Question

The figure is just mass A on top of mass B. There are no forces or anything shown.

Block A, of mass 3.2 kg, is on block B, of mass 7 kg, as shown in the above figure. The lower block is on a frictionless surface while the coefficient of static friction is 0.25 between the two blocks.

a) If they are moving at constant velocity, what is the frictional force between A and B?

b) What is the maximum horizontal force that can be applied to B without A slipping?

I'm not so clear on the topic of friction, so I don't understand how to find it.

Answer 1

frictional force is the normal force multiplied by the coefficient

in this case ... Ff = Ma * g * 0.25

the frictional force keeps A moving with B
... so the max acceleration (without slip) is ... Ma / Ff


the max force is the max acceleration multiplied by the sum of the masses
... f = m a

Answer 2

I figured out a) is 0 N because the velocity is constant