# Relative velocity

1. Jun 29, 2007

### pardesi

consider a disc which is rotating about an axis passing through it;s centre of mass now consider a points on the disc other than the centre of mass which are in line with the centre of mass
what is their relative velocity and what is the velocity of one point from other's frame

2. Jun 29, 2007

### Staff: Mentor

I'm not sure what you are asking. A point a distance r from the center of the disk will have a tangential velocity of magnitude $\omega r$ with respect to the axis.

You want the relative velocity of what with respect to what?

3. Jun 29, 2007

### Danger

I don't know the real scientific answer to this. My own system of logic says that there is zero relative velocity, and each appears stationary to the other.
My reasoning is that two places (say, New York and Los Angeles) are whipping around in a similar manner, but the roads between them don't have to be flexible.

4. Jun 29, 2007

### pardesi

yes i got that now after thinking really too much wavering.....
if i am not wron the velocity depends on the frame of my refrence if the is rotating with the particle then speed of other point is 0 if it isn't rotating and is parallel to the c.m frame it is $$\vec{\omega} (\vec{b}-\vec{a})$$

if i am wrong please do correct me

@doc al i want the relative velocity between two points n line with the cente of rotation

5. Jun 29, 2007

### Staff: Mentor

Ah... I think I understand the question. Imagine two points a & b in a straight line along a radius. Point a is at a distance $r_a$ from the center and point b is at a distance $r_b > r_a$ from the center. Find their relative velocity. Is that the question?

If so, use what I mentioned in the first post to figure their relative velocity. (It's not zero!)

6. Jun 29, 2007

### pardesi

is the relative veolcity sam as the velocity of this point as seen from the others frame

7. Jul 8, 2007

### newTonn

This is avery fantastic question.Relative velocity should be zero,because displacement of one point with respect to other at any time is zero.

8. Jul 8, 2007

### masudr

I believe pardesi's second post gets it right: it is non-zero in any inertial frame.

9. Jul 8, 2007

### newTonn

Anyhow both the points are non-inertial-because of change in direction(acceleration)

10. Jul 8, 2007

### rcgldr

However, a person will weigh less in Los Angeles than New York, because LA is closer to the equator, and the linear velocity is higher. The centripal force, V^2/R is higher the closer you get to the equator, and zero at the poles. (This would be true if the earth were truly spherical).