3D Velocity Of A Moving Object To A Stationary Object

[phys123]

There are 2 objects. Object 1 is stationary and Object 2 is in motion. Object 1's position in the 3D world is (x=0,y=0,z=0). I have Object 2's velocity (also in x,y,z directions). I need to calculate the speed at which Object 2 is moving towards (or away from) Object 1. Keep in mind that this will not be Object 2's speed, because it is not moving directly towards Object 1. It is moving on it's own path, but I need to know the change in speed towards Object 1. Hope I explained that good enough, if not then ask. How can this be done?

 

[Doc Al]

You'll need Object 2's position. At any moment, find the component of Object 2's velocity parallel to its position vector with respect to the origin. That will be the rate of change of the distance between the Objects.

 

[phys123]

I do have Object 2's position, forgot to mention that. And what you are saying is exactly what I need to do. Can you go into detail on how I would calculate this? My physics isn't very strong

 

[Doc Al]

Not much physics here, just a bit of math. Find the unit vector pointing from 1 to 2--that gives you the direction. To find the component of 2's velocity in that direction, take the scalar product of the velocity with that unit vector.