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please show the steps on working the problem so i can do it on my own for the te

ID: 2007659 • Letter: P

Question

please show the steps on working the problem so i can do it on my own for the test. thank you!
A 3.900kg block of wood rests on a steel desk. The coefficient of static friction between the block and the desk is µs=0.455 and the coefficient of kinetic friction is µk=0.205. At time t=0, a force F=10.7 N is applied horizontally to the block. State the force of friction applied to the block by the table at the following times:









t=0 _______N




t > 0 _______N




Consider the same situation, but this time the external force F is 21.6N. Agian state the force of friction acting on the block at the following times:




t=0 __________N




t > 0 __________N

Explanation / Answer

the maximum static friction f max = sMg = (0.455)(3.9)(9.8) = 17.39 N

since applied force F is smaller than f max, so the block does not move,

f - F = 0

f = F

t = 0, 10.7 N

t > 0, 10.7 N

b)

applied force F > fmax, so block will start to move

t = 0, f = fmax = 17.39 N

t > 0, f = kMg = (0.205)(3.9)(9.8) = 7.84 N