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Moving object

  1. Jan 29, 2014 #1
    1. The problem statement, all variables and given/known data
    An object moving on the x axis with a constant acceleration increases its x coordinate by 80 m in a time of 5.0 s and has a velocity of +20 m/s at the end of this time. Determine the acceleration of the object during this motion.
    x = 80m t = 5.0 second v = + 20m/s a = while in motion

    2. Relevant equations
    80 m = 100 m + 1/2 a 25 s2
    -1.6 m/s2


    3. The attempt at a solution
    Supppos to be positive I don't know why I get negative acceleration?
     
  2. jcsd
  3. Jan 29, 2014 #2
    Can we apply this law?
    Velocity is positive direction and if the velocity increases with time, then acceleration is said to be positive. In this case, if the velocity is reducing with time, then the acceleration said to be negative
     
  4. Jan 29, 2014 #3

    BvU

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    Equation is not good. Where does the 100 come from?
    This time, constant acceleration during 5 seconds gives an increase in velocity. There is an equation for that.
    The other equation is the full expression for x(t). Two equations, initial velocity and acceleration unknown. Should be solvable.
     
  5. Jan 29, 2014 #4
    is it Xt = X0 + V0 . t + 1/2 . a .t^2?
     
  6. Jan 29, 2014 #5
    100 m came from 20 m/s . 5 s
     
  7. Jan 29, 2014 #6

    BvU

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    #4 is correct.

    In #5 you get meters allright, but that expression is for uniform (constant speed) motion. In this case the thing accelerates, so the average speed in these 5 sec was less.


    In #4 you don't know X(5) or X(0), but you know their difference, which is all you need.
    You also don't know V(0) so you can't plug in a numerical value.

    You still need an equation for V(t), where you plug in the knowledge that V(5 sec) = 20 m/s.
     
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