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Problems with calculating the acceleration

  1. Sep 15, 2013 #1
    1. The problem statement, all variables and given/known data
    A block moves 11 meters in 5 seconds on a straight line. Calculate the acceleration.


    2. Relevant equations
    [itex]v^2_x=v^2_{0x}+2a_x(x-x_0)[/itex]

    [itex]v_x=v_{0x}+a_xt[/itex]

    [itex]x=x_{0}+v_{0x}t+\frac{1}{2}a_{x}t^2[/itex]

    3. The attempt at a solution
    [itex]a_x=\frac{(\frac{11m}{5s})^2}{2\cdot 11m}=0.22m/s^2[/itex]

    [itex]a_x=\frac{(\frac{11m}{5s})}{5s}=0.44m/s^2[/itex]

    [itex]a_x=\frac{11m}{1/2\cdot (5s)^2}=0.88m/s^2[/itex]

    Why am I getting 3 different answers? In my textbook, it lists the equations as being practically the same (straight-line motion with constant acceleration).
     
  2. jcsd
  3. Sep 15, 2013 #2

    lewando

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    Assuming V0x = 0, then Vx ≠ 11/5 m/s.
     
  4. Sep 15, 2013 #3
    yes, V0x=0 because it starts at rest. But how do you know the velocity is not 11/5 m/s then?
     
  5. Sep 15, 2013 #4

    lewando

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    11/5 m/s is the average velocity. Vx is the instantaneous velocity at the 5m point.
     
  6. Sep 15, 2013 #5
    Ohhhh.
    And because of that, I cannot use any of the formulas which includes the Vx therefore the only correct equation for my current problem is the 3rd, correct?
     
  7. Sep 15, 2013 #6

    lewando

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