Physics Force & Motion BUMHS 2022
PMDC Verified Question 78 of 137
A stone is dropped from a building's top 80 m high from the ground. In how much time will it touch the ground?
A
2 s
B
4 s
C
8 s
D
10 s
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: 4 s
Concept:

An object dropped from rest strictly falls under the constant acceleration of gravity. We use the second kinematic equation.

Formula:

$$ S = v_it + \frac{1}{2}gt^2 $$

Solution:

  • The initial velocity is \( v_i = 0 \).


  • The displacement is \( S = 80 \text{ m} \).


  • Taking gravity as \( g = 10 \text{ m/s}^2 \) (standard simplification for this problem type).


  • Substitute values: $$ 80 = (0)t + \frac{1}{2}(10)t^2 $$


  • $$ 80 = 5t^2 \implies t^2 = \frac{80}{5} = 16 $$


  • $$ t = \sqrt{16} = 4 \text{ seconds} $$


Why other options are incorrect:

Forgetting the square root gives 16s. Forgetting the \( 1/2 \) in the formula gives \( \sqrt{8} \approx 2.8 \text{s} \). Calculating linearly without squared time gives 8s.

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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