# But what does it mean?

1. Jun 29, 2012

### GrayBush

Hey there, not sure if I am posting this in the right place, so I apologize if I am not.

So after some calculation I came up with an average human cell yielding
8.9876e+3 joules if annihilated.

But what does that figure mean in the real world? I suppose I am looking for an analogy here along with a more refined number. Is it equal to the energy of somebody dropping a brick or a D battery or...

2. Jun 29, 2012

### Muphrid

http://en.wikipedia.org/wiki/Orders_of_magnitude_(energy) should give you some idea of what's in the neighborhood. I'm not sure if you mean by "annihilation" that the cell is converted into its rest energy by $E = mc^2$ or some more subtle process.

But really, you should be able to figure out the potential energy changes from dropping objects by $mgh$ also, for a rough estimate. I figure a 100 kg man dropped from a height of 10 meters (about 3 stories) is in the same ballpark as far as the energy goes.

If you really want to blow your mind, though, you can see from wiki that this is only on the order of ~2 calories. Yes, calories from food. Or about the same energy as that stored in a AA battery.

3. Jun 29, 2012

### GrayBush

Yeah, it was resting energy figured using E=mc^2. Okay, so if translated into kinetic energy what would that (8.9876e+3 joules) be comparable to?

4. Jun 29, 2012

### Staff: Mentor

Save yourself some grief and decide that if you're looking for "comparable", you'll be OK with +/- 20%... And think about 1e+4 joules instead of 8.9876e+3 joules because it's way less work and won't change the comparables.

Now you can try it in a few standard equations. For example:

Gravity at the surface of the earth: $E=mgh$: 104J is what happens when you drop a 10kg weight from a height of 100 meters, or a 100kg weight from a height of 10 meters, or a 1000kg weight from a height of one meter.

Kinetic energy: $E=\frac{1}{2}mv^{2}$: 104J is 200 kg moving at 10m/sec (imagine a horse running into you at full speed), or 2 kg moving at 100m/sec (a cannonball from an 18th century light cannon at long range), or 20 grams moving at 1000m/sec (a modern rifle bullet).