# Kinetic energy terminology

1. Mar 31, 2015

I think its correct to state that:
"Kinetic energy is a property of a moving object."
But would it be correct to state that:
"Kinetic energy is carried by a moving object"? If not is there a better alternative terminology to use?
Thank you.

2. Mar 31, 2015

### Snapp'tappin27

Almost all objects have Potential Energy. When all kinetic energy is fully in motion, then KE=TE (total energy) and is not "holding" any un-used Kinetic Energy.

As long as KE is not equal to Total Energy, an object is still "carrying" some KE, but it is held in the form of Potential Energy (energy that is available, but not being used).

Thus, PE+KE=TE

I hope that helps.

3. Mar 31, 2015

### Staff: Mentor

Both phrasings work about equally well for me. Neither is exactly mathematically rigorous (and to be fair, they don't pretend otherwise) so there's only so far they go... but as far as they go, they're both fine.

4. Mar 31, 2015

Thank you. I posed the question because I'm feeling a little uncomfortable with the idea of something carrying kinetic energy energy and I cant yet pin down why I feel uncomfortable with it. I think I'm trying to get a picture in my mind of what kinetic energy actually is. I have no problems with the maths and everything else its just the concept of kinetic energy.

5. Mar 31, 2015

### davenn

I don't really like the term "carrying KE" makes it sound like the KE is a load

I would always prefer to see it stated ... a moving mass has KE

Dave

6. Mar 31, 2015

### bjbbshaw

As you suspect, you have to be careful with kinetic energy.

If you express the kinetic energy of a free moving mass simply as $\frac{m v^2}{2}$ the kinetic energy is relative to the speed of the mass with respect to the observer. From it you can calculate the force and distance or time required to change the speed relative to the observer's frame of reference. Without that context, the kinetic energy of a free particle is arbitrary.

In a bound system, however, the kinetic energy is measured with respect to the other masses in the system and is therefore "absolute" within the system. The total energy is the kinetic energy plus the potential energy within the bound system.

Rotational and vibrational (as in molecules) kinetic energy is "absolute" with respect to any inertial frame of reference within which masses are rotating or vibrating.

In every case we say that a mass "has" kinetic energy or we talk about "the kinetic energy of the mass". The kinetic energy is really a property of the system within which the mass is moving, however.

7. Mar 31, 2015

### Staff: Mentor

That's one of the better ways of knowing that you've pushed the non-precise phrasing about as far as it can be pushed

To be fair to the language, the "carrying" phrasing does work a bit better when your primary goal is to transfer the kinetic energy to something else. You'll hear people talking about bullets and anti-tank projectiles "delivering" energy to the target, seems natural enough in that context.

8. Mar 31, 2015

### nasu

@bjbbshaw

You should be careful with your statements. :)
The kinetic energy is frame dependent. The fact that for a bound system we prefer the system of the center of mass of the system (for example) does not make the kinetic energy "absolute".
Rotational and vibrational KE is no more absolute than the KE of other types of motions. It's just that usually you subtract the translational KE which is pretty much changing the frame from "any inertial frame" to the CM frame. So you are just implying that if we calculate always in the same frame then the KE is frame independent. At least this is how we understand you statment about "absolutness".

9. Apr 1, 2015

10. Apr 1, 2015

### Snapp'tappin27

But cars carry fuel (PE) which is is stored KE. So, in some cases KE is a load- at least in the terms we are talking about here, on this particular post.

I really don't care if you want to look at it one way and me look at it another. I just want to affirm that I'm not an idiot.

11. Apr 1, 2015

### davenn

WOW where the hell did that come from ??!!

I nor anyone else suggested that you were !!

D

12. Apr 6, 2015

### Snapp'tappin27

That's fine. I was just saying that we called it carrying KE so that I could explain it even though that terminology isn't really correct. No hard feelings here.