Physics newtons laws

Question

A 1.8 kg block is pulled across a level surface by a horizontal force of 4 N. Determine the approximate coefficient of friction between the block and the surface if the block moves at constant velocity.

A 0.25

B 0.50

C 0.75

D 1.30

Answer 1

Because the block moves with constant speed, the external force is equal to the frictional force:

f = F.

From another hand, this force

f = μN,

where μ - coefficient of friction and N - normal reaction force.

The external force acts in horizontal direction, hence N = mg.

Now we can write

F = μmg.

From here coefficient of friction

μ = F / mg

Substituting the numbers we get

μ = 0.22

I took g = 10, this is why my answer is not exact. I recommend to do so in all multiple choice questions. Now you look for the answer the closest to yours. And you see that the right answer is (a)!