Forces P and Q make 120° with one another. Their resultant is perpendicular to P and P = 4 N. Find Q (N).
Physics · Mixed Chapter
JEE Main Practice

Question

Forces P and Q make 120° with one another. Their resultant is perpendicular to P and P = 4 N. Find Q (N).

G1 Physics · Accepted Answer

Step 1: Resultant vector R⃗ = P⃗ + Q⃗. Since R⃗ ⟂ P⃗, (P⃗ + Q⃗)·P⃗ = 0. Step 2: Hence P² + PQ cos120° = 0. Step 3: cos120° = −½, so P² − PQ/2 = 0 ⇒ Q = 2P = 8 N.