What are the accelerations in the multiple-pulley problem?

  • Thread starter subhradeep mahata
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In summary: That would be astonishing. In the absence of other forces (as for the 3m block) the ratio would be constant, since that is the mass.Please quote the whole passage, word for word, in case there is some misunderstanding.@hutchphd All right, so what are the changes to be made in the approach?@haruspex Actually it just says in a pink little box named "trick" that the tension times acceleration of a block is constant in case of a pulley. No other references.@haruspex Actually it just says in a pink little box named "trick" that the tension times acceleration of a block is constant in case of a pulley. No other references.
  • #1
subhradeep mahata
120
13

Homework Statement


upload_2019-1-29_21-24-3.png

The figure shows three blocks with their masses m,2m and 3m and two pulleys. Assuming that the pulleys and strings are massless and ideal, calculate the tension in lower string and acceleration in 3m block.

Homework Equations

The Attempt at a Solution


upload_2019-1-29_21-26-40.png

We have to find T. Now, let's assume that the 2m block moves downwards with acceleration a, and the m block move upwards with acceleration a as well.
Accordingly,
2mg-T=2ma and T-mg=ma
adding them, we get a=g/3
substituting it in the first equation, T=4mg/3.
Also, 2T=3m * acc
On solving, we get acc =8g/9, which is acceleration of 3m block.
But, my book says that the product of tension of the segment of the string and the corresponding acceleration of the block is constant. But my answers don't seem to match this criteria.
Neither do i know the meaning of the statement or how it has been derived, nor can I find any mistake in my method(if there is any). Kindly please address the issue.
 

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  • #2
subhradeep mahata said:
assume that the 2m block moves downwards with acceleration a, and the m block move upwards with acceleration a as well.

The free pulley is not fixed in space.
 
  • #3
subhradeep mahata said:
my book says that the product of tension of the segment of the string and the corresponding acceleration of the block is constant.
That would be astonishing. In the absence of other forces (as for the 3m block) the ratio would be constant, since that is the mass.
Please quote the whole passage, word for word, in case there is some misunderstanding.
 
  • #4
@hutchphd All right, so what are the changes to be made in the approach?
 
  • #5
@haruspex Actually it just says in a pink little box named "trick" that the tension times acceleration of a block is constant in case of a pulley. No other references.
 
  • #6
subhradeep mahata said:
@haruspex Actually it just says in a pink little box named "trick" that the tension times acceleration of a block is constant in case of a pulley. No other references.
Then I suggest you pay no attention to it. So with zero tension, the block accelerates infinitely fast?
Sounds like a misprint.
 
  • #7
haruspex said:
Then I suggest you pay no attention to it.
Ok, understood.
Now, am I correct in my procedure?
 
  • #8
subhradeep mahata said:
@hutchphd All right, so what are the changes to be made in the approach?
The lengths of the strings being fixed, there is a kinematic relationship between the accelerations of the three blocks.
E.g., suppose block m moves down by x and block 2m moves down by y. How far must block 3m move?
 
  • #9
haruspex said:
suppose block m moves down by x and block 2m moves down by y. How far must block 3m move?
Actually, I have serious issues in figuring out these kind of things. Any suggestions? Maybe i have difficulty in visualizing these.
Also, I kind of do not understand how the arrangement works. If m moves down, shouldn't 2m move up?
 
  • #10
subhradeep mahata said:
If m moves down, shouldn't 2m move up?
Not necessarily if the pulley moves down too.

One way to figure these out is just to assume a linear relationship between the positions - in this case the horizontal position of m3 (x3 from pulley, say) and the vertical positions of the other two (x1, x2 from their pulley).
So you take the relationship to be x1+b x2+c x3=0.
Now consider moving just two blocks at a time. Hold m3 still and move m1 up a bit. How much does x2 change in relation to x1? What does that tell you about b?
Now do the same holding m2 still and seeing what happens to m3.

An alternative method is to ascribe lengths to the strings and write down equations representing that they are constant.

Bear in mind that x1 and x2 are positions relative to the pulley, so their second derivatives are accelerations relative to the pulley, not accelerations in the lab frame.
 
  • #11
subhradeep mahata said:

Homework Statement


The figure shows three blocks with their masses m,2m and 3m and two pulleys. Assuming that the pulleys and strings are massless and ideal, calculate the tension in lower string and acceleration in 3m block.

Homework Equations

The Attempt at a Solution


View attachment 237979
We have to find T. Now, let's assume that the 2m block moves downwards with acceleration a, and the m block move upwards with acceleration a as well.
Those two blocks have the same magnitude of acceleration, one down, the other up with respect to the hanging pulley. But the pulley also accelerates, so what are the accelerations with respect to the rest frame of reference?
 

Related to What are the accelerations in the multiple-pulley problem?

What is a multiple-pulley problem?

A multiple-pulley problem is a physics problem that involves multiple pulleys and the calculation of forces and tensions within a system of connected ropes or cables. It is commonly used to demonstrate principles of mechanical advantage and the concept of work.

What is the significance of a multiple-pulley problem?

Multiple-pulley problems are important in understanding the principles of mechanical advantage, which is the idea that using a system of pulleys can make it easier to lift or move heavy objects. These problems also help to demonstrate the relationship between force, work, and energy.

How do you solve a multiple-pulley problem?

To solve a multiple-pulley problem, you first need to draw a diagram of the pulley system, labeling the pulleys and ropes. Then, you can use the principles of mechanical advantage and Newton's laws of motion to calculate the forces and tensions within the system. It is important to carefully consider the direction of forces and use the correct equations to solve the problem.

What are some common misconceptions about multiple-pulley problems?

One common misconception is that adding more pulleys will always make it easier to lift an object. However, the number of pulleys alone does not determine the mechanical advantage. The arrangement and configuration of the pulleys also play a significant role. Additionally, some people may think that the tension in the ropes is equal throughout the system, but this is not always the case as the direction and magnitude of forces can vary at different points.

How do multiple-pulley problems relate to real-life applications?

Multiple-pulley problems have many real-life applications, such as in cranes, elevators, and other lifting machines. They are also used in rock climbing and sailing to demonstrate the principles of mechanical advantage. Understanding multiple-pulley problems can also help engineers design more efficient and effective machines and systems.

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