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Position vectors

  1. Apr 9, 2004 #1
    I really need helpo with this question. I am having to revise over easter for the three week break and i need a solution to this. I have all the past exam papers but the lecturere doesnt have any past paper solutions and his lecture notes are vague. TO top it off i wondered if there were any good books with worked examples and questions?

    Here is my problem.

    At time t, the position vector of a point P, on a fairground ride is given by:

    r(t) = (2cost + cos4t)i + (2sint + sin4t)j m

    Find its acceleration, F, at time t. Show that:

    |f|^2 = 260 + 64 cos3t m^2s^-4

    Ok thats the problem.

    Now i have managed to get the velocity by differentiation and then the acceleration which works out to be:

    a(t) = (-2cost-16cos4t)i + (-2sint-16sin4t)j

    But i cant then get this expression to work out to |f|^2 = 260 + 64 cos3t...

    Can anyone help?
    Last edited: Apr 9, 2004
  2. jcsd
  3. Apr 9, 2004 #2

    matt grime

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    What have you found |F|^2 to be? You're missing a 4 from inside the first cos in the post.
  4. Apr 9, 2004 #3
    It is now corrected.

    I got a couple of totally different asnwers to |f|^2 that i reckokn are really wrong.

    4cos^2(t) - 256cos^2(4t) + 4sin^2(t) + 256sin^2(4t) Is one of my answers that i think looks halfway there....
  5. Apr 9, 2004 #4

    matt grime

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    I think you need to remember that (x+y)^2 = x^2+2xy+y^2 and not x^2+y^2, and that (-x)^2=x

    because you're missing two terms in that answer and there's a minus sign, which could just be a typo.

    Remember your trig identities too.
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