Where'd the energy come from?

**qraal** · Nov10-09, 04:02 AM

Originally Posted by Jeff Reid

The other issue you're ignoring is the point of application of that force. A car applies a force to the pavement it moves on, an airplane applies a force to the air it moves through, in these cases, the power is related to the force times speed at the point of application of that force. However a rocket doesn't interact with the space it moves through. Instead the force is generated internally, by expelling a bit of itself backwards at high speed. In this case the point of application of force is at the rocket nozzle, any remaining onboard fuel is accelerated along with the rocket, and the power generated is a function of how much and how fast the onboard fuel is accelerated, not related to the rockets speed relative to some other object or frame of reference in space.

Doesn't matter. The rules still rule regardless of the reference frame one picks. What doesn't work is comparing a measurement in one reference frame against a measurement in another and then wondering why they apparently disagree.

**FoxCommander** · Y, 06:19 PM

Originally Posted by trivia1

A rocket in space, of mass 1kg, accelerates at 2m/s squared. Between t=0 and t=1 it's change in KE is 0.5j, between t=999 and t=1000 it's change in KE is 999.5j. Yet the rocket motor power output hadn't changed. What explains the massive difference in KE transferred to the rocket?

well concider the speed that the object is going at t=999, it is going 1998 m/s. which is pretty fast. the amount of work done by the rocket is the force of the rocket * the distance traveled. If it is going that fast for one second, not even counting acceleration, its distance will be 1998 meters (with acceleration it turns out to be 1999), thats almost 2Km in just 1 second! Now multiply that to the force, which is 2N and you get 3998Joules. which is also a very big amount of energy, for the size.

You can see why even a small force over a great distance can become alot of energy. this is why the difference is so big.