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Friction on an inclined plane/pulley

  • #1

Homework Statement


Two masses (mA = 2.5kg) and (mB = 4.0kg) are on inclines and are connected together by a string as shown in the figure . The coefficient of kinetic friction between each mass and its incline is = 0.30.

Picture: http://img209.imageshack.us/img209/1416/incline1.jpg [Broken]

Q:If mA moves up and mB moves down, determine the acceleration.
---

Homework Equations



Net Fy= Fn-mg(cos theta)= may
Net Fx= Fn-mg(sin theta)-uFn= max
Ffr= uFn

The Attempt at a Solution


1. I just can't figure this out...here is what I have so far:
F(mA)= 2.5(g)*sin(51)-2.5(g)cos(51)(.3)= 19-4.6= 14.4N
The downward force of mA is 14.4N

For block B:
F(mB)= 4(g)sin(21)- 4(g)cos(21)(.3)= 14-11= 3N
Block B is falling downward at a force of 4N

So how in the world can the system be moving to the right? Even if it was, there would still be a tension of 14N, divide that be the weight (6.5) and you get 2.1 m/s^2...which is not the right answer.
---

Any help is appreciated guys...I'm lost here. And also one more if your filling altruistic

Homework Statement



A 3.0kg block sits on top of a 5.0kg block which is on a horizontal surface. The 5.0-kg block is pulled to the right with a force (F) as shown in the figure . The coefficient of static friction between all surfaces is 0.59 and the kinetic coefficient is 0.39.

Picture: http://img191.imageshack.us/img191/753/giancolich05p031.jpg [Broken]

Q: a) What is the minimum value of F needed to move the two blocks?
b)If the force is 10% greater than your answer for (a), what is the acceleration of each block?


Homework Equations



Same equations.

The Attempt at a Solution


Fn= 5kg+3kg(9.8)= 78.4
78.4*(.59)=46.3
...so the block should require 46.3 N of force to move. Add that to the tension of the string from the 3kg block on top:
3kg*9.8=17.3
...the top block should require 17.3N to move +46.3= 63.6N...which is also wrong.

Homework Statement





Homework Equations





The Attempt at a Solution

 
Last edited by a moderator:

Answers and Replies

  • #2
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,147
492
For question 1, you appear to have omitted the tension force in your equations. Either both blocks move to the right, with the same magnitude of acceleration, or both move to the left, with the same magnitude of acceleration. You really don't know until you plug in the numbers, to solve two equations with 2 unknowns.
 
  • #3
But wouldn't they both be pulling in opposite directions, making the block on the left lead the acceleration?

I'm not sure how you can get the entire system to accelerate to the right since there is more force on the left side.
 
  • #4
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,147
492
Can you get the system to accelerate to the left? If so, what is the acceleration? (I don't believe the problem is worded correctly).
 
  • #5
17
0
the system could be moving to the right and decelerating.
 
  • #6
17
0
if you resolved this question, can u plz post the answer. i want to see if i solved it correctly. thanks.
 

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