Can someone me out with free body diagrams

[mystmyst]

**a horizontal force with magnitude A is acting on a pulley as shown. the two masses p and P are moving together, same acceleration. p is trying to fall to the left but the tension from the rope and the static friction is keeping it in place.

Everything else you need to know is in my two images:

[PLAIN]http://img153.imageshack.us/img153/2193/mymechanicsquestion.png

Free body diagram:

[PLAIN]http://img14.imageshack.us/img14/7249/isolatedstatic.png Thanks!

 

[tiny-tim]

hi mystmyst!

(first, i don't understand what the pulley is attached to … and if the blocks aren't relatively moving, how can there be any tension in the rope? )

(1) you can always do either separate diagrams (and equations) or a single one; if there's no relative motion, there's no point in separate diagrams

your N_P down is wrong, an fbd should only have the forces on the body, so only the upward force in this case (and anyway N_p = N_P … Newton's third law)

btw, use m and M not p and P … p and P are too similar, especially in handwriting

(2) yes, if the pulley is frictionless, the tensions are the same, and if there's no acceleration then A = 2T

(3-5) i don't really understand your questions

anyway, a friction force goes on the diagram in exactly the same way whether it's kinetic or static

 

[mystmyst]

A horizontal force with Magnitude A is acting on the pulley.
Since there is static friction between p and P, the bodies are moving together.
p is trying to fall off to the left but the tension from the rope and the friction is keeping it in place.


1) what is relative motion?

Why is my Np down wrong? Np is acting upward on the large mass which means it has the equal reaction downward for the small mass. Newton's 3rd law?

2) So, even though the bodies are moving, does A=2T? or does it must mean that A > 2T since if A=2T then the bodies wouldn't be moving?

3) u answered

4+5) I am asking if those vectors are correct. It's another Newtons 3rd law of motion. since there friction to the right on the small mass, there's frictoin to the left on the large mass. and 5 is, since there's frictoin on the large mass to the right, there's frictoin on the ground to the left. I'm asking if that's correct.

 

[tiny-tim]

A horizontal force with Magnitude A is acting on the pulley.

yes, but what is the axle of the pulley attached to?

or is it completely free, except that A is pulling it?

1) what is relative motion?

i meant, if one block is moving relative to the other block

Why is my Np down wrong? Np is acting upward on the large mass which means it has the equal reaction downward for the small mass. Newton's 3rd law?

you can't have both force on the same diagram: one is on the body, the other is from the body: you can only have forces on the body

2) So, even though the bodies are moving, does A=2T? or does it must mean that A > 2T since if A=2T then the bodies wouldn't be moving?

moving never matters in a force diagram, only accelerating

if there's no acceleration, the forces will sum to zero (A = 2T)

4+5) I am asking if those vectors are correct. It's another Newtons 3rd law of motion. since there friction to the right on the small mass, there's frictoin to the left on the large mass. and 5 is, since there's frictoin on the large mass to the right, there's frictoin on the ground to the left. I'm asking if that's correct.

yes, the friction in the diagram for one body will always be equal and opposite to the friction v in the diagram for the other body

 

[mystmyst]

O. I think I get it. The N_P down is wrong for the small mass but I should have a N_p down for the large mass.

 

[tiny-tim]

Yup!

 

[mystmyst]

yay tiny-tim! you rockk!

PF should really get a thank you button. and also a like button...