# Homework Help: Why is KE never negative?

1. Mar 14, 2006

### jcais

Why is KE never negative?

2. Mar 14, 2006

### arildno

What's the expression of KE you're familiar with?

3. Mar 14, 2006

### Astronuc

Staff Emeritus
Because like absolute temperature it is or isn't.

Think of counting numbers with respect to objects, like apples and oranges.

Does it mean anything to say that one has -5 oranges or apples? Of course, not!

With respect to vectors, vectors do have + or - values, but that has to do with orientation, not magnitude.

Often - numbers are relative to some reference number, as in -30°C, which is relative to 0°C, which on the absolute temperature scale is ~243 K (mks, or SI).

4. Mar 15, 2006

### vaishakh

Energy of no kind is ever negative. KE is an energy. So it cannot be negative. Pls show me somewhere when energy is negative.

5. Mar 15, 2006

### Hootenanny

Staff Emeritus
Without very little scientific knowlage it is very simple to observe why kinetic energy is never negative. Kinetic energy is given by;
$$E_k = \frac{1}{2}mv^2$$
Now, the only two variables are mass and velocity.

(1) Is it possible to have a negative mass?

(2) Is it possible to have a negative velocity?

(3) What happens when you square a negative number?

I hope this helps

Last edited: Mar 15, 2006
6. Mar 15, 2006

### Chi Meson

Since total energy is a matter of frame of reference, it is possible for an object to be observed to have a negative quantity of potential energy. Such as an object on the surface of a planet will have a negative potential energy, and the electron of a hydrogen atom is said to be at the -13 eV energy level.

This is, again, relative to the observer, and subject to conventional definitions of the "zero" point of potential energy, but no matter where this zero point is set, there will be something that could be observed to be at a negative level.

7. Mar 15, 2006

### jcais

Thank you

Thank you to all for your assistance :-)

8. Mar 15, 2006

### LightMage

K.E is a scalar product..Meaning it has only a MAGNITUDE to it and no direction..For other quantites like Force or velocity, those are vectors, meaning that it has both DIRECTION and MAGNITUDE..hence, for such quantities, there is a -ve sign existing to tell you its direction from the equilibrium..