Resultant force magnitude via parallelogram law

Steps:

• Use formula for resultant R = √(F₁² + F₂² + 2 F₁ F₂ cos θ), where θ is angle between forces.
• Substitute F₁ = 10 N, F₂ = 10 N, θ = 120° (cos 120° = -1/2).
• Compute inside: 10² + 10² + 2(10)(10)(-1/2) = 100 + 100 - 100 = 100.
• R = √100 = 10 N.

Why B is correct:

• Matches parallelogram law calculation, where obtuse angle reduces resultant to equal one force.

Why the others are wrong:

• A: Underestimates; ignores partial reinforcement from 120° angle.
• C: Matches 60° angle (acute, full reinforcement), not 120°.
• D: Assumes 0° angle (forces aligned), ignoring separation.

Final answer: B