Pulley Problem - Please give me hand here

In summary, the question involves two blocks, A (44 N) and B (22 N), connected by a rope over a pulley. Block A is on a table, and Block B is hanging off the edge. Block C is on top of Block A. The question asks for the minimum weight of Block C needed to keep Block A from sliding off the table, given a coefficient of static friction of 0.20 between Block A and the table. The solution involves finding the tension in the rope, which is equal to the weight of Block B (22 N), and setting it equal to the frictional force on Block A, which is equal to the weight of Block C. Solving for the weight of Block C gives a
  • #1
freecorp777
4
0
Here's a pulley problem that has got me stopped; I'd appreciate any help that's offered.

There are two blocks, A (44 N) and B (22 N) connected via a rope that stretches over a pulley. Block A is resting on a table, and block B is hanging over the edge. Block C is positioned on top of Block A.

The question reads: "Given that the coefficient of static friction between Block A and the tabel is 0.20, what is the minimum weight of Block C to keep Block A from sliding off the table?"(The answer that is given is 66 N, but I can't seem to get this)I tried starting off with:
Block A=M=44N Block B=m=22N
(let F stand for the friction force)
T-F=Ma and mg-T=ma
Solving for Tension and setting them equal gives:
mg-ma=F+Ma
Rearranging some more and solving for a gives:
a= (mg-F)/(m + M)
The Tension on Block A then becomes M(mg-F)/(m + M) = 8.8 N
So then in order for Block A to resist motion, the Friction between it and the table must be equal to T, so F=T=8.8 .20 * (4.489 + C) * 9.8 = 8.8 but then the mass of C ends up being zero. Where did I go wrong?
 
Last edited:
Physics news on Phys.org
  • #2
Several problems here. One, you are trying to prevent block A from sliding, so the acceleration is zero. Two, you are given the weights of the blocks, not their masses. (22N is a force, not a mass.)

Hint: Since the entire affair is in equilibrium, what must be the tension in the rope?
 
  • #3
Well, in order for Block A *not* to move, the the frictional force on Block A must be equal to the tension right?
I redid the work for the acceleration and tension.

T = (Mmg + mF) / (M + m) M=4.489 kg m=2.245 kg

The tension (not taking Block C into account yet) is 17.6 N.
Set this equal to u(M + C) g (the frictional force) and solve for C, and I get a mass of 8.97 kg, so a weight of 88 N, which is still off.
 
Last edited:
  • #4
freecorp777 said:
Well, in order for Block A *not* to move, the the frictional force on Block A must be equal to the tension right?
This is true.

Now answer my question: What's the tension in the rope? If you are looking at the situation correctly, you should be able to answer without doing any calculations.

Hint: The hanging mass is in equilibrium.
 
  • #5
Yes, I think I got it. The tension is equal to the weight of the hanging block (22 N), since hte other block is resting on the table.
It's in equilibrium, so a=0, so the whole term on that side of the equal sign becomes 0. Then T=mu * m (of Block A and C) * 9.8, then solve for the mass of Block C.
 
  • #6
Exactly. But don't waste time finding the masses; you are given the weights (w = mg) and that's all you need.
 

FAQ: Pulley Problem - Please give me hand here

How does a pulley work?

A pulley is a simple machine that is used to lift or move heavy objects. It consists of a wheel with a groove around its circumference and a rope or cable that runs through the groove. When a force is applied to one end of the rope, the object on the other end is lifted or moved.

What is the advantage of using a pulley in a lifting system?

One of the main advantages of using a pulley in a lifting system is that it can help to reduce the amount of force needed to lift an object. By distributing the weight of the object over multiple ropes and pulleys, the force needed to lift the object is divided, making it easier to lift.

How do you calculate the mechanical advantage of a pulley system?

The mechanical advantage of a pulley system can be calculated by dividing the output force (the weight of the object being lifted) by the input force (the force applied to the rope). The number of ropes supporting the weight also affects the mechanical advantage - the more ropes, the greater the mechanical advantage.

Can a pulley change the direction of a force?

Yes, a pulley can change the direction of a force. When a rope is pulled down, the direction of the force is downwards. When the same rope is wrapped around a pulley and pulled, the direction of the force changes to upwards. This is useful in lifting systems where the object needs to be moved in a different direction.

What are the different types of pulleys?

There are three main types of pulleys: fixed, movable, and compound. Fixed pulleys are attached to a surface and do not move, while movable pulleys can move up and down with the load. Compound pulleys consist of a combination of fixed and movable pulleys and provide a greater mechanical advantage than the other two types.

Similar threads

Replies
102
Views
6K
Replies
15
Views
4K
Replies
6
Views
9K
Replies
2
Views
652
Replies
8
Views
2K
Back
Top