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Cable pulling a car

  1. Feb 8, 2010 #1
    1. The problem statement, all variables and given/known data

    A 1010 kg car is held in place by a light cable on a very smooth (frictionless) ramp, as shown in the figure . The cable makes an angle of 31.0 degrees above the surface of the ramp, and the ramp itself rises at 25.0 degrees above the horizontal.

    Draw a free-body diagram for the car.
    Draw the force vectors with their tails at the front bumper of the car. The location and orientation of your vectors will be graded. The exact length of your vectors will not be graded.

    The vectors used for the FBD are:

    Fthrust - The thrust force
    FN - The normal force
    FT - The tension force
    w - the weight
    f - the force of friction

    carnewcopy.jpg

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    So the two i am unsure about are the BLUE vectors (the tension and the thrust)

    Can anyone help me with this please and let me know if any other vectors i drew are incorrect.

    Thank you :)
     
  2. jcsd
  3. Feb 8, 2010 #2

    mgb_phys

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    Friction will depend on which way the car is moving - it could be pointing up or down the slope.
    Thrust is trickier - you can define it in any direction you want, I would have thought up the slope, parallel with the slope, was most useful but it's not a very well posed question.
     
  4. Feb 8, 2010 #3
    the car doesnt even seem to be moving i guess....

    "A 1010 kg car is held in place by a light cable"

    hhmmm and its a frictionless surface so why would there even be a friction vector in the FBD?

    This question seems very poor to me too.... but i still have to do it somehow.
     
  5. Feb 8, 2010 #4
    So i moved the F_thrust to be along the dotted line but now i just am not sure where to put the friction vector....
     
  6. Feb 8, 2010 #5

    mgb_phys

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    If it's a frictionless slope there isn't one.
    Note stationary doesn't imply no friction. If you park a real car on a slope you better hope there is some friction force between the tires and the road!
     
  7. Feb 8, 2010 #6
    "Note stationary doesn't imply no friction. If you park a real car on a slope you better hope there is some friction force between the tires and the road!"

    In this case the friction would be up the slope correct?

    This is what i am confused about though.
    Its for my online hw so it gives me the vectors i need and i just have to place them in the right direction..... so i think i have to use the friction vector which doesnt make any sense because if its on a frictionless slope then how can there be a friction force?
     
  8. Feb 8, 2010 #7
    Here is the new FBD i drew up.

    Does it look right? Do any of the vectors need to be placed somewhere else or does this look ok??

    Thank you

    carnewcopycopy.jpg

    Draw a free-body diagram for the car.
    Draw the force vectors with their tails at the front bumper of the car. The location and orientation of your vectors will be graded. The exact length of your vectors will not be graded.

    The vectors used for the FBD are:

    Fthrust - The thrust force
    FN - The normal force
    FT - The tension force
    w - the weight
    f - the force of friction
     
  9. Feb 8, 2010 #8
    Could i also get some help finding the normal force....

    How hard does the surface of the ramp push on the car?

    isn't the normal force just the y component vector of the weight?

    so Fn = (m)(g)sin(90-25)
    Fn = 8971 N

    but this is wrong? where did i go wrong with this?

    thanks for any help
     
  10. Feb 8, 2010 #9

    mgb_phys

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    The normal force always acts normal to the surface (ie at right angles)
    Friction always acts to prevent movement
     
  11. Feb 8, 2010 #10
    The diagram i drew above (post #7) is wrong according to my online hw.

    can you see where it is wrong?

    Thank you
     
    Last edited: Feb 8, 2010
  12. Feb 8, 2010 #11

    mgb_phys

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    You are right,
    The friction could be either up or down - or you could just change the sign.
    Thrust could be in any direction you want - I would say your choice makes most sense.

    Don't worry too much - often the online tests are pretty dumb. Even textbook questions can be wrong more often than you woudl think - quite often the questions are copied/cut down from longer questions and some vital part is missed. Don't let it get to you.
     
  13. Feb 8, 2010 #12
    Well the only reason why i am worrying is cause if i dont get it right i will get a zero for this question and i really dont want that :( plus there are 2 other parts to this question that i am having trouble with and i think the correct FBD will help me visual how to get the answers to them.
     
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