Understanding Friction: Solving for Forces on Boxes A and B

[IvaNMK]

Homework Statement

[PLAIN]http://img510.imageshack.us/img510/3893/58096739.jpg

Homework Equations

F = ma, friction...

The Attempt at a Solution

I just want to understand the problem so I can do it.

1a. If I apply 18N to box A, does it actually move? or it just reaches the max friction and still stay put?

1b. how many forces should be on box B? I see 2 forces, the kinetic friction given by A on B and the static (or kinetic?) friction between box B and the ground

 

[tiny-tim]

Hi Iva!

1a. If I apply 18N to box A, does it actually move? or it just reaches the max friction and still stay put?

The 18 N applied force in the question is stated to be the minimum force needed to get box A to move (ie, "to overcome the maximum static friction").

There's two answers to your question! …
i] no, it's only "on the point of moving" or "about to move", to quote various exam questions on static friction
ii] yes, it moves imperceptibly slowly at first, but then the coefficient of kinetic friction takes over, so it accelerates.

1b. how many forces should be on box B? I see 2 forces, the kinetic friction given by A on B and the static (or kinetic?) friction between box B and the ground

Yes, only 2 (horizontal) forces on B …

their total is the F in F = ma for B.