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Homework Help: Particle in plane, find position vector

  1. Feb 18, 2013 #1
    A particle of mass m moves in the x-y plane; its acceleration in m/s^2 is given by the
    expression ->a = −6iˆ for t ≥ 0 secs . At t = 0 secs, the object is at the point (x(sub 0), y(sub 0) = (36,0); coordinates are in meters. At t = 4 secs , the object is at the point x(sub 0), y(sub 0) = (36, 40) (Hint: to answer all of the following questions, use the given information to find the position vector r of the particle).

    a. At what time (if any) in seconds is the particle at its maximum possible positive value of x? If there is no such maximum, state clearly why this is so

    b. At what time (if any) in seconds is the particle moving parallel to the x-axis? If the particle is never moving in this way, explain clearly how you deduced this.

    c. Find the time t > 0 in seconds at which the particle crosses the y-axis.

    I don't understand how to do any of this. Can someone please show me or send me in the right direction at least? This is for calc based physics 1 and nothing like this has been taught to me.
  2. jcsd
  3. Feb 18, 2013 #2


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    Surely you have been taught about vectors and about basic kinematics. The problem can be broken into two kinematics problems, one horizontal and one vertical.
  4. Feb 18, 2013 #3
    You are given the acceleration and the initial position. You are not given the initial velocity, though, so the motion is not fully defined. However, you are given the position at some later time, so you can determine the initial velocity and thus all the details of motion.
  5. Feb 18, 2013 #4
    True, but not this in particular. I got it now. I wasn't familiar with solving for the constants in both the i and j direction.
  6. Feb 18, 2013 #5


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    When solving problems one is supposed to apply the principles that have been learned to new situations.
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