We say that a tree is stationary but is it in motion with respect to some other object?
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Sure. If you drive past a tree, the tree is in motion with respect to your car.
Relativity says that all constant-velocity reference frames are equally valid.
However, your car doesn't have constant velocity for very long. The tree's reference frame is useful because it contains many objects - trees, road, buildings - that never accelerate.
(Actually, they ARE accelerating as the Earth rotates, so from a more universal perspective the tree's current reference frame is no more useful than the car's. But for those of us also on Earth, the rotation cancels out)
Relativity says that all constant-velocity reference frames are equally valid.
However, your car doesn't have constant velocity for very long. The tree's reference frame is useful because it contains many objects - trees, road, buildings - that never accelerate.
(Actually, they ARE accelerating as the Earth rotates, so from a more universal perspective the tree's current reference frame is no more useful than the car's. But for those of us also on Earth, the rotation cancels out)
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