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Finding the velocity and position of a particle from a graph

  1. Jul 13, 2011 #1
    1. The problem statement, all variables and given/known data

    Consider the plot below describing the acceleration of a particle along a straight line with an initial position of −15 m and an initial velocity of −3 m/s.

    6ie0py.png

    A. What is the velocity at 5 s? Answer in units of m/s.
    B. What is the position at 5 s? Answer in units of m.

    2. Relevant equations

    v = v0 + at

    3. The attempt at a solution

    For part A, V = -3 + 1(5)
    = 2

    For part B, V = -15 + 2(5)
    = -5

    ---------------

    Both of these answers are wrong, and I'm not sure why...There was another question with this graph that asked to find the position and velocity of the particle at 1 s, and this worked for that, so I'm not sure what to do now...
     
  2. jcsd
  3. Jul 13, 2011 #2

    SteamKing

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    Staff Emeritus
    Science Advisor
    Homework Helper

    The equation v = v0 + at is valid only when a is constant. From the diagram, you can easily see that a is not constant over the range of time.

    You have not indicated any equations to determine position as a function of velocity or acceleration.

    Are you sure you know how to relate position, velocity, and acceleration?
     
  4. Jul 13, 2011 #3

    gneill

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    Staff: Mentor

    According to the diagram the acceleration at time t=0 is not 1. And the acceleration changes at time t=2. So you'll have to break up the motion into two parts, the first covering the time interval from t=0 to t=2, the second covering the interval from t=2 to t=5.

    For part B you're looking for a position, not a velocity. There's another kinematic equation that applies when you want to find the distance given the initial position, velocity, and acceleration.
     
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