Physics · Kinematics · chapter 2 of 14

Class 11 Physics Chapter 2: Motion in a Straight Line MCQs with answers

10 multiple-choice questions on Motion in a Straight Line, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
A physics student treats a bus travelling 60 km between two towns as a point object. This is a fair approximation because
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Answer: (b) the size of the bus is much smaller than the distance it moves

A body may be treated as a point object when its size is much smaller than the distance it moves in the time considered; a bus about 10 m long covering 60 km qualifies. Speed, mass and the shape of the road do not decide this.

Question 2
On the position-time graph of a body, the velocity of the body at a particular instant is given by
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Answer: (c) the slope of the tangent to the graph at that instant

Velocity is the limit of Δx/Δt as Δt → 0. The chord through two points of the graph becomes the tangent in this limit, so the velocity is the slope of the tangent at that instant. The slope of the line from the origin gives only the average velocity from t = 0.

Question 3
A jogger runs 300 m due east along a straight track and then turns and runs some distance due west along the same track. Over the whole run her average speed is twice the magnitude of her average velocity. How far did she run westward?
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Answer: (a) 100 m

Let the westward run be w. Path length = 300 + w; magnitude of displacement = 300 − w. Both are divided by the same time, so 300 + w = 2(300 − w), 3w = 300 and w = 100 m.

Question 4
A hockey ball moving at 6 m s⁻¹ towards the east is struck and moves at 4 m s⁻¹ towards the west 2 s later. Taking east as positive, the average acceleration of the ball in this interval is
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Answer: (d) −5 m s⁻²

ā = (v₂ − v₁)/(t₂ − t₁) = (−4 − 6)/2 = −5 m s⁻², that is, 5 m s⁻² towards the west. Taking −1 m s⁻² ignores the reversal of direction.

Question 5
A ball rolling at 3 m s⁻¹ along a straight corridor hits a wall and rebounds along the same line at 3 m s⁻¹. Which statement about the collision is correct?
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Answer: (a) The ball's velocity changed, so it was accelerated during the contact.

Velocity has magnitude and direction. The speed stayed 3 m s⁻¹ but the velocity changed from +3 m s⁻¹ to −3 m s⁻¹, a change of 6 m s⁻¹; a change in velocity in any time means an acceleration.

Question 6
A portion of the position-time graph of a car is a curve that bends downward (its slope keeps decreasing). In that portion the acceleration of the car is
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Answer: (b) negative

The slope of the x-t graph is the velocity. A curve bending downward has a decreasing slope, so the velocity decreases and the acceleration is negative; a straight line would mean zero acceleration.

Question 7
The velocity of a cyclist on a straight road rises uniformly from 4 m s⁻¹ to 12 m s⁻¹ in 5 s. The displacement of the cyclist in these 5 s is
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Answer: (c) 40 m

Displacement is the area under the v-t line, a trapezium: ½ (4 + 12) × 5 = 40 m. Using only the final velocity (12 × 5) gives 60 m and only the initial velocity gives 20 m.

Question 8
A body starts from rest and moves with uniform acceleration along a straight line. Its position-time and velocity-time graphs are respectively
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Answer: (d) a parabola, and a straight line inclined to the time axis

For uniform acceleration x = ½at² (a parabola) and v = at (a straight line through the origin, inclined to the time axis). The pair in option (a) describes uniform motion.

Question 9
For a particle in one-dimensional motion, which of the following quantities can take a negative value?
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Answer: (c) displacement

In one dimension the sign of displacement shows its direction, so it can be negative. Path length, speed and average speed are magnitudes or totals of path and are never negative.

Question 10
A trolley is given a push up a smooth inclined track. It slows down, stops for an instant and rolls back. At the instant it stops, its acceleration is
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Answer: (b) the same as just before it stopped, directed down the track

The trolley moves with a constant acceleration down the track throughout; zero velocity at an instant does not mean zero acceleration. If the acceleration were zero at that instant, the trolley would stay at rest instead of rolling back.

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