Friction on an inclined plane/pulley

In summary, the problem involves two masses (mA = 2.5kg and mB = 4.0kg) connected by a string on inclines with a coefficient of kinetic friction of 0.30. The acceleration of the system is unknown when mA moves up and mB moves down. The equations used to solve the problem are the net force equations in the y and x directions and the equation for friction. The solution involves finding the forces for each block and accounting for the tension in the string. For the second problem, involving a 3.0kg block on top of a 5.0kg block with a coefficient of static friction of 0.59 and a coefficient of kinetic friction of 0.39
  • #1
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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.
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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.
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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.
 
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  • #2
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
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
the system could be moving to the right and decelerating.
 
  • #6
if you resolved this question, can u please post the answer. i want to see if i solved it correctly. thanks.
 

1. What is friction and how does it affect an object on an inclined plane?

Friction is a force that opposes motion between two surfaces in contact. When an object is on an inclined plane, friction acts in the opposite direction of the object's motion, making it more difficult to move the object up the plane.

2. How does the angle of the inclined plane affect friction?

The steeper the angle of the inclined plane, the greater the friction force will be. This is because as the angle increases, the component of the object's weight that is parallel to the plane also increases, resulting in a larger friction force.

3. What role does the weight of the object play in friction on an inclined plane?

The weight of the object affects the normal force between the object and the inclined plane. The greater the weight of the object, the greater the normal force, which in turn increases the friction force. This is why it is more difficult to move heavier objects up an inclined plane.

4. How does the presence of a pulley affect friction on an inclined plane?

A pulley can reduce the amount of friction on an inclined plane by changing the direction of the force required to move the object. By redirecting the force, the pulley can make it easier to move the object up the plane, reducing the effect of friction.

5. What are some ways to reduce friction on an inclined plane?

There are a few ways to reduce friction on an inclined plane. One way is to use a lubricant, such as oil or grease, which can create a slippery surface and reduce the friction force. Another way is to use a smoother surface for the inclined plane, such as a sheet of plastic or metal, which will have less resistance and therefore less friction. Finally, using a pulley system can also help reduce friction by changing the direction of the force needed to move the object.

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