Newton's laws of motion -- finding the velocity of a Block in a pulley system

Click For Summary

Homework Help Overview

The discussion revolves around determining the speed of block B in a pulley system when the cord at point A is pulled down at a specified velocity. The problem is situated within the context of Newton's laws of motion and involves analyzing the relationships between the velocities of different components in the system.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss creating variable names for unknown velocities and attempt to relate them through equations. Some explore using distances instead of velocities to derive a velocity ratio. Questions arise regarding the movement of the pulley in relation to the movement of block B, with various interpretations of how much the pulley moves as block B rises.

Discussion Status

The discussion is ongoing, with participants sharing their reasoning and calculations. Some have proposed potential relationships between the velocities and distances, while others express uncertainty about their conclusions. There is a mix of interpretations regarding the mechanics of the system, and no consensus has been reached yet.

Contextual Notes

Participants are navigating a complex setup involving multiple pulleys and blocks, leading to various assumptions about the relationships between the components. The discussion reflects the challenges of visualizing the system's mechanics and the implications of different assumptions on the outcome.

Vv anand
Messages
41
Reaction score
1

Homework Statement


Determine the speed with which block B rises in figures if the end of cord at A is pulled down with a speed of 2ms^-1

Homework Equations


Given Velocity downwards at A=2m/s

The Attempt at a Solution


Really stuck... Couldn't even start the question solving...I know all policies and therefore i am not demanding the question to be solved but just a hint on how to start will surely help
 

Attachments

  • 14987150441211737520228.jpg
    14987150441211737520228.jpg
    23 KB · Views: 1,109
  • 1498715174224-1852538468.jpg
    1498715174224-1852538468.jpg
    22 KB · Views: 828
Physics news on Phys.org
Vv anand said:
a hint on how to start
Create some variable names for the different unknown velocities. See what equations you can write to relate them.
 
haruspex said:
Create some variable names for the different unknown velocities. See what equations you can write to relate them.
Yes that i hv already done... Will start again.. Thx
 
Yea i started again but ended with nothing..I can show u my work if u want
 
This is what i did...i m stuck after this!
 

Attachments

  • 1498749332200-1079336475.jpg
    1498749332200-1079336475.jpg
    37.9 KB · Views: 1,065
I don't know if it is allowed, but I used distances instead of velocities.
Just took a starting position, moved the block B 1 unit distance and calculated the distance each other bit moved if the string stayed tight.
That gave me a velocity ratio.
 
If mass B moves up ##\delta##, how much does the pulley that supports A move down?
 
Chestermiller said:
If mass B moves up ##\delta##, how much does the pulley that supports A move down?
Is it 3##\delta##
 
Vv anand said:
Is it 3##\delta##
No. Look at the diagram carefully.
 
  • #10
Chestermiller said:
No. Look at the diagram carefully.
Oh is it 4 ##/delta##
 
  • #11
Vv anand said:
Oh is it 4 ##/delta##
Just because of the middle string attached to the mass coming from the upper left pulley
 
  • #12
Vv anand said:
Oh is it 4 ##/delta##
No. Look carefully. The wire between pulley
 
  • #13
I get that the pulley supporting A moves down ##\delta## if mass B moves up ##\delta##. This focuses on the wire that passes over pulley D.
 
  • #14
Chestermiller said:
I get that the pulley supporting A moves down ##\delta## if mass B moves up ##\delta##. This focuses on the wire that passes over pulley D.
Sir I am coming to the solution that since pulley supporting B moves up the pulley supporting a moves down 3##/delta## as i hv to ke
 
  • #15
Vv anand said:
Sir I am coming to the solution that since pulley supporting B moves up the pulley supporting a moves down 3##/delta## as i hv to ke
Yes sir...i got that the ##/pulley## is moving delta downwards
 
  • #16
Vv anand said:
Yes sir...i got that the ##/pulley## is moving delta downwards
Wrote that upper comment by mistake...
 
  • #17
So if ##v_B## is the upward velocity of mass B, what is the downward velocity of the pulley that supports A?
 
  • #18
Chestermiller said:
So if ##v_B## is the upward velocity of mass B, what is the downward velocity of the pulley that supports A?
Sir vb
 
  • #19
Vv anand said:
Sir vb
But this brings me to the solution that vb=2/3.
 
  • #20
Vv anand said:
But this brings me to the solution that vb=2/3.
Yes. So?
 
  • #21
But sir the solution is 0.5 m/s
 
  • #22
Vv anand said:
But sir the solution is 0.5 m/s
I guess I don't agree.
 
  • #23
Chestermiller said:
I guess I don't agree.
I confirm 0.5m/s.
 
  • #24
When I first looked at this, I got VB=2VA/3, but when OP asked for more info on how, I checked and am now convinced that it is VA/4
When I pull with force T, I find a mechanical advantage of 4:1, by just looking at all the tensions. (assuming massless blocks, light frictionless strings, etc.)

Since you noted the blocks move equally, surely the section from C to E shortens twice as fast as the other two legs?

Perhaps you could expand on your reasoning?
 
  • Like
Likes   Reactions: Vv anand
  • #25
The set-up is sufficiently complicated that it is best to take a very disciplined approach.
There are four distances of interest: AC, CE, CD, DE.
We need four equations to relate them. Some equations will be that one distance is the sum of others, while others will be that a sum of distances is constant.
@Vv anand , what equation relates:
AC, CE, DE?
CD and DE?
AD, AC and CD?
CD, CE and DE?
 
  • #26
Merlin3189 said:
When I first looked at this, I got VB=2VA/3, but when OP asked for more info on how, I checked and am now convinced that it is VA/4
When I pull with force T, I find a mechanical advantage of 4:1, by just looking at all the tensions. (assuming massless blocks, light frictionless strings, etc.)

Since you noted the blocks move equally, surely the section from C to E shortens twice as fast as the other two legs?

Perhaps you could expand on your reasoning?
Like this?
 

Attachments

  • Screenshot_20170629-075513.jpg
    Screenshot_20170629-075513.jpg
    9.9 KB · Views: 410
  • #27
Vv anand said:
Like this?
Yes, that works.
 
  • #28
@haruspex I'm trying to get an understanding of where I went wrong. Was I correct in assessing that, if ##v_A## is the downward velocity of A and ##v_B## is the upward velocity of B, then the rate of increase in length of the segment of wire between A and the pulley that supports A is ##v_A-v_B##?

EDIT: Ooops. That's not correct. I really messed up on this one.

Chet
 
Last edited:

Similar threads

Newton's Second Law (Pulley Sustem)
  • · Replies 3 ·
Replies
3
Views
2K
Ideal Vs Real Mass Pulley system
  • · Replies 10 ·
Replies
10
Views
5K
Choosing what consists of a "system" in Newton's laws of motion
  • · Replies 28 ·
Replies
28
Views
2K
Newton's Third Law Problem: Two masses, a rope and a pulley
  • · Replies 21 ·
Replies
21
Views
11K
Newton's laws of motion -- pulley system
  • · Replies 14 ·
Replies
14
Views
3K
Pulleys with Torque: Free Body Diagram Analysis
  • · Replies 34 ·
2
Replies
34
Views
4K
System of two pulleys and three masses
  • · Replies 22 ·
Replies
22
Views
7K
Newton's laws of motion -- Constaint motion
  • · Replies 13 ·
Replies
13
Views
4K
Newton’s Law of Motions: tension forces in a pulley
  • · Replies 4 ·
Replies
4
Views
2K
Two masses and two pulleys problem
  • · Replies 33 ·
2
Replies
33
Views
10K