Finding mass

  1. 1. The problem statement, all variables and given/known data
    You apply a force of .35N up to lift a fork, the resulting acceleration is .15m/s2. What is the mass in grams.


    Please help I don't know where to start with this simple question.
     
  2. jcsd
  3. PhanthomJay

    PhanthomJay 6,228
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    Start with identifying the net force acting on it, and use Newton's 2nd law. Please show an attempt.
     
  4. The net force would not be zero and I am only given the applied force. Without a mass I don't know how I can get the force of gravity.

    So as far as I can tell I have .35 Fg = m .15m/s2
     
  5. SammyS

    SammyS 8,269
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    Let mass = m & use symbols where needed.

    What does Newton's 2nd Law say?
     
  6. 0.35 Fg = m .15m/s2
    0.35 - (m 9.8 m/s2) = m 0.15ms2

    The second law says that if the net force is not zero there is an acceleration in the direction of the force.
     
  7. SammyS

    SammyS 8,269
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    You apply a force of 0.35 N upward.

    If the mass of the for is m, what force is gravity exerting downward on the fork?

    What is the net force being exerted upon the fork?
     
  8. Wouldn't I need the mass of the fork (m) to calculate the force exerted by gravity?
     
  9. SammyS

    SammyS 8,269
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    You will have "m" in two places in your equation. Use algebra to solve for m.

    I repeat:
    What is the net force being exerted upon the fork?​
     
  10. I can get up untill
    Fapp - m a(gravity) / a(applied) = m
    Now I am a bit confused on my next move. Is this right so far? Since m on the left is being multiplied by the acceleration of gravity I think I should divide to get rid of it. But once I do the right side would cancel out to zero.
     
  11. SammyS

    SammyS 8,269
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    Fapp - m g = m a

    0.35 - m g = m (0.15)

    Not sure what you use for g: 10 m/s2 or 9.8 m/s2 or 9.81 m/s2

    Put in the appropriate number for g & solve for m.
     
  12. Yes, but as I said earlier I can get up until

    0.35 - m g / 0.15 = m

    I can't successfully eliminate the LH m. I tried dividing and adding it to the RH
     
  13. PhanthomJay

    PhanthomJay 6,228
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    Before you divide by 'a', your correct equation, per post 5 2nd line, was

    0.35 - (m 9.8 m/s2) = m 0.15ms2

    leaving off units, then

    0.35 - (9.8m) = 0.15m

    Now this is algebra, add 9.8m to both sides

    0.35 = 9.95m

    Now solve for m and convert to grams.
     
  14. Thanks, that makes allot of sense.
     
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