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Homework Help: A kinematics problem

  1. Apr 4, 2012 #1
    1. The problem statement, all variables and given/known data

    Displacement of a particle moving in a straight line is
    s= 6 + 12t - 2t^2
    find the distance traveled till t=5.

    2. Relevant equations

    firstly i am having confusion in correctly interpreting the equation. The equation gives us the displacement in 't' th second.(correct?)
    if i put t=0 in the eq i get s=0....now what does that mean? the particle is at x=6 at t=0?

    at t=6.5 s=0 and before that s always comes out to be positive
    as the particle is moving in straight line we can just add up the displacements till t=5 to get distance covered till t=5

    3. The attempt at a solution

    but how to add up displacments. the equation gives the displacement in t th sec.
    like if i put t=5 in the equation i get s=16 but this is the displacement in 5th second only..i need the displacements in all time b/w 0 to 5 seconds
    i also referred to the solution provided in the book which goes as follows:
    ds/dt = v= 12-4t
    ( now i dont agree ds/dt would be the velocity...plz explain)
  2. jcsd
  3. Apr 4, 2012 #2


    User Avatar

    Staff: Mentor

    I suspect that by "displacement" they mean "distance from the origin". Then the equation

    s= 6 + 12t - 2t^2

    would in fact be the position of the particle with respect to time, and ds/dt would indeed be its velocity.
  4. Apr 4, 2012 #3
    assuming that the answer comes correct.....thanks.......but i want to kill the author....it discourages if one cant solve a prob even after studying deeply :((
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