# Work Kinetic E and Potential E

1. Feb 22, 2005

### hotmail590

Can work change the energy of a system?

Work cannot change the energy of a system.

The above is false but i am not sure how to explain it

I understand that work = negative potential energy
W = -U

Also work = Change of Kinetic Energy
W = (triangle)K

Last edited: Feb 22, 2005
2. Feb 23, 2005

### hotmail590

Can anyone help me out?

3. Feb 23, 2005

### Tom Mattson

Staff Emeritus
Not in general. In general, work is given by:

$$W=\int_C\vec{F} \cdot d \vec{s}$$

Not all forces have a corresponding potential energy function. Such forces are called nonconservative. This term should have been presented to you either in class or in your textbook. An example of a nonconservative force is friction.

That is a lead for you to follow. Try to think about that in the context of your question.

4. Feb 23, 2005

### hotmail590

Does that mean that if there is work, then the energy system is nonconservative and if there is no work then the energy system is conservative?

5. Feb 23, 2005

### Tom Mattson

Staff Emeritus
No. Both conservative forces and nonconservative forces can do work.

Last edited: Feb 23, 2005
6. Feb 23, 2005

### Tom Mattson

Staff Emeritus
Let's put it this way. Take your system to be an inclined plane of height H and a block of mass M. Let the surface of contact between the block and the plane have a coefficient of kinetic friction $\mu_K$. Now let the block slide down the plane from rest.

Question: Will the block's KE at the bottom equal its PE at the top? If not, then where did the missing energy go? What force was responsible for the loss?

This should be easy to answer.

7. Feb 23, 2005

### xanthym

Also, the distinction should be made between INTERNAL work (that performed by forces derived soley from elements internal to the system) and EXTERNAL work (which is derived from forces external to the system).

INTERNAL work does NOT change total system energy. Only EXTERNAL work can change total system energy. Thus, the system under consideration (and what it contains and does not contain) must be defined prior to determining the effect of a given force's work on total system energy.

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Last edited: Feb 23, 2005