# a question about vectors, suvat

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#1
How to show they do not collide in c, and is find the position vector of B relative to A means the vector of B minus vector of A?
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1 year ago
#2
If you have done part a) correctly, you will indeed find that subtracting the position vector of A from the position vector of B gives you the expression shown in part b). As for question c), if the ships collide at some time then the two ships must have the same position vector at that time, meaning that the position vector of B relative to A (or A relative to B) would be 0j + 0i. To have 0j, t would have to equal 1. If t = 1, you would have 1i. This means the position vector of B relative to A is 0j + 1i at this time, so the ships are not in the same position as eachother at this time. Because t=1 is the only time you can have 0j, this means the two ships can never be in the same position as eachother.
I hope that makes sense.

Can I ask what unit this question is from, and which exam board and subject? I know this thread is posted under Physics but it reminds me of my A-level maths days.
Last edited by awkwardshortguy; 1 year ago
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#3
(Original post by awkwardshortguy)
If you have done part a) correctly, you will indeed find that subtracting the position vector of A from the position vector of B gives you the expression shown in part b). As for question c), if the ships collide at some time then the two ships must have the same position vector at that time, meaning that the position vector of B relative to A (or A relative to B) would be 0j + 0i. To have 0j, t would have to equal 1. If t = 1, you would have 1i. This means the position vector of B relative to A is 0j + 1i at this time, so the ships are not in the same position as eachother at this time. Because t=1 is the only time you can have 0j, this means the two ships can never be in the same position as eachother.
I hope that makes sense.

Can I ask what unit this question is from, and which exam board and subject? I know this thread is posted under Physics but it reminds me of my A-level maths days.
it is IAL edxel M1, I understand c now, but I cannot understand why b relative to a should be b-a, not a-b?
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1 year ago
#4
(Original post by Wuxinyu)
it is IAL edxel M1, I understand c now, but I cannot understand why b relative to a should be b-a, not a-b?
Thank you, and I think it is like this:
Imagine a number line going from -2 to 8. The position of 8 relative to -2 is 10 to the right, or '+10'. If you subtract -2 from 8, that gives you +10, which is the right answer. If you subtracted 8 from -2 that would give you -10, which we know is wrong. So the rule must be that we subtract the thing that the other thing is relative to. The 8 is relative to the -2, so we subtract the -2. B is relative to A so we subtract the A. I hope that makes sense.
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