Physics Fluid Dynamics PMDC Conceptual Practice
PMDC Verified Question 36 of 37
When a heavy coin is dropped from a very short distance above the ground, it strictly does not achieve terminal velocity before hitting the ground. What is the fundamental physical reason for this?
A
The weight of the coin heavily decreases due to local gravity changes.
B
The surrounding air resistance becomes completely negligible near the ground.
C
The mass of the coin is too small to overcome air resistance.
D
The coin hits the ground before air resistance has time to grow equal to its weight.
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: The coin hits the ground before air resistance has time to grow equal to its weight.
Concept:

Terminal velocity is exclusively reached when the upward drag force (which increases with speed) grows large enough to exactly cancel out the downward force of gravity (weight).

Formula:

$$ F_{\text{net}} = W - F_{\text{drag}} $$

Solution:

  • A heavy coin has a significant downward weight.
  • As it falls, it accelerates, and the opposing drag force begins to increase from zero.
  • Because the drop distance is very short, the coin simply hits the ground before it can accelerate to a speed fast enough for the drag force to match its heavy weight.
  • Thus, the net force remains positive downwards the entire brief trip.


Why other options are incorrect:

Weight (\( mg \)) is constant over short drops, ruling out A. Air resistance exists but hasn't scaled up yet, ruling out B. The coin's mass is large, making it harder (not easier) for drag to catch up, ruling out C.

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