For example, if Jack and Jill, who both weigh 20kg each, are moving in the same direction at 1m/s, they should both have 10J of kinetic energy each, and they are both moving at 0m/s relative to each other and so there is a difference of 0J of kinetic energy between Jack and Jill. If Jack accelerates to 2m/s, while Jill stays at 1m/s, Jack now has 40J of kinetic energy, while Jill still has 10J, so Jack has 30J more kinetic energy than Jill. However, relative to Jill, Jack is moving at 1m/s, and so should only have 10J more energy than her. What am I missing?
In: Physics
You’re not missing anything. Kinetic energy is frame-dependent, which means that it is totally fine for different reference frames to disagree about how much kinetic energy an object has.
In Jill’s frame of reference Jack does only have 10J more energy; from a stationary observer, he has 30J more energy.
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