Hello, I've come up with a simple, imaginary situation that bugs me. Could someone help resolve my confusion? Let's say that a spaceship is traveling through space, and two people- observer 1 and observer 2- are watching it. Both observers can monitor the ship's speed and the amount of fuel in its "gas tank." Initially, the ship is moving at speed v with respect to observer 1, and is at rest with respect to observer 2. Now the spaceship accelerates, so it is moving at speed v with respect to observer 2 and speed 2*v with respect to observer 1. Observer 2 notes that 0.5*m*v2 (m is the mass of the ship) joules of energy are withdrawn from the fuel during this period of acceleration, but to observer 1 it appears that 0.5*m*((2v)2 - v2), or 0.5*m*3v2 joules of energy are consumed. It can't be true that observer 1 records three times more fuel used than observer 2; what accounts for this "inconsistency?" Thank you!