Why is W Tot not equal to W Glider on Spring in Newton's Third Law?

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Homework Help Overview

The discussion revolves around understanding the total work done in a system involving a glider attached to a spring, particularly in the context of Newton's Third Law. Participants are examining the relationship between the work done by various forces, including friction and the spring, and how these relate to the total work as defined in their textbook.

Discussion Character

  • Conceptual clarification, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are questioning why the textbook's expression for total work excludes the work done by the glider on the spring. They are exploring the implications of Newton's Third Law on the work calculations and whether all forces in the system need to be considered to apply the formula for total work.

Discussion Status

There is an ongoing exploration of the definitions and implications of total work in the context of the problem. Some participants suggest that the total work should account for all forces acting on the glider, while others clarify that the work done by the glider on the spring cancels out with the work done by the spring on the glider. The discussion is productive, with participants seeking clarity on the relationships between the forces involved.

Contextual Notes

Participants are working under the constraints of a textbook example that defines total work in a specific way, which has led to confusion regarding the inclusion of all forces in the calculations. There is a focus on understanding how friction interacts with the spring and glider during acceleration.

Selfless_Gene
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Homework Statement


Hello I am solving a problem that requires me to find the max distance of a glider with spring attached to it. I am getting confused on Work total, in an example in my textbook W total is the W friction + W spring on glider = W total. My question is why did the textbook example not include W glider on spring. I know these are action reaction pairs from Newtons third law, but is this valid to say W tot is only equal to W friction + W spring?

Thank You

Homework Equations


W tot = K2 - K1

The Attempt at a Solution

 
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Selfless_Gene said:

Homework Statement


Hello I am solving a problem that requires me to find the max distance of a glider with spring attached to it. I am getting confused on Work total, in an example in my textbook W total is the W friction + W spring on glider = W total. My question is why did the textbook example not include W glider on spring. I know these are action reaction pairs from Newtons third law, but is this valid to say W tot is only equal to W friction + W spring?

Thank You

Homework Equations


W tot = K2 - K1

The Attempt at a Solution

You need to be clear what total is being considered. Total work done by what, or done on what?
"W friction + W spring on glider" suggests you want the total work done by the spring, and that this includes some work done against friction whilst accelerating the glider. Is that right?
Work done by the glider on the spring is equal and opposite to that done by the spring on the glider, so if you add them they will cancel out.
 
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haruspex said:
You need to be clear what total is being considered. Total work done by what, or done on what?
"W friction + W spring on glider" suggests you want the total work done by the spring, and that this includes some work done against friction whilst accelerating the glider. Is that right?
Work done by the glider on the spring is equal and opposite to that done by the spring on the glider, so if you add them they will cancel out.

When I refer to total work my textbook has a formula and which is W tot = K2-K1. So to be more clear my question is in order to use this formula do we have to consider all the work done in this system by all forces? I ask this because in the example they used W tot = W fric + W spring on glider . It did not include W glider on spring.

Thank You
 
Selfless_Gene said:
my textbook has a formula and which is W tot = K2-K1
That appears to be expressing the increase in KE. That would be right if it means the total work done on a mass.
If the mass is a glider being accelerated by a spring that encounters friction, it would equal "W spring on glider". It would not include the work done by the spring against friction.
Selfless_Gene said:
in the example they used W tot = W fric + W spring on glider
That would be right for the total work done by the spring. It would not equal K2-K1.
Now, I don't know where exactly this friction acts during acceleration. Maybe some acts directly on the spring while other friction acts on the glider:
Code:
Total work done by spring
    ├→ Work done by spring against friction during acceleration
    ↓
Work done by spring on glider
    ├→ Work done by glider against friction during acceleration
    ↓
Total work done on glider
    =  increase in glider KE
Could you post the section of the textbook, maybe as an image?
 

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