Two Pulleys, two weights and a string

• Petersilie23
In summary: But in summary, the conversation is about the use of free body diagrams in solving physics problems and how to properly apply them to find the necessary variables. The conversation also emphasizes the importance of putting in effort and not relying on others to provide answers.
Petersilie23
Homework Statement
There are two rolls and two weights. I need to define how much the right weight needs to weigh, so that both weights are in balance. The right weight is a little bit lower, but i don´t know if that has any impact. The right roll has double the radius of the left roll. Thanks for your answers! Greetings from Germany!
Relevant Equations
No

Welcome to PF.

What do you think so far? Are you familiar with drawing a Free Body Diagram (FBD) for each weight?

Petersilie23
No never heard of it. I am thinking between 25kg and 50kg, but have no idea how you can find a fast answer in a test for example. Do you have an idea, because i´ll soon have my Physics exam.

Petersilie23 said:
No never heard of it. I am thinking between 25kg and 50kg, but have no idea how you can find a fast answer in a test for example. Do you have an idea, because i´ll soon have my Physics exam.
A free body diagram considers one rigid component of the system. It illustrates all the forces that act directly on it.
Usually you need a FBD for each of several components. For two components that act on each other, each FBD will show that force, but acting oppositely.
The next step is to turn the diagrams into equations by considering the net forces in two directions (3 for a 3D problem), the torques they exert, and any resulting accelerations.

In the present case you have four components. For the pulleys, you do not care about the forces at their centres - they are just whatever they need to be to keep the pulleys in place - but you do care about the torques.

Petersilie23 and berkeman
Thanks for your answers: i’ll try ist. But Two questions: What impact has the double radius of the right roll: Does the right weight need to be heavier or more light. Another question: is it important that the right weight is further down than the left one and what impact does it have: Needs the weight to be more heavy or more light?

Petersilie23 said:
Thanks for your answers: i’ll try ist. But Two questions: What impact has the double radius of the right roll: Does the right weight need to be heavier or more light. Another question: is it important that the right weight is further down than the left one and what impact does it have: Needs the weight to be more heavy or more light?
You continue to ask questions without showing any effort on your part. That's not how this forum works. We love helping people figure out how to do things but we expect some effort to be made. We do not spoon feed answers. You have been told to do a FBD. Learn how to do that and do it. That's always the first step in solving these kinds of problems.

Petersilie23 said:
But Two questions: What impact has the double radius of the right roll: Does the right weight need to be heavier or more light. Another question: is it important that the right weight is further down than the left one and what impact does it have: Needs the weight to be more heavy or more light?
Drawing the FBDs and solving the equations is how you answer those questions.

1. How does the use of two pulleys affect the system?

The use of two pulleys in this system allows for a mechanical advantage, making it easier to lift heavier weights with less force. The pulleys also change the direction of the force applied, allowing for a more efficient transfer of energy.

2. What is the purpose of the string in this system?

The string in this system serves as a medium to transfer the force from one weight to the other. It also helps to distribute the force evenly, preventing one weight from bearing all the weight.

3. How does the weight of the pulleys affect the system?

The weight of the pulleys can affect the overall mass of the system, which in turn can affect the amount of force needed to lift the weights. Heavier pulleys may require more force to overcome their weight and lift the weights, while lighter pulleys may require less force.

4. Can the distance between the pulleys affect the system?

The distance between the pulleys can affect the mechanical advantage of the system. A shorter distance between the pulleys will result in a higher mechanical advantage, making it easier to lift the weights. However, a longer distance may require more force to lift the weights.

5. What factors can affect the efficiency of this system?

The efficiency of this system can be affected by factors such as the weight and friction of the pulleys, the weight of the weights, and the tension of the string. Any resistance or loss of energy due to these factors can decrease the efficiency of the system.

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