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# M4 Relative velocity watch

1. In M4, I don't understand the concept of fixing targets. So if we have two particles, P and Q, each moving at Vp and Vq respectively, the velocity of P relative to Q is pVq. If P intercepts Q, then pVq points towards the inital position of Q. How is this true?
2. (Original post by jaspreetb)
In M4, I don't understand the concept of fixing targets. So if we have two particles, P and Q, each moving at Vp and Vq respectively, the velocity of P relative to Q is pVq. If P intercepts Q, then pVq points towards the inital position of Q. How is this true?
This may help.

If you were an observer sitting on Q, and observing P, then for them to meet, P would appear to be heading directly for Q, i.e. along the line PQ.

From the outside, pVq only initially points to the initial position of Q. After a moment P and Q will both have moved. pVq no longer points at the initial position of Q, rather without pVq changing, it now points to the new (current) position of Q.

We can think of pVq as stopping Q and observing P's movements from Q's point of view. It is an apparant motion from Q's viewpoint, and that remains directly towards Q.
3. (Original post by ghostwalker)
This may help.

If you were an observer sitting on Q, and observing P, then for them to meet, P would appear to be heading directly for Q, i.e. along the line PQ.

From the outside, pVq only initially points to the initial position of Q. After a moment P and Q will both have moved. pVq no longer points at the initial position of Q, rather without pVq changing, it now points to the new (current) position of Q.

We can think of pVq as stopping Q and observing P's movements from Q's point of view. It is an apparant motion from Q's viewpoint, and that remains directly towards Q.
So does pVq ALWAYS point towards Q if they are due to intercept?
4. (Original post by jaspreetb)
So does pVq ALWAYS point towards Q if they are due to intercept?
Yes.

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