# Relativistic energy of a ball

1. Apr 14, 2010

### astro2cosmos

according to mass-energy equivalent theorem, Regardless of whether the object is at rest or moving, the object of mass m having energy E=mc2.

suppose a ball of mass m is placed on ground, then how much energy this ball have???
Is it equal to E=mc2 ??
now if we place this ball above the ground up to height h, Is this mean all Energy of ball (i.e E=mc2) is converted to potential energy (mgh) ???
if so this ball have so much tremendous energy!!!!
how it can be possible?????

2. Apr 14, 2010

### utesfan100

If the ball went up to h all by itself, then yes. Usually ball's don't do that, and the rest energy of a small fraction of its mass would usually blast the ball well above escape velocity if it did.

Usually the energy to raise the ball h is added by some other force, thus the rest energy of the ball is constant.

3. Apr 14, 2010

### starthaus

No, it isn't. But , if the ball is made out of a radioactive material and you let it sit on your desk, it will release an energy:

$$\Delta E=c^2 \Delta m$$

Now, this can be a tremendous amount of energy due to the huge value of the conversion factor $$c^2$$

If it is radioactive, this is how it is possible. Be careful when you play with radioactive tennis balls :-)

4. Apr 14, 2010

### GRDixon

mc<sup>2</sup> is descriptive of the isolated ball. mgh is an energy of the ball/earth system. Unfortunately, this potential energy is often said to be part of the ball's energy (e.g. in quantum theory).

5. Apr 15, 2010

### astro2cosmos

what is the "descriptive of the isolated ball"?? then does the total energy of the ball contain both quantities i.e (T.E = mc2 + mgh)??
but in this case the quantitative value of mgh is very much higher than the mc2 in terms of Joule.!!!!!!
wat is confusion!!!!