While in an airplane flying at 100 km/h you look down from the window and spot another airplane flying at the same speed in the opposite direction. relative to you, the speed of the spotted plane is?

If you are flying at the north with 100km/h, and you spot another plane flying south also with speed of 100 km/h, in non relativistic motion (because the speeds are small compared with c)  the relative velocity at which you see the second plane is the difference between the velocity of the second plane and your velocity.

So if we define the north as positive, your velocity is v1 =  100km/h, and the other plane's velocity is v=2 -100km/h (because it is traveling in opposite direction)

Then the relative velocity is V = v2 - v1 = -100km/h - 100km/h = -200 km/h.

So you see the second plane with a velocity of -200km/h.

more than, less

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