Vectors in kinematics (maths a level) help Watch

curlyyy
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Need some help with this question, q10 part b

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m.aurelius
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Do you know what the answer is? Because i
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m.aurelius
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I think I got 5 seconds but I’m not sure if my working is worth 5 marks
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curlyyy
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Yh it’s 6 sec
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m.aurelius
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I keep the wrong answer sorry (5 seconds total but 2 seconds further elapsed). I’m not sure what I’m doing wrong. I worked out point B by calculating displacement (s=ut + 0.5at^2) and then doing the same equation but now working out t with (14i-2j) for my initial velocity.
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mqb2766
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(Original post by m.aurelius)
I keep the wrong answer sorry (5 seconds total but 2 seconds further elapsed). I’m not sure what I’m doing wrong. I worked out point B by calculating displacement (s=ut + 0.5at^2) and then doing the same equation but now working out t with (14i-2j) for my initial velocity.
(Original post by curlyyy)
Yh it’s 6 sec
What is the relative position of B compared to A (j component)?
Use
s = (u+v)*t/2
Then its moving with a constant velocity in the j direction of -2ms^-1 so how much time will elapse before its brought back to zero.
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curlyyy
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(Original post by mqb2766)
Use
s = (u+v)*t/2
Then its moving with a constant velocity in the j direction of -2ms^-1 so how much time will elapse before its brought back to zero.
Ohh! I think I got it:

The relative position of B compared to A is 28.5i + 12j
So then using r=r0+vt, and looking only at j components:
r=0
r0=12
v=-2
so t=6

That took me ages, thank you so much both, really appreciate your help
Also, do you usually do all the qs in the book or topic questions from past papers? and do you know where to get topic questions for maths Edexcel?
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mqb2766
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(Original post by curlyyy)
Ohh! I think I got it:

The relative position of B compared to A is 28.5i + 12j
So then using r=r0+vt, and looking only at j components:
r=0
r0=12
v=-2
so t=6

That took me ages, thank you so much both, really appreciate your help
Also, do you usually do all the qs in the book or topic questions from past papers? and do you know where to get topic questions for maths Edexcel?
Generally for questions:
mathsgenie
physics and maths tutor
dr frost maths
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m.aurelius
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(Original post by curlyyy)
Ohh! I think I got it:

The relative position of B compared to A is 28.5i + 12j
So then using r=r0+vt, and looking only at j components:
r=0
r0=12
v=-2
so t=6
Sorry, what equation did you use there? Is r0 your j displacement? And what’s r? I got the relative position of B but didn’t know what equation to use after that.
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curlyyy
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(Original post by m.aurelius)
Sorry, what equation did you use there? Is r0 your j displacement? And what’s r? I got the relative position of B but didn’t know what equation to use after that.
I used r=r0+vt where r is the position vector when it's due east to A, so when the j component is 0. r0 is the initial position vector so when it hits point B, I sort of thought of it like A is the origin so B in relation to A is 28.5i + 12j, so r0 would be 28.5i + 12j. Then v is velocity of B, and t, time.
Then, I only compared the j components to get the time.
Have you seen that equation before? It's ch8 in the stats and mechanics book (if you do edexcel). I only know it bc I did this last year and am resitting, so it may be that you're teacher hasn't covered it yet at school.
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m.aurelius
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(Original post by curlyyy)
I used r=r0+vt where r is the position vector when it's due east to A, so when the j component is 0. r0 is the initial position vector so when it hits point B, I sort of thought of it like A is the origin so B in relation to A is 28.5i + 12j, so r0 would be 28.5i + 12j. Then v is velocity of B, and t, time.
Then, I only compared the j components to get the time.
Have you seen that equation before? It's ch8 in the stats and mechanics book (if you do edexcel). I only know it bc I did this last year and am resitting, so it may be that you're teacher hasn't covered it yet at school.
Hm I don’t think I’ve seen that before? I do ocr so maybe I just haven’t covered that yet. Thanks for the explanation
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