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Math Help - Mechanics and Vector Help

  1. #1
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    Mechanics and Vector Help

    I'm trying to do some revision before classes start again tomorrow but I'm struggling a little bit with this one, any help would be greatly appreciated! I've included my working so far:

    A particle moves in 2D with a vector velocity which at time t is ' v = (-0.4ti + 5j) m/s '.


    At time t=0 the particle is at (3,2)

    a) Find the acceleration vector at time t

    b) Find the position vector at time t

    c) Find the speed at time t = 10




    I found the acceleration vector at time t to be -0.4i m/s^2


    I can't figure out how to find the position vector at time t and also not sure how I'd carry on to do part c.


    I've integrated to get: 'r = -0.2ti + 5tj + c' and i know at time t=0, 'r = 3i + 2j' but I'm not sure what to do to finish off the question. Thank you for any help!

    edit: I found b) to be 'r = (3-0.2t)i + (5t+2)j' but I was wondering if anybody could still help me with c)?
    Last edited by mikev; April 14th 2013 at 10:48 AM.
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  2. #2
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    Re: Mechanics and Vector Help

    Seriously? You have done the hard part! The speed at time t= 10 is just the length of the velocity vector at t= 10. Replace t with 10, calculate the velocity vector and the find its length.
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  3. #3
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    Re: Mechanics and Vector Help

    Yeah, sorry! I was a bit of an idiot and wrote the wrong bit down I was stuck on.... I got the speed to be 9 m/s.

    The bit I was stuck on was 'Find the time at which it crosses the Y-axis'

    I'm such an idiot!! Thank you for the help on that one anyway!
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