Re: the two forms of energy in question here.

Posted by Southern Man on Jun 16, 2002 at 08:02
(209.102.128.207)

Re: the two forms of energy in question here. (paul baughn)

If I lift 100 Kg of apples to a height of 10 meters they have a certain amount of energy. If I observe 100 Kg of oranges moving at 10 meters per second they have a certain amount of energy. If the energies are equal are apples then oranges? Remember they both weight 100 Kg!

Obviously not! That is why apples aren’t oranges – because the energies can’t be balanced because apples aren’t oranges.

Now imagine a 100Kg watermelon dropped from a height of 10 meters. Pardon me – I’ll be back later. Got to get something to eat right now.

Mumph, mumph, mumph. That’s better. Now to your math. You lifted 100Kg to 10 meters and calculated 980J. Then you proposed the same mass moving at 10 M/s and calculated a different energy. So? Why not calculate the same mass moving at 1e10 m/s? Then you would get a much larger difference. The reason you get a difference is because you are calculating different unrelated energies. 100Kg at 10M elevation does not produce 100Kg traveling at 10m/s at zero elevation.

In your water example the problem is the same. The water in the pipe doesn’t accelerate to 1m/s in 1 second. It might take several hours to get to a steady state condition. And all this time you have to keep supplying water. To get 500K joules of kinetic energy you have to supply enough water at 10 meters elevation. How much? Well the energy requirement would be 500K joules. When you get to that steady state condition ALL of the water in the pipe will contain the 500K J but you are only extracting 1 meter of water every second that has to be replaced. The rest is just moving down the pipe due to what we call inertia. You only have to replace the energy of the water that you removed from the system.