The motion of any object is only meaningful when given relative to something else. Relativity in motion assures us that the laws of Physics don't vary depending on how much inertia an object has. This is important because everything has some amount of inertia. Standing on the surface of the earth, we feel that we are not in motion, but because the earth is moving, so are we. If we measure the speed of a ball thrown atop a train, we can either measure the speed of the ball with respect to the train’s motion or with respect to the motion of the Earth.
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The title says it all: this blog features physics videos found everywhere on the web: animations, demonstrations, lectures, documentaries.