Physics Vectors & Equilibrium PMDC Conceptual Practice
PMDC Verified Question 3 of 50
Two forces of magnitude 20 N and 50 N act simultaneously on a body. Which of the following forces cannot be a resultant of these two forces?
A
40 N
B
30 N
C
20 N
D
70 N
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: 20 N
Concept:
The range of possible values for the resultant of two vectors.

Formula:
$$|F_1 - F_2| \le R \le F_1 + F_2$$

Solution:
Given \(F_1 = 50\text{ N}\) and \(F_2 = 20\text{ N}\):
  • The minimum possible resultant magnitude occurs when the two forces act in opposite directions (\(\theta = 180^\circ\)):
    $$\text{Min Resultant} = 50\text{ N} - 20\text{ N} = 30\text{ N}$$
  • The maximum possible resultant magnitude occurs when they act in the same direction (\(\theta = 0^\circ\)):
    $$\text{Max Resultant} = 50\text{ N} + 20\text{ N} = 70\text{ N}$$
The magnitude of the resultant force must fall within the range \([30\text{ N}, 70\text{ N}]\). Since 20 N is less than 30 N, it cannot be a possible resultant force.

Why other options are incorrect:
  • 30 N is the minimum limit (opposite direction).
  • 40 N is within the valid range.
  • 70 N is the maximum limit (same direction).

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