Physics Work & Energy ETEA 2017
PMDC Verified Question 95 of 108
When a force retards the motion of a body the work done is:
A
Zero
B
Negative
C
Positive
D
Positive or negative depending upon the magnitude of force and displacement
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Negative
Concept:

A retarding force is one that opposes the direction of motion (e.g., friction or braking force). In this case, the angle between force and displacement is 180°.

Formula:

$$ W = Fd \cos \theta $$

Solution:

  • Angle \( \theta = 180^{\circ} \)


  • \( \cos(180^{\circ}) = -1 \)


  • \( W = Fd(-1) = -Fd \)


  • The work done is fundamentally negative because energy is being removed from the moving body.


Why other options are incorrect:

Positive work implies the force aids motion (speeding the object up). Zero work implies force acts perpendicularly. Option D is incorrect because retardation strictly implies opposition.

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