Three mass pulley problem-Neat

In summary, the conversation discusses a physics problem involving blocks and friction. The main question is about finding the acceleration of the blocks under certain conditions. There is also a discussion about the existence and direction of a friction force between the blocks. One of the speakers mentions the need for access to old AP Physics Mechanic tests for future preparation.
  • #1
nightflarer
2
0
Block 1 of mass m_1 is placed on block 2 of mass m_2 which is then placed on a table. A string connecting block 2 to a hanging mass M passes over a pulley attached to one end of the table. The mass and friction of the pulley are negligible. The coefficients of friction between blocks 1 and 2 and between block 2 and the tabletop are nonzero and are given in the following table (coefficient between blocks 1 and 2 for static friction is u_s1, for kinetic it's u_k1, for coefficient between block 2 and tabletop, static friction coefficient is u_s2, for kinetic it's u_k2.

Basically, you're supposed to find the acceleration of block 1 and 2, assuming M is large enough that as the hanging block descends, block 1 is slipping on block 2. There are some other questions too.

This was a test question, and I got it right for the most part. One of the questions was whether or not there existed a friction force f_1 exerted on block 1 by block 2, and if so, in what direction. I said there would be one of magnitude m_1*g*u_s1, and that's what a lot of my classmates thought since that's how the teacher had taught us in a problem we did a few weeks ago. However, now she says there is no friction force. What's up?

By the way, does anyone know where I can get access to old AP Physics Mechanic tests, especially the free responses? They'd be a good help for me for preparing for tests in the future. Thanks a lot.
 
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  • #2
nightflarer said:
One of the questions was whether or not there existed a friction force f_1 exerted on block 1 by block 2, and if so, in what direction. I said there would be one of magnitude m_1*g*u_s1, and that's what a lot of my classmates thought since that's how the teacher had taught us in a problem we did a few weeks ago. However, now she says there is no friction force. What's up?
Assuming your description of the problem is accurate, of course there's a friction force between the two blocks. But since you earlier said that block 1 is slipping on block 2, why did you use the coefficient of static friction?
 
  • #3
Doc, the part I was referring to was a different section where M was small enough so that the blocks remain at rest when released. Sorry if that was unclear.
 
  • #4
Big difference. If the blocks don't move there will be no friction force between them. (Just like if you rest a book on a horizontal surface: What's the friction force on it? None. If you try to push it and it resists, then there's a friction force involved.)
 

1. What is a three mass pulley problem?

A three mass pulley problem is a physics problem that involves three masses connected by a string or rope passing over a pulley. The masses are typically at different heights and the goal is to determine the acceleration of the masses and the tension in the string.

2. How do you solve a three mass pulley problem?

To solve a three mass pulley problem, you can use Newton's second law of motion, which states that the sum of the forces acting on an object is equal to its mass multiplied by its acceleration. You will also need to consider the forces of tension in the string and the force of gravity on each mass. By setting up and solving equations for each mass, you can determine the acceleration and tension values.

3. What are the key concepts involved in solving a three mass pulley problem?

The key concepts involved in solving a three mass pulley problem include Newton's second law of motion, forces (such as tension and gravity), and the concept of a pulley (which changes the direction of the tension force). It is also important to understand the relationships between the masses, the tension in the string, and the acceleration of the system.

4. What are the common challenges when solving a three mass pulley problem?

One of the common challenges when solving a three mass pulley problem is setting up and correctly identifying all of the forces acting on each mass. It can also be difficult to determine the direction of the acceleration and tension, as they may vary depending on the arrangement of the system. Additionally, properly setting up and solving the equations can be a challenge for some students.

5. Are there any real-life applications of the three mass pulley problem?

Yes, the three mass pulley problem has many real-life applications, such as in engineering and construction. For example, cranes and elevators use pulley systems to lift heavy loads. By understanding the principles involved in a three mass pulley problem, engineers can design and optimize these systems for efficiency and safety.

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