3
$\begingroup$

Suppose we have a car A moving along a straight path and another car B moving in a circular path. I know from the formula I have studied that the relative velocity of A as observed by B will not simply be

$v_b-v_a$

But I still can not get a 'feel' of why this is so. It just seems so natural to think that the above expression should be the expression for relative velocity in ANY frame regardless of whether it rotates or not, after all, relative velocity is the velocity of one object with respect to other and it seems like a simple difference of velocities should do the trick!

$\endgroup$
4
  • 1
    $\begingroup$ Taking the difference is fine as long as the velocities are vectors. $\endgroup$
    – Javier
    Apr 10 '17 at 1:39
  • $\begingroup$ Can you provide an example of what you mean? ie an example in which the simple difference does not work. $\endgroup$ Apr 10 '17 at 2:50
  • $\begingroup$ @sammygerbil I think the OP is confused about frames. ‘the above expression should be the expression for relative velocity in ANY frame whether it rotates or not'. It would be the same in any inertial frame of reference! $\endgroup$ Apr 10 '17 at 4:14
  • $\begingroup$ Related to relative velocity in rotating frame of reference and Relative velocity is not the simple difference of individual velocities? posted by the same OP. $\endgroup$ Apr 10 '17 at 10:26
3
$\begingroup$

I too had some difficulty grasping the concept. However, I present here an example that helped me understand the concept and hope it helps you too. Suppose, you and your friend are sitting at diametrically opposite ends of a rotating turntable.

enter image description here

Lets take an instant where your friend's velocity is the vector $v$ wrt an stationary observer beside the table. Thus, your velocity must be $-v$. And by the formula you mention, the relative velocity of your friend wrt your frame of reference should have been

$v-(-v) = 2v$.

But you will always see your friend at the opposite end of the table stationary, not moving an inch. You see the surrounding in motion, but not your friend, implying the velocity of your friend relative to you is always 0.

The formula you've mentioned is just a special case of relativity where two people have their velocities always along the same straight line wrt a stationary observer.

$\endgroup$
0
1
$\begingroup$

I'll give you an idea. Suppose you're sitting on a turntable and your friend is rotating it. Suppose you are at the centre of the turntable. You will see your friend moving right? You are in a rotating frame and both of you are not moving in the ground frame so the difference in the velocities is zero but you still see him moving.


Hope it gives you an intuition.

$\endgroup$
1
  • $\begingroup$ awesome! i finally understand WHY that formula doesn't apply to rotating frames! $\endgroup$ Apr 14 '17 at 2:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.