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J. Perelman
"Entertaining physics". Book 1.
Chapter 1. Speed. The addition of movements


Imagine a rowing boat floating in the lake and let the arrow and in our drawing depicts the direction and speed of its movement. Intercept goes sailing boat; arrow b represents its direction and speed. If you, the reader, will ask, whence this boat has sailed away, you certainly will immediately indicate the point M on the shore. But if the same question apply to passengers rowing boats, they would indicate a completely different point. Why?

Sailing boat going across a row. Arrows a and b - speed. That will see the rowers?

This happens because the passengers can see the boat is not moving at right angles to the path of your boat. They do not feel their own movement: they think that they are in place, and everything is moving at their own speed, but in opposite direction. So for them sailboat moves not only in the direction of arrow b, but also in the direction of the dotted line andback to the movement of the boat (see Fig. below). Both movements sailing boats - real and perceived - are formed by the parallelogram rule. As a result, the passengers of the boat seems to be sailing the boat moves along the diagonal of the parallelogram constructed on b and and. That is why passengers it seems that sailing boat sailed away from the shore not at the point M, and at some point N, far ahead of the motion of a rowing boat.

Rowers seems that the sailing boat is not across them, and obliquely from the point N, and not on M

Moving along with the Earth in its orbit and seeing the rays of the stars, we judge the position of the source of these rays is just as wrong as passengers rowing boats erroneously determine the place of departure sailing. So the stars seem to us to be slightly shifted forward along the path of motion of the Earth. Of course, the speed of the Earth is negligible compared to the speed of light (10,000 times less); therefore, the apparent displacement of the stars is negligible. But it can be detected by using astronomical instruments. This phenomenon is called the aberration of light.

If these questions interest you, try without changing the conditions of our problem on the boat, to say:

1) what is the direction of a rowing boat for passengers sailing?

2) which receives a rowing boat, in the opinion of the passengers sailing?

To answer these questions, you need on-line and to construct a parallelogram of velocities; the diagonal it will show that passengers sailing boats rowing seems to be floating in an oblique direction, as if to approach the beach.

Entertaining physics J. Perelman


System Orphus


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