A heavy box of mass 50 kg is moving on a horizontal surface. If co-efficient of kinetic friction between the box and horizontal surface is 0.3 then force of kinetic friction is :

Solution

Physics sir · 3 steps

On a flat floor the normal force is just mg, so kinetic friction is μₖ mg. Multiply and you are done.

1. Normal force. The surface is horizontal and nothing else pushes up or down, so the normal force balances the weight: N = mg = 50 × 9.8 = 490 N.

   fₖ = μₖ N = μₖ mg

2. Put in numbers. fₖ = 0.3 × 490 = 147 N.
3. So. The force of kinetic friction is 147 N, option (b).

The idea to keep. Kinetic friction is always μₖ times the normal force, not times the mass. On a level surface with no extra vertical pushes, N = mg.

Answer from the NTA final answer key. Solution by PrepWiser.