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
    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

  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 9,266
    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.
    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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