Conservation of Mechanical Energy

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Homework Help Overview

The discussion revolves around the conservation of mechanical energy in a scenario involving a block of mass m, its kinetic energy before interacting with a spring, and the potential energy stored in the spring upon compression. Participants are exploring concepts related to energy transformation, friction, and the implications of energy loss.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the calculation of kinetic and potential energy, the implications of energy loss due to friction, and the interpretation of negative energy values. Questions arise regarding the correctness of reported energy values and the understanding of energy conservation principles.

Discussion Status

Some participants have provided guidance on interpreting energy changes and the relationship between kinetic and potential energy. There is an ongoing exploration of how to accurately account for energy loss and the work done by friction, with various interpretations being considered.

Contextual Notes

Participants mention the lack of exact measurements for variables and the challenge of determining the coefficient of friction. The discussion reflects uncertainty about the definitions and calculations related to energy in the context of the problem.

sunny17
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Homework Statement


This is my first time posting here guys so correct me if I am wrong. This question was on my test and I wanted to know if I did it right. There is a block of mass m and it is moving at a certain velocity before pressing against a spring which compresses distance x.

Find Change in Kinetic Energy
Change in Potential Energy
Is this a conservative energy. Why?
Energy lost to Friction
coefficient of Friction.

Homework Equations


KE = 1/2mv^2
PE of Spring = 1/2kx^2

The Attempt at a Solution



I don't know the exact measurements for the variables but I used the kinetic energy formula to find intial KE and the final should be 0 because it turns into Potential correct? I got negative for that.

I found change in potential by setting intial to 0 and using the equation.

I then noticed that kinetic and potential weren't equal and potential was smaller so there was no conservation of energy.

The loss in energy is friction? I then used this to find the coefficient. Is this correct?
 
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Welcome to Physics Forums.

The approach you describe sounds right, although it's difficult to say for sure without seeing exactly what you did.
 
PeroK said:
Welcome to Physics Forums.

The approach you describe sounds right, although it's difficult to say for sure without seeing exactly what you did.


Energy cannot be negative correct? I got negative Kinetic Energy but reported it as positive would this be right?
 
sunny17 said:
Energy cannot be negative correct? I got negative Kinetic Energy but reported it as positive would this be right?

Energy is a bit like money. If you lose it, you could describe that as a negative change.
 
PeroK said:
Energy is a bit like money. If you lose it, you could describe that as a negative change.

Then say I get Intial KE as 100J and Final is 0 because it converts to PE then Change in KE is -100?
Assuming the PE is 80J.

If I were to find the work done by Friction would it be -100 - 80? = -180J?
I thought it would be 100 - 80 = 20J.
 
sunny17 said:
Then say I get Intial KE as 100J and Final is 0 because it converts to PE then Change in KE is -100?
Assuming the PE is 80J.

If I were to find the work done by Friction would it be -100 - 80? = -180J?
I thought it would be 100 - 80 = 20J.

It really just like bookkeeping. You start with KE = 100J and end with PE = 80J, so:

1) There is a loss of 20J.
2) The change in energy is -20J.

These are equivalent.

It's then s equally valid to say:

a) The work done by friction is 20J
b) The work done by friction is -20J.

a) assumes you know that friction takes energy out of the system and b) makes this more explicit.
 
PeroK said:
It really just like bookkeeping. You start with KE = 100J and end with PE = 80J, so:

1) There is a loss of 20J.
2) The change in energy is -20J.

These are equivalent.

It's then s equally valid to say:

a) The work done by friction is 20J
b) The work done by friction is -20J.

a) assumes you know that friction takes energy out of the system and b) makes this more explicit.

Thank You. That makes sense so much more sense now.
 

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