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Homework Help: Using vectors as a function of time

  1. Sep 3, 2015 #1
    1. The problem statement, all variables and given/known data

    Vector P is a function of time t.

    In a specified coordinate system P is given by: P= (A+Bt)i + (A+Ct-Dt^2)j and P is in meters when t is seconds

    Given these values (shown without their units): A=15.0, B=7.00, C=20.0, D=4.00

    (a) Calculate the time when P points in the +x direction.

    (b) Calculate the magnitude of P at the time calculated in part a.

    (c) Calculate the time when P make an angle of -45 degrees with the +x axis.

    (d) Calculate the magnitude of P at the time calculated in part c.

    2. Relevant equations
    magnitude= sqrt( quantities squared added together)

    3. The attempt at a solution

    I am mostly confused on part a and c. So for a, would you find where t=0 in the j direction which would leave you with only the i direction which is the +x? For part c, would have something to do with dot product because it includes an angle of 45 degrees?
  2. jcsd
  3. Sep 3, 2015 #2


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    The convention is that i represents the x direction and j the y direction.
    So for P to point in the x direction the coefficient of j in your equation must be zero. So you need to find what value of t makes that coefficient zero.

    For c the ratio of coefficient of j to coefficient of i has to be -1 (think of the angle that the line from (0,0) to (1,-1) makes with the x axis), so that gives you another equation to solve.
  4. Sep 3, 2015 #3
    Thank you, I will try that soon
  5. Sep 3, 2015 #4
    So for a) 15.0+20.0t-4.00t^2=0 then that comes to 5.66 seconds
    b) sqrt(15.0+7.00 x 5.66)= 54.6 m
  6. Sep 4, 2015 #5


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    Looks about right, except for the mention of the 'sqrt(' function.
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