Conservation of Mechanical Energy

  • #1
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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
Coefficent of Friction.

Homework Equations


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

The Attempt at a Solution



I dont 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 coefficent. Is this correct?
 

Answers and Replies

  • #2
PeroK
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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.
 
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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.


Energy cannot be negative correct? I got negative Kinetic Energy but reported it as positive would this be right?
 
  • #4
PeroK
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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.
 
  • #5
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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.
 
  • #6
PeroK
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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.
 
  • #7
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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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