# Conservation of Mechanical Energy

## 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?

PeroK
Homework Helper
Gold Member
2020 Award
Welcome to Physics Forums.

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

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?

PeroK
Homework Helper
Gold Member
2020 Award
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.

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.

PeroK
Homework Helper
Gold Member
2020 Award
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.

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.