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Question is about the acceleration

  1. Mar 19, 2015 #1
    1. The problem statement, all variables and given/known data
    The system shown is released from rest and moves 50 cm in 1.0 s. What is the value of M? All surfaces are frictionless.

    X = 0.5 m
    t = 1sec
    m1 = 3kg
    m2 = M

    However, my question is about the acceleration. I'm just curious why I cannot use vf = v0 + at equation instead of x = x0 + v0t + 0.5at^2

    2. Relevant equations
    x = x0 + v0t + 0.5at^2
    vf = v0 + at

    3. The attempt at a solution

    X = X0 + V0t + 0.5at^2
    0.5m = 0 + 0 + 0.5a(1)^2
    a=1m/s^2

    M*g = (3kg + M)a
    M(g-a) = 3a
    M = 3a/(g-a) = 0.34kg

    So I got M = 0.34kg. I know this is the answer but now my question is the acceleration.
    I know I got 1m/s^2 from X = X0 + V0t + 0.5at^2 equation, but why can I not use Vf = V0 + at equation?
    If I use Vf = V0 + at then I get a=0.5m/s^2 since vf = 0.5m/s and v0 = 0m/s and t=1.
    Why do they have different acceleration?
     

    Attached Files:

  2. jcsd
  3. Mar 19, 2015 #2
    Where did you get Vf = 0.5 m/s from?

    Chet
     
  4. Mar 19, 2015 #3
    Since it moved 0.5m in 1 sec, isn't it 0.5m/s??
     
  5. Mar 19, 2015 #4
    That's the average velocity, not the final velocity. The velocity at time zero is 0 m/s and the final velocity at the end of 1 sec is 1m/s. So, what's the average of these two?

    Chet
     
  6. Mar 19, 2015 #5
    0.5! Thank you so much :)
     
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