Chapter 2: Motion in a Straight Line Short Questions | physics Class 11 CBSE

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Chapter 2: Motion in a Straight Line Short Questions | physics Class 11 CBSE | ByteNotes.in

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Question 1

Quick Recall

Define instantaneous velocity and state how it is determined from a position-time graph.

Instantaneous velocity is the limit of average velocity as the time interval Δt approaches zero: v = lim(Δt→0) Δx/Δt = dx/dt.
It represents the rate of change of position with respect to time at a specific instant.
On a position-time graph, instantaneous velocity at any point equals the slope of the tangent drawn to the curve at that point.

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Question 2

Quick Recall

Distinguish between average speed and instantaneous speed.

Average speed is the total path length divided by the total time interval and is always greater than or equal to the magnitude of average velocity.
Instantaneous speed is the magnitude of instantaneous velocity at a particular instant of time.
Unlike average speed, instantaneous speed is always exactly equal to the magnitude of instantaneous velocity at that instant.

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Question 3

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What is the significance of the area under a velocity-time graph?

The area under the velocity-time curve between two time instants t1 and t2 equals the displacement of the object during that time interval.
For an object moving with constant velocity u, the area under the v-t graph is a rectangle of height u and base T, giving displacement = uT.
This result holds generally for any motion, including uniformly accelerated motion, where the area under the v-t curve (a trapezium) gives total displacement.

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Question 4

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Under what condition is the point object approximation valid?

The point object approximation is valid when the size of the object is much smaller than the distance it moves in a reasonable duration of time.
For example, a railway carriage moving between two distant stations can be treated as a point object because its length is negligible compared to the distance travelled.
This approximation simplifies calculations by ignoring rotational and internal motion of the object, treating it as having all mass concentrated at a single point.

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Question 5

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State the three kinematic equations of motion for uniform acceleration.

The first equation relates final velocity to initial velocity and acceleration: v = v0 + at.
The second equation gives displacement as a function of time: x = v0t + ½at².
The third equation relates final velocity to initial velocity and displacement: v² = v0² + 2ax.

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Medium

Question 6

Quick Recall

Can a particle have zero velocity and non-zero acceleration simultaneously? Justify with an example.

Yes, a particle can have zero velocity and non-zero acceleration at the same instant.
A particle thrown vertically upward has zero velocity at its highest point, but its acceleration continues to be g = 9.8 m s⁻² directed downward at that instant.
This shows that zero velocity does not imply zero acceleration; the particle is momentarily at rest but is still being acted upon by gravity.

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Question 7

Quick Recall

How does the sign of acceleration indicate the nature of motion of a particle?

The sign of acceleration depends on the chosen positive direction of the axis, not directly on whether speed is increasing or decreasing.
If acceleration is in the same direction as velocity, the particle's speed increases (e.g., object falling downward with downward taken as positive).
If acceleration is opposite to the direction of velocity, the particle's speed decreases; for example, a ball thrown upward decelerates even though gravity's acceleration is constant.

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Question 8

Quick Recall

Define average acceleration and state its SI unit.

Average acceleration over a time interval is defined as the change in velocity divided by the time interval: ā = (v2 - v1)/(t2 - t1) = Δv/Δt.
It represents the average change in velocity per unit time over the given interval.
The SI unit of acceleration is metre per second squared (m s⁻²), and on a v-t graph it equals the slope of the line joining the two corresponding points.

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Medium

Question 9

Quick Recall

What does the shape of a position-time graph indicate about the acceleration of an object?

A straight line on the x-t graph indicates zero acceleration (uniform motion), since the slope (velocity) remains constant.
A curve bending upward (concave up) on the x-t graph indicates positive acceleration, as the slope is increasing with time.
A curve bending downward (concave down) on the x-t graph indicates negative acceleration, as the slope is decreasing with time.

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Medium

Question 10

Quick Recall

Write the expression for stopping distance of a vehicle and explain what it depends on.

The stopping distance is given by ds = v0²/(2a), where v0 is the initial velocity and a is the deceleration (magnitude) due to braking.
Stopping distance is proportional to the square of the initial velocity; doubling the initial speed increases stopping distance by a factor of four.
It also depends on braking capacity (deceleration): a larger deceleration results in a shorter stopping distance, making it an important factor for road safety.

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Question 11

Quick Recall

What is free fall? State the value of acceleration due to gravity used for free fall calculations.

Free fall is the motion of an object released near the Earth's surface under the influence of gravity alone, with air resistance neglected.
The magnitude of acceleration due to gravity is g = 9.8 m s⁻², directed vertically downward; it is taken as constant when the height of fall is small compared to Earth's radius.
Free fall is a special case of uniformly accelerated motion, where initial velocity is zero and acceleration equals g throughout the motion.

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Medium

Question 12

Quick Recall

Define relative velocity of object A with respect to object B moving along the same straight line.

The relative velocity of object A with respect to object B is defined as vAB = vA - vB, where vA and vB are their respective velocities.
When both objects move in the same direction, the magnitude of relative velocity is |vA - vB|, which is less than either individual speed.
When objects move in opposite directions, the magnitude of relative velocity is |vA + vB|, which exceeds either individual speed.

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Question 13

Quick Recall

Explain why acceleration and velocity cannot change abruptly at any instant in realistic motion.

In realistic physical situations, the functions describing position, velocity, and acceleration are differentiable at all points, meaning they vary continuously.
Sharp kinks seen in idealized x-t, v-t, and a-t graphs imply non-differentiability, which is physically impossible in real motion.
Therefore, acceleration and velocity always change continuously — abrupt (instantaneous) changes in these quantities do not occur in practice.

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Chapter 2: Motion in a Straight Line | Short Questions

Define instantaneous velocity and state how it is determined from a position-time graph.

Distinguish between average speed and instantaneous speed.

What is the significance of the area under a velocity-time graph?

Under what condition is the point object approximation valid?

State the three kinematic equations of motion for uniform acceleration.

Can a particle have zero velocity and non-zero acceleration simultaneously? Justify with an example.

How does the sign of acceleration indicate the nature of motion of a particle?

Define average acceleration and state its SI unit.

What does the shape of a position-time graph indicate about the acceleration of an object?

Write the expression for stopping distance of a vehicle and explain what it depends on.

What is free fall? State the value of acceleration due to gravity used for free fall calculations.

Define relative velocity of object A with respect to object B moving along the same straight line.

Explain why acceleration and velocity cannot change abruptly at any instant in realistic motion.

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