Three blocks and a spring on an inclined plane....

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Homework Help Overview

The discussion revolves around a physics problem involving three blocks and a spring on an inclined plane, focusing on the application of the Work Energy Theorem and the calculation of work done by the spring. Participants are exploring concepts related to energy conservation, tension in the spring, and the dynamics of the blocks on the incline.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants discuss the tension in the spring when one block loses contact and the total energy in the system. There are attempts to apply the Work Energy Theorem and questions about the relationships between different variables involved in the equations formed by the original poster.

Discussion Status

The discussion is ongoing, with participants providing guidance on considering various energy components and questioning the correctness of equations formed. There is a focus on clarifying the relationships between variables and ensuring the dimensions of terms in the equations are appropriate.

Contextual Notes

Participants note the importance of defining variables clearly and ensuring that the equations account for all relevant forces and energies in the system. There is an emphasis on understanding the implications of the spring force and gravitational potential energy in the context of the problem.

navneet9431
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Homework Statement


IMG_20180831_000121.jpg

See question number 1.

Homework Equations


Work Energy Theorem,
work done by all the forces=change in K.E.

The Attempt at a Solution


I tried solving this question this way,
Screenshot_2018-08-31-11-42-56-108_com.hashlearn.now.jpg

IMG_20180831_114823.jpg
please help me calculate the Work Done by spring here??
I will be thankful for any help!
 

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It might be simpler to start from the final situation. When A just loses contact, what is the tension in the spring?

(In future, please use thread titles that indicate the topic.)
 
It would be,
-0.6x=1*10*sin(37)
So what's next?
haruspex said:
It might be simpler to start from the final situation. When A just loses contact, what is the tension in the spring?
 
navneet9431 said:
It would be,
-0.6x=1*10*sin(37)
So what's next?
Actually I asked for the tension, but you went a step further and found the extension.
So what is the total energy in the system at that point?
 
It would be zero,Right?
haruspex said:
Actually I asked for the tension, but you went a step further and found the extension.
So what is the total energy in the system at that point?
 
navneet9431 said:
It would be zero,Right?
You have three components to consider: spring PE, GPE and KE. They are not all zero.
 
I think I need to apply the work energy theorem.
But on which block ?
haruspex said:
You have three components to consider: spring PE, GPE and KE. They are not all zero.
 
In particular, what do you think KE might be if mC is minimum mass to just lose contact?
(Interesting problem. The answer is independent of no less than two parameters that you'd think at first blush would matter.).
 
navneet9431 said:
I think I need to apply the work energy theorem.
That comes later. First try to say what the total energy is at the end. Take the initial height of block B as your zero for GPE. Invent variables as necessary for unknowns.
 
  • #10
haruspex said:
That comes later. First try to say what the total energy is at the end. Take the initial height of block B as your zero for GPE. Invent variables as necessary for unknowns.
IMG_20180831_214927.jpg

I tried solving it this way this time...,
I have formed two equations,
IMG_20180831_232621.jpg

Are these two equations useful in finding "m"?
Or,do I need to find something more?
I will be thankful for help!
 

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  • #11
navneet9431 said:
View attachment 230083
I tried solving it this way this time...,
I have formed two equations,
View attachment 230087
Are these two equations useful in finding "m"?
Or,do I need to find something more?
I will be thankful for help!
In eqn (1),your first term is missing a factor. It does not appear to have the dimension of energy.
What is the relationship between the x on the left and xi and xj on the right?

In eqn (2), you seem to have "mH" (or m+1?), but I do not know what this is supposed to be. And are you sure about that minus sign?
 
  • #12
haruspex said:
In eqn (1),your first term is missing a factor. It does not appear to have the dimension of energy.
What is the relationship between the x on the left and xi and xj on the right?

In eqn (2), you seem to have "mH" (or m+1?), but I do not know what this is supposed to be. And are you sure about that minus sign?
In the first equation W_gravity on B=1*x*g*sin(37),is it correct now?
And in equation 2
It's not "mH",It's (m+1)[sorry!For my poor handwriting]
Yes I think there would be a minus sign because the formula for spring force is "-k*x".
Is it correct?
 
  • #13
navneet9431 said:
In the first equation W_gravity on B=1*x*g*sin(37),is it correct now?
And in equation 2
It's not "mH",It's (m+1)[sorry!For my poor handwriting]
Ok, you fooled me in both because you left out the units. When you use a symbol like m for mass you do not need to state units because m has dimension, but when you plug in a specific value like 1kg that's what you must write, 1kg, not just "1".
navneet9431 said:
Yes I think there would be a minus sign because the formula for spring force is "-k*x".
That's fine if you are taking xi as negative.
But you did not answer my other question: what is the relationship between the xi and xj you have in the second equation and the x you have in the first equation?
 
  • #14
haruspex said:
But you did not answer my other question: what is the relationship between the xi and xj you have in the second equation and the x you have in the first equation?
The relation between xi, xj and x would be "xi+xj=x".
IMG_20180901_075151.jpg

Please tell how to proceed further!
 

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