# Homework Help: Relative Velocity

1. Jun 16, 2010

### dstepp

I want to calculate the instantaneous relative velocity between two objects. Object A is at location (x1,y2) moving towards the origin (0,0) at velocity V. How do you calculate object A's relative velocity to object B at location (x2,y2)?

2. Jun 16, 2010

### mathman

You need to specify velocity of B relative to the origin. If the speeds are not near the speed of light, it is simply a vector subtraction of B velocity from A velocity.

If the difference is near the speed of light, you need to take into account special relativity effect - check Google.

3. Jun 16, 2010

### dstepp

B is a stationary observer. I am trying to calculate how fast object A is approaching B, even though object A is moving towards the origin.

I haven't been in a physics class since college, but remember that it probably has something to do with the law of cosines. I don't think it is just a simple vector subtraction.

I appreciate any help. Thank you.