Physics Force & Motion SZABMU 2022
PMDC Verified Question 63 of 137
Two cars travelling on straight road in opposite direction with speed \( 70 \text{ kmh}^{-1} \) and \( 60 \text{ kmh}^{-1} \), their relative velocity will be:
A
\( 10 \text{ kmh}^{-1} \)
B
\( 130 \text{ kmh}^{-1} \)
C
\( 65 \text{ kmh}^{-1} \)
D
\( 5 \text{ kmh}^{-1} \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: \( 130 \text{ kmh}^{-1} \)
Concept:

Relative velocity measures how fast one object is moving from the perspective of another. When objects move in opposite directions, their approach (or separation) rate is the sum of their individual speeds.

Formula:

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

Solution:

  • Let the first car's velocity be \( \vec{v}_1 = +70 \text{ km/h} \).


  • Since the second car moves in the opposite direction, \( \vec{v}_2 = -60 \text{ km/h} \).


  • The relative velocity magnitude is \( |\vec{v}_{rel}| = |70 - (-60)| = 130 \text{ km/h} \).


Why other options are incorrect:

Subtracting the magnitudes (10 km/h) applies only if they are travelling in the same direction. Averaging them (65 km/h) has no physical meaning here.

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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