Question

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?

Answer 1

Answer:

Observed airplane is at rest, speed = 0 km/hr

Explanation:

 When two bodies with velocity $V_1$ and $V_2$ moves opposite to each other, their relative velocity is the sum of  their individual velocity.

 That is relative velocity = $V_1+V_2$

  In this case, $V_1 = 100 km/hr$ and Relative velocity = 100 km/hr

 Substituting

     100 = 100 + $V_2$

     So, Velocity of airplane flying opposite direction, $V_2$ = 0 km/hr

      So that observed airplane is at rest.

Answer 2

Answer: 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.