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Newton's second law problem

  1. Oct 16, 2007 #1
    I have 2 problems that i cant do

    You apply a force of 0.35N up to lift a fork. The resulting acceleration is 0.15m/s2 up. Determine the mass of the fork (the answer is 35g-why?)

    A hot air balloon has acceleration of 1.10m/s2 down. The mass is 315kg.
    What is the upward force on the system?
    The balloonist wished to change acceleration to zero. Determine the mass of the ballast that must be discarded overboard (air resistance is neglected)

    please help!!
     
  2. jcsd
  3. Oct 16, 2007 #2
    i thought that Fnet=Fa-Fg=ma so i said that Fa=0.35N and a=0.15m/s2.

    Then Fg=mg so 0.35N=m(9.8)
    m=0.036kg but the answer is 35g so do u have to subtract 9.8ms2 from the given a?
    I drew a FBD but i still am confused as to how to do it.
     
  4. Oct 16, 2007 #3

    Doc Al

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    Staff: Mentor

    fork problem

    This is correct.
    Also true. Here's how you combine them:
    Fa - Fg = ma
    Fa - mg = ma
    Fa = mg + ma
    Fa = m(g + a)

    Continue and solve for m.
     
  5. Oct 17, 2007 #4
    thank you very much-these concepts are hard to wrap my head around!
     
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