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Basic Newtons law's help

  1. Nov 8, 2007 #1
    [​IMG]

    Frictionless surface, pulled along a horizontal line, string has no mass

    Calculate the mass m


    The answer says 17,1 kg, i don't know if it is correct,
    can sum1 plz give a solution with the steps:smile:
     
  2. jcsd
  3. Nov 8, 2007 #2
    The answer is correct. You have to show some work for us to help you.
     
  4. Nov 8, 2007 #3
    I dont know how to put it.

    I can work out the force of the string and the accel but i cant do it the other way around

    i have taken the answer and tried to work backwards and see if i get 80N force on the sting between the block

    For block m (17,1 kg's)

    Fr = ma
    F2 = 17,1a

    For 15 kg mass

    Fr = ma
    150 - F1 = 15a
    F1 = 150 - 15a

    Newton's 3rd law F1=F2

    150 - 15 = 17,1a

    32,1 a = 150
    a = 4,67289..

    F = ma
    F = 17,1 * 4,67289..
    F = 79,906..
    F = 80 N

    so the answer is correct, but i cant seem to work out the mass
    I dont know where to start,

    I dont know how to get the acceleration:confused:
     
  5. Nov 8, 2007 #4
    Have you tried drawing the free body diagram for the 15 kg mass?
     
  6. Nov 8, 2007 #5
    You calculate the acceleration by isolating the 15 kg mass, and taking into consideration the applied force and the tension force.
    Fnet = Fapp - Ft
    ma = 150 - 80
    15a = 70
    a = 4.67
     
  7. Nov 8, 2007 #6
    Now to find the unknown mass, you have to use the entire free body diagram. This time you have to ignore the tension force.

    Fnet = Fapp
    ma = 150
    (15 + m)(4.67) = 150
    (15 + m) = 32.1
    m = 32.1 - 15
    m = 17.1 kg
     
  8. Nov 8, 2007 #7
    You must first isolate the 15 kg mass and find its acceleration. So now, you can use this value to determine the mass of the unknown mass.
     
  9. Nov 8, 2007 #8
    Thanks guys
    this is the other sullution i got:
    F = ma
    a = F/m
    call the object of mass 15kg object 1
    call the object of mass m kg object 2
    acceleration of system (object 1 + object 2) = acceleration of object 1 = acceleration of object 2 = 150N / (15kg + m kg)
    acceleration of object 2 = 80N / m kg
    therefore:
    150 / (15 + m) = 80 / m
    multiply through etc
    1200 + 80m = 150m
    1200 = 70m
    m = 17.14
     
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