Originally Posted by
MikeSnow
Wait, what? Speed is relative. As a result, kinetic energy is relative as well. Let's say you are sitting down in a train moving at a constant speed of 100 km/h relative to the ground, in a straight line. You as a passenger would have zero kinetic energy relative to the train frame of reference. And if the train wouldn't have any windows (and assuming you can't rely on noise either), you wouldn't even be able to tell if the train is stationary or is moving.
Now imagine you have a small drone, and you fly it inside that train. Again, it would be irrelevant if the train were moving at 100km/h or 200km/h relative to the ground, or if it were stationary. It would have no effect on the flight of the drone inside the train, and the G forces and aerodynamic effects it experiences.
Kinetic energy (and potential energy too) values are relative to the frame of reference you choose, however changes in kinetic energy during a collision are not. No matter which inertial frame of reference you will compute the collision in, the dissipated energy during the collision will always be 1/2*m*v^2.
For example if you consider the frame of the Earth, the initial energy is 1/2*m*v^2, final energy is zero, so energy dissipated is 1/2*m*v^2. If you consider the frame going at the same speed as the airplane, the initial kinetic energy is that of the Earth, and that kinetic energy becomes slightly less after the collision due to the impact (and small change of momentum of the Earth). The change is still 1/2*m*v^2