Physic Homework...College Intro?

Two blocks, each of mass m = 2.5 kg, are pushed along the horizontal surface of a table by a horizontal force P of magnitude 6.9 N, directed to the right, as shown in the figure below. The blocks move together to the right at constant velocity.

(a) Find the frictional force exerted on the lower block by the table.

(b) Find the coefficient of kinetic friction between the surface of the block and the table.

(c) Find the frictional force acting on the upper block.


(a) Since the blocks are being moved at constant speed, then the forces are "balanced" making the friction force equal to the force P, so P = f = 6.9N.

(b) Kinetic friction is given as:

f = μn

Solved for μ:

μ = f / n

= f / 2mg------------------>treating the two blocks as one

= 6.9N / 2(2.5kg)(9.8m/s2)

= 0.14

(c) The frictional force acting on the upper block is static friction and must also equal the force P or it would slide to the left. The question doesn't ask for it, but the static friction coefficient must be:

μ ≤ f/mg

≤ 6.9N / (2.5kg)(9.8m/s2)

μ ≤ 0.28

Hope this helps.