Physics Force & Motion NUMS 2022
PMDC Verified Question 90 of 137
Two busses moving at 100 km/h and 80 km/h respectively cross each other while moving in opposite direction. Velocity of one bus relative to other bus is:
A
100 km/h
B
20 km/h
C
80 km/h
D
180 km/h
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: 180 km/h
Concept:

When two objects move in directly opposing directions, the speed at which the gap between them closes is the arithmetic sum of their respective speeds.

Formula:

$$ \vec{v}_{rel} = \vec{v}_1 - \vec{v}_2 $$

Solution:

  • Assign a directional axis. Let Bus 1 move forward: \( \vec{v}_1 = +100 \text{ km/h} \).


  • Bus 2 moves opposite: \( \vec{v}_2 = -80 \text{ km/h} \).


  • The velocity of Bus 1 relative to Bus 2: $$ \vec{v}_{12} = 100 - (-80) = 180 \text{ km/h} $$


  • To an observer on either bus, the other bus appears to be flying past at 180 km/h.


Why other options are incorrect:

20 km/h would be their relative velocity if they were racing in the SAME direction.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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