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I've been struggling with this question for a while. Imagine an object moving from planet A to planet B at a constant speed of lets say 10 m/s. If everything suddenly doubled in size, I would imagine that the initial speed of the object would remain the same since an increase in size would not account for an increase in speed. So if everything increased in size we would notice that all the moving objects in the universe would suddenly appear to move at a slower speed. Is my thinking wrong here? Dying to get an answer to this.

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closed as unclear what you're asking by CuriousOne, Carl Witthoft, user81619, DarenW, John Rennie Sep 14 '15 at 5:39

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Despite the comments, this question can be answered with physics. You didn't specify what you mean by either "everything" or "doubles in size" so I'll assume you meant all massive objects double in length along each orthogonal spatial dimension while maintaining their form and composition.

The apparent speed and actual speed are unchanged. A radar dish on Planet A still records the craft departing at 10m/s because the speed of light is unchanged. The only difference as far as that tracking goes is that the ship will be closer to the dish than expected (by the sum of the planet's radius and half the ship's length) assuming the doubling is radially symmetric from the geometric center of each object.

What will change is when the ship comes in to land at Planet B. Each object has increased in mass eightfold, so the gravitational attraction between them is 64 times the original amount, but the ship is only carrying eight times the original fuel and has eight times the original kinetic energy, and thus won't be able to land as gently as planned.

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