) m/s, \(v = 0\), \(a = -6\) m/s²

(a) Time to stop: \[v = u + at \Rightarrow 0 = 30 - 6t \Rightarrow t = \frac{30}{6} = \boxed{5 \text{ s}}\]
(b) Stopping distance: \[v^2 = u^2 + 2as \Rightarrow 0 = 900 + 2(-6)s \Rightarrow s = \frac{900}{12} = \boxed{75 \text{ m}}\]
Verification: \(s = ut + \frac{1}{2}at^2 = 30(5) + \frac{1}{2}(-6)(25) = 150 - 75 = 75\) m. Confirmed.

Exercise 2.11

Two trains A and B are 200 km apart. Train A starts from station P towards Q at 40 km/h. Train B starts from Q towards P at 60 km/h at the same time. Find (a) when they meet, (b) where they meet (distance from P).

Solution:
The trains move toward each other, so relative speed = 40 + 60 = 100 km/h.

(a) Time to meet: \[t = \frac{\text{separation}}{\text{relative speed}} = \frac{200}{100} = \boxed{2 \text{ hours}}\]
(b) Distance from P: \[d_A = 40 \times 2 = \boxed{80 \text{ km from P}}\]
Verification: Distance covered by B = 60 × 2 = 120 km. Total = 80 + 120 = 200 km. Confirmed.

Exercise 2.12

A stone is dropped from the top of a building. It takes 3 seconds to reach the ground. Find the height of the building and the velocity of the stone just before it hits the ground. (\(g = 9.8\) m/s²)

Solution:
\(u = 0\), \(t = 3\) s, \(g = 9.8\) m/s²

Height: \[h = \frac{1}{2}gt^2 = \frac{1}{2}(9.8)(9) = 4.9 \times 9 = \boxed{44.1 \text{ m}}\]
Impact velocity: \[v = gt = 9.8 \times 3 = \boxed{29.4 \text{ m/s}}\]
Check: \(v^2 = 2gh = 2 \times 9.8 \times 44.1 = 864.36\), \(\sqrt{864.36} = 29.4\) m/s. Confirmed.

Exercise 2.13

A ball thrown vertically upward returns to the thrower after 6 seconds. Find (a) the initial velocity, (b) the maximum height reached, and (c) the position of the ball at \(t = 4\) s. (\(g = 9.8\) m/s²)

Solution:
Total time = 6 s. By symmetry, time to reach max height = 6/2 = 3 s.

(a) Initial velocity:
At max height, \(v = 0\): \(v = u - gt \Rightarrow 0 = u - 9.8 \times 3\) \[\boxed{u = 29.4 \text{ m/s}}\]
(b) Maximum height: \[h = ut - \frac{1}{2}gt^2 = 29.4(3) - \frac{1}{2}(9.8)(9) = 88.2 - 44.1 = \boxed{44.1 \text{ m}}\]
(c) Position at t = 4 s:
The ball reaches max height at \(t = 3\) s and is now falling back. At \(t = 4\) s: \[x = ut - \frac{1}{2}gt^2 = 29.4(4) - \frac{1}{2}(9.8)(16) = 117.6 - 78.4 = \boxed{39.2 \text{ m above the ground}}\] The ball is 39.2 m above the starting point (or 44.1 − 39.2 = 4.9 m below the maximum height, which equals \(\frac{1}{2}g(1)^2\) — i.e., it has fallen 4.9 m in 1 second from the top).

Exercise 2.14

Read off the x-t graph below and answer: (a) What is the velocity at \(t = 2\) s? (b) Is the velocity uniform during \(t = 0\) to \(t = 4\) s? (c) What is the average velocity from \(t = 2\) s to \(t = 6\) s?

t (s) x (m) 2 4 6 8 10 20 30 40
Solution:
(a) Velocity at t = 2 s:
Between \(t = 0\) and \(t = 4\) s, the graph is a straight line from \(x = 0\) to \(x = 40\) m. \[v = \frac{\Delta x}{\Delta t} = \frac{40 - 0}{4 - 0} = \boxed{10 \text{ m/s}}\]
(b) Is the velocity uniform from t = 0 to 4 s?
Yes, the x-t graph is a straight line in this interval, indicating a constant slope and hence uniform velocity of 10 m/s.

(c) Average velocity from t = 2 s to t = 6 s:
At \(t = 2\) s: \(x = 20\) m (midpoint of the line from 0 to 40).
At \(t = 6\) s: \(x = 40\) m (end of horizontal segment).
\[\bar{v} = \frac{40 - 20}{6 - 2} = \frac{20}{4} = \boxed{5 \text{ m/s}}\]

Exercise 2.15

A person walking at 5 km/h finds that rain appears to fall vertically. When the person doubles their speed to 10 km/h, the rain appears to come at an angle of 45 degrees to the vertical. Find the actual speed and direction of the rain.

