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Frames of Reference In physics it is always crucial to have a frame of reference to study the motion of an object or particle. In early studies earth was thought to be the one "true" system of reference. But then it was discovered that earth circled around the sun, and that it rotated. This means that earth couldn't be the "true" frame of reference because it was in motion. The "true" frame of reference is a frame of reference which is inertial, this means that either it is in constant velocity motion (no acceleration) or it is not moving. Finding such a frame of reference was impossible for scientist, so it was concluded that the movement of an object or particle is always relative to the frame of reference it is compared to.. Let's suppose that there is a boat traveling downstream in a river. The streams velocity is Vs=5[m/s] and the boats velocity is Vb=20[m/s] parallel to the streams velocity. Two observers are set to determine the motion of a ball which will be rolled over the boats deck, one on the ground and another one on the boat's deck We'll call the observer on the boat's deck observer "1" and the one on the ground observer "2". Observer one rolls a ball over the boat's deck which has a velocity V1 and this velocity is perpendicular to the boat's velocity (this means the ball's motion will be perpendicular to the boat's motion).
Observer "1" will notice that the ball travels describing a straight line from its original position until it gets to the other side of the boat's deck. In the meantime observer "2" will notice that the ball describes a curve. What we conclude from this is that the movement described by the ball is relative to what we compare it to. Since the boat and the observer "1" are moving they will see that the ball moves perpendicular relative to the boat, but relative to the ground and to observer "2" described a curve. So which of the two did the ball truly describe? Our general answer to this question would be that the ball described actually a curve. Considering the earth's rotation movement relative to space, it would be false to affirm that the ball truly described a curve. The only true statement would be that the ball described a curve relative to the ground. It wouldn't be accurate even to say that the ball described a curve relative to the earth because the river is on earth and yet relative to the river the ball described a different curve from the one it described relative to the ground.
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