Physics Vectors & Equilibrium PMDC Conceptual Practice
PMDC Verified Question 4 of 50
The sum of the magnitudes of two forces acting at a point is 16 N. If the resultant force is 8 N and its direction is perpendicular to the smaller force, then the magnitudes of the individual forces are:
A
6 N and 10 N
B
8 N and 8 N
C
4 N and 12 N
D
2 N and 14 N
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: 6 N and 10 N
Concept:
Vector addition using right-triangle properties when the resultant is perpendicular to one of the component forces.

Formula:
$$F_1 + F_2 = 16\text{ N}$$
$$F_1^2 + R^2 = F_2^2$$

Solution:
Let the smaller force be \(F_1\) and the larger force be \(F_2\). Thus, \(F_2 = 16 - F_1\).
Since the resultant \(R = 8\text{ N}\) is perpendicular to \(F_1\), these vectors form a right-angled triangle where \(F_2\) is the hypotenuse:
$$F_1^2 + 8^2 = (16 - F_1)^2$$
$$F_1^2 + 64 = 256 - 32F_1 + F_1^2$$
$$32F_1 = 256 - 64$$
$$32F_1 = 192 \implies F_1 = 6\text{ N}$$
Calculating the larger force:
$$F_2 = 16 - 6 = 10\text{ N}$$

Why other options are incorrect:
  • 8 N and 8 N is incorrect because if both are 8 N, their sum is 16 N but they cannot form a right triangle with a perpendicular resultant of 8 N.
  • 4 N and 12 N, and 2 N and 14 N are incorrect because these pairs do not satisfy the Pythagorean relation with a third side of 8 N. For example, \(4^2 + 8^2 = 80 \neq 12^2\).

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