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Homework Help: Problem with Work Done by Friction, Velocities-Extremely Hard

  1. Mar 16, 2008 #1
    Problem with Work Done by Friction, Velocities--Extremely Hard

    1. The problem statement, all variables and given/known data
    Here is the known data: The track has to be 1500m max, less track can be used. There is a minimum of 3 hills (you decided how steep and how high they are). There are two turns (track makes one lap). The coefficent of friction around the turns is zero (you decide the radius of curvature of the turns and the angle of the bank of the turns so your sled doesn't go off track). There is a massless cable on a frictionless pulley that pulls your sled to the top of the first hill with an acceleration of 0.5m/s^2. The mass of the sled is 350kg, with 2 people it is a max of 600kg. The sled can never exceed 25m/s. At the end of the track there is a natural braking system with a coefficent of fricition of 0.3.

    A)Since the sled had an accelaeration going up that first hill, it has an initial velocity upon reaching the first hill. Determine that velocity assuming the sled started from rest.

    B) Determine work done by fricition going up each hill, down each hill, and along any straightaway of a fully loaded sled.

    C) Dtermine the velocity at the top and bottom of each hill, and at the beginning and end of each straight away.

    D) Based on the velocity of your sled going into each turn determine the agnle required to bank the turn to keep your sled on the track

    E) Based on the velocity of your sled coming into the final straightaway before it stops, dtermine how much track is needed for the frictional forces to do their work and bring that sled to a safe stop.

    2. Relevant equations

    Not sure, I suck at physics and our teacher has done a good job of teaching

    3. The attempt at a solution

    Need help in any form I can get it. Been working on this by myself for a week and have gotten no where
  2. jcsd
  3. Mar 16, 2008 #2
    will someone please help? I just need to be able to start on this. Thanks!
  4. Mar 16, 2008 #3
    pretty please?
  5. Mar 16, 2008 #4
    The questions sound like they're asked in incremental order. So, for part a, figure out the net acceleration of the car as it goes up the hill. The tension in the rope pulls the car up, and it acts along the hill. Gravity acts straight downwards. So, find the net acceleration up the hill by doing some vector and trig stuff. Then use kinematics to figure out the velocity when it reaches the top.

    b: Work is (simply) force times distance. Figure out the frictional force acting on the car and the distance through which it acts. (although the problem seems to state that there is no friction)

    c: Kinematics, similar to part a. Or, conservation of energy for a simpler approach.

    d: Draw a free-body diagram and break all forces into their components. Solve for theta.

    e: I'd use the work-kinetic energy theorem.

    edit: I think this might be too much help though. The hardest part of a problem like this is to think it through, figuring out how to set it up and start solving it.
  6. Mar 16, 2008 #5
    to much help?

    Yeah it sucks to NEVER have had physics before, have a teacher who DOESN'T teach, and then be given a problem like that. I don't understand physics at all and I reach out for help and you give me really nothing. I can look up formulas, that's easy, not help. It's the application of those formulas that is hard and WHY you are taught at school instead of it being an independent study course. So thanks for no help. Your response is like you being sick and going to see the doctor and the doctor telling you "here are your symptoms, you figure it out"
  7. Mar 16, 2008 #6
    Woah, no need to lash out like that. I don't want to hold your hand through a long problem like this, so I outlined the steps I would use to solve it, ie. helping you set it up. But anyway, the first step in any problem like this is to draw a free-body diagram. Depending on whether you know the hill's height (distance up the y-axis), or the hill's length (distance on the s-axis, which runs up the hill), you'll want to find the net acceleration of the car in the y- or s- direction. Either way you'll have to resolve one of the forces (tension or gravity) into a sloped component.

    So let's say you know the hill's height h. Draw the FBD and write out Newton's 2nd law for the forces in the y direction. Gravity points straight down, and the tension points in the direction of the slope. What we want to find is the acceleration in the y-direction, so you must resolve the tension force onto the y-axis. Then you can use kinematics to find the final velocity after it accelerates at this net acceleration a distance h. Was that explicit enough? It's going to take a long time to write this out for every part.
  8. Mar 16, 2008 #7
    Sorry it's just frustrating being a student who goes to class everyday, pays for that class, and gets nothing out of it. I also go to a school that doesn't have physics tutoring or SI sessions. Our teacher is crap and is NEVER prepared for his classes. That helps, it's really the work done by friction part that I don't understand
  9. Mar 16, 2008 #8
    Use the definition of work. The force that friction does is constant on each hill, since it is u * n, where n is the normal force (which is NOT m*g here, since you're on a slope.) So, the work done by friction is just that constant force acting through some distance. But, the distance through which it acts is not the vertical height of the hill, but the length of the hill.
  10. Mar 16, 2008 #9
    Yeah it looks like I'm going to have to find someone here to walk me through it, thanks for trying through
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