# Homework Help: Basic Relative Motion Question

1. Aug 30, 2010

### mohabitar

I'm watching a lecture video describing the following situation:

There is a red ball and a green ball on a collision course. The green ball moves with a constant velocity 6 m/s due East, while the red ball moves with a constant velocity 2 m/s due west.

They are saying that the velocity of the green ball in the frame of the red ball is 8 m/s. I'm not understanding why it's 8. Obviously they are adding the two velocities together, but why? One would think that since green is headed east and red is headed west, that we should subtract the values. What's the reasoning behind this? Thanks.

2. Aug 30, 2010

### diazona

Imagine yourself moving along with the red ball. Maybe that will make it a bit clearer.

For a slightly more detailed explanation: to find relative velocity, you should actually subtract the two velocities. But velocity is a vector, which means it's not just the number of m/s that matters, it's the direction. For objects moving along a line, you indicate the direction by a sign: to the right (east) is positive, and to the left (west) is negative. So in your example, the green ball has velocity +6 m/s, but the red ball has velocity -2 m/s. When you subtract those, (green)-(red), you get +8 m/s. Thus the velocity of the green ball in the frame of the red ball is +8 m/s.

To see why you subtract instead of adding, imagine a new situation, a red ball and a green ball both moving to the east at 4 m/s. Both of them are moving in the same direction, so they both have the same sign on the velocity: both velocities are +4 m/s. You agree that the velocity of the green ball in the frame of the red ball is 0, right? Obviously adding the numbers doesn't give you the right answer. But subtracting them does.

3. Aug 30, 2010

### mohabitar

Thanks a lot for your answer. Really helped clear things up. However, what exactly does 'in the reference of' or 'in the frame of' really mean?

4. Aug 30, 2010

### diazona

It's kind of like "as if you were traveling along with". So when they ask for the velocity of the green ball in the frame of the red ball, they want to know what velocity you would observe the green ball to have if you were moving along with the red ball.