How firiction works here

shalikadm

I am trying to figure out what happens here...and now I've stuck on this...
Here it goes....,

situation 1

Both contact A with B and A with ground are rough
We increase the force P applied here from zero.When there's sufficiently big μ_s between A and B than that of A and ground,there's an ability there to move both A and B together at the same speed.What I see here is that there's a friction there even they are moved by the force at a regular velocity.When A travels at a regular speed there must be a force applied on this to cancel out the friction on A by the ground.Actually its the friction applied by B on A that cancels out the friction by ground on A.So there must be a friction between A and B when even traveling at a regular velocity.Am I correct ?

situation 2

Contact of A with B are rough.BUT A with ground is smooth.
A and B is put in a smooth slope as shown in the image.When we take both A and B together as a one system,there's no friction (friction s between A and B cancels when taken together as one system). So the only force is the resolution of weight parallel to the slope.So the acceleration on this system is gsinθ..Here's where I can't proceed on.We have told that there isn't a friction between A and B when they are going down the slope.The reason that was told for this is that the resolution of weight of B parallel to the slope is used for to keep the gsinθ acceleration parallel to the slope.So there's no any effort of B to slide on A-so there's no friction between them.But to explain this to my self I want to know that B 's acceleration is gsinθ.Is there someone who can explain me how B get an acceleration of gsinθ.I'm afraid to think that the acceleration of B is same as the whole system..I see acceleration is a vector...so a_A+a_B=a_system like thing is haunting my mind...We see such things in vectors like momentum....momentums of objects of a system is added to the momentum of system (algebraically as a vector)..So in bomb explosion the momentum of the system is zero...like that thing...please explain this to me...

Please try to give me a short and clear explanation(a best one)..
I'm in a hurry
Thanks !

CWatters

Yes. It's the static friction between B and A. They aren't moving with respect to each other.

Situation 2 is different to situation 1. In situation 1 the force P is only acting on block B. In situation 2 the "force" (acceleration) of gravity is acting equally on both blocks. Their acceleration and velocity is the same. There is no force causing one block to move (or try to move) relative to the other. So no frictional forces between them.

The acceleration is the same for A and B for the same reason as this..

http://www.youtube.com/watch?v=5C5_dOEyAfk

If you tied the feather to the hammer they would still fall at the same rate.

shalikadm

Thanks a lot Cwatters !