Solution:
Let the rain fall at velocity \(v_r\) at angle \(\theta\) with the vertical. The rain has horizontal component \(v_h\) and vertical component \(v_v\).

Case 1: Person walks at 5 km/h. Rain appears vertical, meaning the relative horizontal component is zero: \[v_h - 5 = 0 \Rightarrow v_h = 5 \text{ km/h}\] The rain has a horizontal component of 5 km/h (in the direction the person walks).

Case 2: Person walks at 10 km/h. Rain appears at 45 degrees to the vertical: \[\tan 45° = \frac{v_h - 10}{-v_v} \quad \text{(but let's be careful with signs)}\] Actually, relative horizontal velocity of rain w.r.t. person = \(5 - 10 = -5\) km/h (rain appears to come from the front). Relative vertical velocity = \(v_v\) (downward). \[\tan 45° = \frac{|-5|}{v_v} = \frac{5}{v_v} \Rightarrow v_v = 5 \text{ km/h}\]
Actual rain speed: \[v_r = \sqrt{v_h^2 + v_v^2} = \sqrt{25 + 25} = \sqrt{50} = \boxed{5\sqrt{2} \approx 7.07 \text{ km/h}}\]
Direction: \[\theta = \tan^{-1}\left(\frac{v_h}{v_v}\right) = \tan^{-1}\left(\frac{5}{5}\right) = \boxed{45° \text{ with the vertical}}\] The rain actually falls at 45 degrees to the vertical, in the direction the person is walking.

Did You Know?

Frequently Asked Questions - NCERT Exercises and Solutions: Motion in a Straight Line

What are the key NCERT exercise types in Chapter 2 Motion in a Straight Line?
NCERT Class 11 Physics Chapter 2 Motion in a Straight Line exercises cover conceptual MCQs, short numerical problems (2-3 marks), long numerical derivations (5 marks), and assertion-reason questions. The MyAiSchool solution set provides step-by-step worked solutions for every NCERT exercise question in the chapter, aligned with the CBSE board exam pattern. Students should focus on dimensional analysis, formula application, and unit consistency to score full marks.
How should students approach numerical problems in Motion in a Straight Line?
For numerical problems in NCERT Class 11 Physics Chapter 2 Motion in a Straight Line: (1) list all given data with units, (2) write the relevant formula(e), (3) substitute values carefully, (4) calculate with proper significant figures, (5) state the final answer with the correct unit. Always draw a free-body or schematic diagram where relevant. The MyAiSchool solutions follow this 5-step CBSE-aligned format consistently.
What are the most-asked CBSE board questions from Chapter 2?
From NCERT Class 11 Physics Chapter 2 (Motion in a Straight Line), the most-asked CBSE board questions test conceptual understanding of fundamental principles, application of derived formulas to standard scenarios, and proof-style derivations. 5-mark questions usually combine derivation + application. The MyAiSchool exercise set tags each question by board frequency so students can prioritize high-yield problems before exams.
How do I check the dimensional correctness of my answer?
Dimensional correctness in NCERT Class 11 Physics is verified by ensuring the LHS and RHS of any equation have the same dimensional formula (M^a L^b T^c). For Chapter 2 Motion in a Straight Line problems, write the dimensions of each quantity, substitute, and simplify. If dimensions match the equation is dimensionally valid (necessary but not sufficient). The MyAiSchool solutions include dimensional checks in every numerical answer.
What are common mistakes students make in Chapter 2 exercises?
Common mistakes in NCERT Class 11 Physics Chapter 2 Motion in a Straight Line exercises include: (1) unit conversion errors (CGS vs SI), (2) sign convention mistakes for vectors, (3) forgetting to consider all forces in free-body diagrams, (4) algebra errors during derivations, and (5) misreading the problem statement. The MyAiSchool solutions highlight these traps with red-flag annotations so students learn to avoid them.
How does the MyAiSchool solution differ from other NCERT solution sets?
MyAiSchool Class 11 Physics Chapter 2 Motion in a Straight Line solutions use NEP 2024-aligned pedagogy with Bloom Taxonomy tagged questions (L1 Remember to L6 Create), step-by-step working with reasoning, dimensional checks, interactive simulations, and Competency-Based Questions (CBQs) for board exam practice. Each solution is verified against NCERT and CBSE marking schemes for accuracy.
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