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Kinematics

JEE Main level, single correct, medium

140 of 308

Question 140 of 308

Two cars $A$ and $B$ are moving in the same direction along a straight line with speeds $100$ km/h and $80$ km/h, respectively such that car $A$ is moving ahead of car $B$ . A person in car $B$ throws a stone with a speed $v$ so that it hits the car $A$ with a speed of $5$ m/s. The value of $v$ is ______ km/h.

Show answer and solution

Answer: (C) $38$

Velocity of car A,

v_A = 100

km/h

Velocity of car B,

v_B = 80

km/h

Velocity of the stone relative to car B is

v

. The velocity of the stone relative to the ground is

v_S = v_B + v = 80 + v

.

The velocity of the stone relative to car A is

v_{S/A} = v_S - v_A = (80 + v) - 100 = v - 20

.

The stone hits car A with a relative speed of

5

m/s. Converting this speed to km/h:

v_{S/A} = 5 \times \dfrac{18}{5} = 18

km/h

Equating the relative speeds:

v - 20 = 18 \Rightarrow v = 38

km/h

Answer:

38

JEE Main question paper PDF

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JEE Main 2025 Apr 2 Shift 1 question paper

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JEE Main Physics practice question on Kinematics (Relative Motion), with the answer and a step-by-step solution. Scan a question from your own book or paper to get a whiteboard video.