A block is moving along the y-axis with velocity 4 m s⁻¹ on a plank relative to the ground, while the plank is moving along the x-axis with velocity 3 m s⁻¹. The unit vector in the direction of friction force acting on the block at this instant is:

Question

G1 Physics · Accepted Answer

Velocity of the block relative to the plank is 𝐯ᵣₑₗ = 4𝐣 - 3𝐢 = -3𝐢 + 4𝐣. Friction on the block opposes this relative sliding, so its direction is 3𝐢 - 4𝐣. Dividing by magnitude 5 gives (3𝐢 - 4𝐣)/5.