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Q & A: speed in space

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Most recent answer: 12/09/2018
Q:
Since speed is always measured relative to some frame that is considered at rest (easy to do on Earth because the ground make a good rest frame), how do you measure speed in space where a rest frame is completely arbitrary? Since F = m*a (we only consider non relativistic speeds), could we say that the speed is equal to a*t once F = 0 (i.e we accelerated by 10m/s^2 for 600 seconds, then we are travelling at 6,000m/s relative to a local point in space)? Acceleration is incompatible with a rest frame by definition, so I'd think it would make a pretty good starting point.
- Anonymous
A:

You've basically given a recipe for assigning a velocity relative to somebody who started off with you but didn't experience the force causing the acceeration. If the acceleration was caused by gravity, rather than a real force, then your recipe would give the velocity relative to something that started off with you but experienced a force that countered the effect of gravity. In practice, I think what you're describing is still velocity relative to the Earth, or whatever object you started on.

Mike W.


(published on 12/09/2018)

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