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Homework Help: Physics G.11 Question on Forces/Newton's Laws and Friction

  1. Jul 9, 2013 #1
    Hi, i am very confused on how your supposed to do these two quesitons. I am currently taking a intensified g11 course during the summer, and i think the teacher is going to fast, and im not sure how im supposed to do this. Please help explain each question out thoroughly, also it would be best if you could help me with what equations i should use.

    1. In the horizontal starting area of a four person boblsed race, the four athletes, with a combined mass including outfits of 295kg, exert a minimum horizontal force of 41 N [fwd] to get the 315kg sled to be gin moving. After the sled has traveled for almost 15m, all four people jump into the sled, and the sled then experiences a kinetic friction of magnitude 66N. Determine the coefficient of :
    (a) static friction
    (b) kinetic friction

    2. To move a 45kg wooden crate across a wooden floor (μ = 0.2), you tie a rope onto the crate and pull on the rope. While you are pulling the rope with a force of 115N, it makes an angle of 15 degrees with the horizontal. How much time elapses between the time at which the crate just starts to move and the time at which you are pulling it with a velocity of 1.4 m/s.

    I know this may be alot of trouble for you, so thank you for taking your time to help me!
  2. jcsd
  3. Jul 9, 2013 #2


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    Hello Michael! Welcome to PF! :wink:

    Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:

    (start with Question 2 … what acceleration do you get?)
  4. Jul 9, 2013 #3


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    1. For part (a), you must draw a free-body diagram of the bobsled. What are all the forces on the sled, and in what directions do they point? For part (b), you must now do a free-body diagram on the full bobsled. What are the forces on the sled? In what direction do they point?

    2. I'm simply going to repeat my suggestions for # 1. In fact, the most important thing I can do for you right now is to point you to the Problem-Solving Strategy, which you can download on this page. I would recommend the mechanics-tailored version for the problems you're doing.
  5. Jul 9, 2013 #4
    Thanks Tim
    So following standard procedures.

    2. Relevant equations
    Fn= Fg - FappSinTheta
    Fn=mg - FappSinTheta
    Ff = Fnμ

    3. The attempt at a solution
    Fn=Fg - FappSinTheta
    =(45kg)(9.8) - 115sin15
    Ff = Fnμ = 411(0.2) = 82.2N

    Im not sure if i did this correctly, i am quite confused about what to do.
  6. Jul 9, 2013 #5


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    You need to write down all the forces in the x direction. What you've written down is Newton's Second Law in the y direction. But there is no motion in that direction, since [itex]115\sin(15^{\circ})<45[/itex]. You must sum the forces in the x direction, and set that equal to the mass times the acceleration in the x direction.
  7. Jul 10, 2013 #6


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    (just got up :zzz:)
    correct …

    the vertical acceleration is zero, so the vertical components of all the forces add to zero, enabling you to find the normal force Fn :smile:
    correct …

    now use the horizontal components of all the forces in good ol' newton's second law (Ftotal = ma), to find the acceleration :wink:
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