Q 11-03-045JEE MainJEE Main 2026 (4 Apr, Shift 2)Medium
Two cars $A$ and $B$ are moving in the same direction along a straight line with speeds $100\ \text{km/h}$ and $80\ \text{km/h}$, 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 $5\ \text{m/s}$. The value of $v$ is ______ km/h.
Answer: (C) $38$
Concept. The stone's velocity relative to car A decides how hard it hits A. The stone leaves car B with B's velocity plus $v$ (relative to B), all along the road.
Step 1. Stone's ground speed. $80+v$ km/h.
Step 2. Relative to A. $(80+v)-100=v-20$ km/h, which must be $5$ m/s $=18$ km/h.
Step 3. Solve. $v=38$ km/h.
Answer. $38$ (Option C).
Solution by Sreeraj P, M.Sc Physics
