Describing Motion Around Us — Class 9 MCQs with Answers

Describing Motion Around Us — important questions & MCQs with answers (Class 9 Science)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Which of the following is a vector quantity?

   ADistanceBSpeedCTimeDDisplacement
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   Answer: Displacement

   Explanation: Displacement has magnitude AND direction, which makes it a vector quantity. Distance, speed, and time carry only magnitude, so they are scalars.

2. Q2 of 6Easy

   The SI unit of velocity is:

   Akm/hBm/sCcm/sDm/s²
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   Answer: m/s

   Explanation: Velocity = displacement ÷ time. With displacement in metres and time in seconds, the SI unit works out to m/s.

3. Q3 of 6Medium

   A bicycle's velocity increases from 5 m/s to 15 m/s in 4 s. The acceleration is:

   A2.5 m/s²B5 m/s²C20 m/s²D0.4 m/s²
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   Answer: 2.5 m/s²

   Explanation: a = (v − u)/t = (15 − 5)/4 = 10/4 = 2.5 m/s².

4. Q4 of 6Medium

   Assertion (A): A velocity-time graph that is a straight line parallel to the time axis shows zero acceleration. Reason (R): The slope of a velocity-time graph gives the acceleration.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: Both statements are true. A line parallel to the time axis has zero slope, and since the slope gives acceleration, the acceleration is zero; R explains A.

5. Q5 of 6Hard

   A body starts from rest with a = 4 m/s². What distance does it cover in the 5th second alone?

   A18 mB50 mC40 mD16 m
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   Answer: 18 m

   Explanation: Distance in the nth second = u + a(2n − 1)/2 = 0 + 4(2×5 − 1)/2 = 4(9)/2 = 18 m. This differs from total distance up to 5 s, which is ½(4)(5²) = 50 m.

6. Q6 of 6Hard

   Assertion (A): A child on a merry-go-round of radius 7 m completes one revolution in 22 s, so the child's average speed is 2 m s⁻¹ (take π = 22/7). Reason (R): The child's average velocity over one complete revolution is equal to the average speed.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is true but R is false

   Explanation: A is true: average speed = 2πR ÷ T = 44 m ÷ 22 s = 2 m s⁻¹. R is false: after one full revolution the displacement is zero, so the average velocity is zero.

Describing Motion Around Us notes: key concepts (Class 9 Science)

Motion is described by how position changes with time, and by the quantities distance, displacement, speed, velocity and acceleration. This chapter distinguishes scalars from vectors, derives the three equations of uniformly accelerated motion, and reads motion from graphs.

Rest and motion are relative

An object is at rest or in motion only with respect to a chosen reference point; a passenger is at rest relative to the bus but moving relative to the road.

Distance and displacement

Distance is the total path length travelled, a scalar; displacement is the shortest straight-line change in position with direction, a vector.

Scalar and vector quantities

Scalars have magnitude only, such as distance and speed; vectors have both magnitude and direction, such as displacement and velocity.

Speed

The distance travelled per unit time, a scalar quantity measured in metres per second.

Velocity

The displacement per unit time — speed with a direction, so it changes if the direction changes even at constant speed.

Average speed and average velocity

Average speed is total distance over total time; average velocity is total displacement over total time, and the two differ whenever the path is not straight.

Uniform and non-uniform motion

Uniform motion covers equal distances in equal intervals of time; non-uniform motion does not.

Acceleration

The rate of change of velocity with time; it is negative (retardation) when the object slows down.

Equations of uniformly accelerated motion

Three relationships linking initial velocity, final velocity, acceleration, time and displacement, valid only when acceleration is constant.

Distance-time graph

The slope gives the speed; a straight line means uniform motion and a horizontal line means the object is at rest.

Velocity-time graph

The slope gives acceleration and the AREA under the graph gives displacement — the two most useful readings in the chapter.

Uniform circular motion

Motion along a circular path at constant speed; it is accelerated motion because the direction, and therefore the velocity, keeps changing.