physics questions?

Question

A force of 30.0 N is required to start a 3.6 kg box moving across a horizontal concrete floor.
a) What is the coefficient of static friction between the box and the floor?

b)If the 30.0 N force continues, the box accelerates at 0.50 m/s2. What is the coefficient of kinetic friction?

Answer 1

The frictional force = m * coefficient of static friction.
30N = Normal Force * coefficient
Normal Force = mg = 3.6 * 9.8
coefficient = 30N / (3.6 * 9.8) = 0.8503 (solution!)

b.) F = ma - Ff
Ff = Normal Force * coefficient
30N = 3.6kg * .5 m/s^2 - 3.6 * 9.8 * coefficient
30N = 1.8N - 35.28 * coeffiecnt
28.2N = - 35.28 * coeffiecnt
coefficient = 0.7993 (solution!)