# Potential plus kinetic energy

1. Aug 20, 2011

### Drew777

What would it mean when you subtract potential energy of a higher elevation from the summation of energy(kinetic + potential of a lower elevation)?

Last edited: Aug 20, 2011
2. Aug 20, 2011

### BruceW

potential energy + kinetic energy = constant. Therefore the summation of energy of the lower position equals the summation of energy of the higher elevation.

So subtracting the potential energy of the higher position (from the summation of energy) gives the kinetic energy of the higher position.

3. Aug 20, 2011

### Drew777

So is it possible to get a negative number as the kinetic energy of the higher elevation?

4. Aug 20, 2011

### rcgldr

Kinetic energy can't be negative. A constant sum for gravitational potential energy and kinetic energy only occurs if there are no other forces involved. If some force other than gravity affects an object, then the work done by that force also affects the total energy.

5. Aug 20, 2011

### Drew777

I understand that. When I take the the potential energy of the higher point and subtract it from the summation of energy I get from (PEf +KEf) I get a negative number. What does this mean?

Last edited: Aug 20, 2011
6. Aug 20, 2011

### Dr_Morbius

It means you are doing the math wrong. You can't do that. What you're doing produces a meaningless number. You have to subtract the potential energy from the summation not from the kinetic energy.

7. Aug 20, 2011

### willem2

kinetic energy can't be negative.
It means that a thrown object will fall back before it gets to this altitude.
If you throw something straight up, it will start to move down again when the kinetic energy is 0.

8. Aug 20, 2011

### Drew777

Sorry I wrote it down wrong. I went back to edit what I meant.

What I have is the potential energy of a low point that I will call PEf and I got the kinetic energy it took for the object to move down the incline(KEf). I added them together to get Ef.
Now I have to take this number Ef and subtract it from the higher point of potential energy, that I will call PE. I ended up with a negative number on all my trials but one. Why would this be?

9. Aug 20, 2011

### rcgldr

10. Aug 21, 2011

### BruceW

KEf + PEf = PE + KE
(rearranged): KEf + PEf - PE = KE

If you then get a negative number for KE, then maybe it is because you measured KEf and PEf at different points.
Also, if the object rolls down the incline, then it will have some rotational energy:
KEf + PEf + KE(rotational) = PE + KE
This might explain where the energy went. Also, if there was a lot of friction, then maybe the energy turned into heat.

11. Aug 21, 2011

### Drew777

What was done is I slid a puck down an air table. I did this 7 times, each with a different weight. I calculated the potential energy of the high point and calculated the potential energy at the low point. The KEf I measured was using 1/2(m)(Vf^2). So the KEf is the kinetic energy from the high point (PEhp) to the lower point(PEf). To get Ef I added PEf + KEf.
Now I need to find Δ E. The equation for Δ E is Ef-PEhp.

Example
PEf + KEf=Ef
2,070,000 ergs + 623,000 ergs=2,690,000 ergs
Ef - PEhp=Δ E
2,690,000 ergs - 2,810,000 ergs= -107,000 ergs
All of my trials came out with a negative number except my last trial. It came out with a +20,000.
I am trying to understand where these numbers are coming from. Is -107,000 just the energy lost from the top to the bottom. If so then where did the positive number come from?
Thanks
Drew

12. Aug 21, 2011

### cepheid

Staff Emeritus
Your phrasing doesn't make sense here. KEf is the kinetic energy that the object has when it is at the lowest point.

In the ideal (frictionless) case, since mechanical energy is conserved, ΔE should be 0 in all cases. (All potential energy lost during the descent is converted into kinetic energy). The fact that you get a negative ΔE means that mechanical energy is lost. This must be a combination of experimental error + the fact that the table is not entirely frictionless. It's probably mostly the latter, a consequence of which is that although total energy (as always) is conserved, mechanical energy (which means kinetic + potential) is not conserved. Some of the mechanical energy is converted into heat by friction. That's where the shortfall comes in: it's the work done by the frictional force.

I don't know why you get a positive ΔE in one instance. This could be an indication that the variation in your results is dominated by measurement error (contrary to what I assumed above, that friction was the dominant effect when it came to the deviation between expected and measured results).

We'd need more information about how you got your numbers in order to comment further.

13. Aug 21, 2011

### Dr_Morbius

By the way, it is bad practice to edit a post you made. The edit you made to your post is now going to confuse readers because our replies no longer make sense. You can make edits for spelling and grammar but you need to do it immediately. Do not change the meaning of the post. If you made a mistake make another post with the correction.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook