Finding mass within a pulley system. Finding coefficient of friction

  1. Ok so I'm in college upgrading, and doing my physics 20. I have a hand in assignment due Wednesday and I can't for the life of me figure out these two questions.

    6. The 4.0kg block shown accelerates across a frictionless horizontal table at 1.5m/s2.
    Find the mass of object m1
    [​IMG]
    A. 0.61kg B. 0.72kg C. 6.0kg D. 26kg

    For m1 I have: FN=Fg. Fg=mg, so 39N. FT-0(no Ff)=ma . So FT is 6.0N. And then I'm lost.


    8. A 45kg toboggan and rider decelerate on level show at 0.53m/s2. What is the coefficient of friction between the toboggan and the snow?

    A. 0.012 B. 0.054 C. 0.22 D. 0.53

    I got that FN=Fg. Fg=mg so 441.45N(rounded to 2SD). Fnet=ma so 24N. And then I'm lost. I know that μ=Ff/FN

    Someone help me please!
     
    Last edited: Oct 11, 2011
  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    You have a combined mass of M1 + M2 being accelerated (together). Write an equation for the force that is needed to accelerate such a mass.
     
  4. I got the mass...I think. FT=ma. 6.0N=m1(9.81m/s2) So m1=0.6112kg

    Now for the other one...
     

  5. ---
    Ok I think I solved it. Ff is Fnet in the direction of acceleration? So since it's decelerating, wouldn't it accelerate in the direction of the friction?

    So Ff=ma
    Ff=24N

    μ=Ff/FN
    μ=24N/441N
    μ=0.05442
     
  6. NascentOxygen

    Staff: Mentor

    Not so fast. You haven't got this one right, yet. You must determine the force needed to accelerate the combined masses, since they accelerate together.
     
  7. But I only need the mass of the one.
     
  8. SammyS

    SammyS 9,056
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    NascentOxygen has asked you this question for a reason. Answering it will help you answer the question asked in the problem. More importantly, it should help you understand how to approach such a problem in general.
     
  9. I'm not having problems with the others, just when I need to find a mass with minimal info.
    Fg1-FT=ma
    Use the total mass of the system. The difference of force of gravity and tension on the rope would be the net force acting on the system.
     
    Last edited: Oct 11, 2011
  10. NascentOxygen

    Staff: Mentor

    The question supplies all the information you need. You have a mass (M1 + M2) being accelerated. The force that is causing the acceleration is the weight of M1.

    One equation, one unknown.
     
  11. NascentOxygen

    Staff: Mentor

    That looks right.
     
  12. Thanks for your help. I tend to overthink these things
     
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