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NDA 2026
Exam
NDA
Year
2026
Subject
General Ability
Chapter
General Science

NDA 2026 · General Ability · General Science

Two forces of equal magnitude simultaneously act at a point. It is...

Two forces of equal magnitude simultaneously act at a point. It is observed that the resultant force is equal in magnitude to the individual forces. Which of the following statements is/are correct?
I. The angle between one of the individual forces and the resultant force is π/3\pi/3.
II. The angle between the individual forces is 2π/32\pi/3.

Options

  1. AI only
  2. BII only
  3. CBoth I and II
  4. DNeither I nor II

Correct answer

C. Both I and II

Step-by-step solution

Let the two forces be F1⃗\vec{F_1} and F2⃗\vec{F_2}, and their resultant be R⃗\vec{R}.

Given that ∣F1⃗∣=∣F2⃗∣=∣R⃗∣=F|\vec{F_1}| = |\vec{F_2}| = |\vec{R}| = F.

The magnitude of the resultant of two vectors is given by R2=F12+F22+2F1F2cos⁡θR^2 = F_1^2 + F_2^2 + 2F_1 F_2 \cos \theta, where θ\theta is the angle between the two forces.

Substituting the given values:

F2=F2+F2+2F2cos⁡θF^2 = F^2 + F^2 + 2F^2 \cos \theta

F2=2F2(1+cos⁡θ)F^2 = 2F^2(1 + \cos \theta)

cos⁡θ=−12\cos \theta = -\dfrac{1}{2}

θ=2π3\theta = \dfrac{2\pi}{3}

Thus, the angle between the individual forces is 2π3\dfrac{2\pi}{3}. Statement II is correct.

Since the two forces are equal in magnitude, their resultant bisects the angle between them. Let α\alpha be the angle between the resultant and one of the forces.

α=θ2=12(2π3)=π3\alpha = \dfrac{\theta}{2} = \dfrac{1}{2} \left(\dfrac{2\pi}{3}\right) = \dfrac{\pi}{3}

Thus, the angle between one of the individual forces and the resultant force is π3\dfrac{\pi}{3}. Statement I is also correct.

Both statements I and II are correct.
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