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Work/Energy equation on inclined plane

  1. Jul 14, 2015 #1
    I've been trying to work out what I'm doing wrong on this question but I just don't get it.

    'A cyclist starting from rest freewheels for 120m down a slope of 1 in 30. At the bottom of the slope the road becomes horizontal, and the cyclist stops without using his breaks after going a further 40m. If the total mass of the bicycle and rider is 72kg, find the resisting force, assumed constant'

    From what I could find 1 in 30 grade is 3.75 a degree decline.

    Potential energy= (mgh)72*10*sin(3.75)(120)
    Force of gravity over distance(mgs) = 72*10*cos(3.75)(120) + (72*10*40)
    Force over distance vs resistance over distance = 72*10*sin(3.75)-R*(72*10*cos(3.75)*120) + (72*10*40) = Crazy wrong answer.

    I've been unable to figure this one out for quite a while, I occasionally go back to it. This is just one of many incorrect answers i've ended up with.
    (The answer given in the book is 18N)

    I've also tried working backwards to find the forces of gravity from knowing the resistance is 18N, didn't help, ended up with ridiculously small numbers.

    Help.
    Shaun.
     
    Last edited: Jul 14, 2015
  2. jcsd
  3. Jul 14, 2015 #2

    CWatters

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    Check that. I think you worked out the slope in % not degrees.
     
  4. Jul 14, 2015 #3
    h=1 s=30
    (h=1*4) /(s=30*4) = 4/120 = height of 4m

    y4/x120 = 0.03 inv Tan(0.03) 1.7 degrees?
     
  5. Jul 14, 2015 #4

    haruspex

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    There is no point in calculating the angle. You are given the sine of the angle (1/30), and you will only be needing trig functions of the angle. They can all be determined directly from the sine without ever knowing the angle in degrees. This will make for less work as well as improving precision.
    Good technique is to work the problem entirely symbolically, ##\theta## for the angle etc., until you have a formula for the answer. Only then should you plug in numbers.
     
  6. Jul 14, 2015 #5
    My issue wasnt so much that but I dont know how to work out the resistance without the velocity, and I cant find the velocity without the resistance, and I am unsure how to deal with the energy situation without one of those in this situation. I've been over this same question now so many times I've lost perspective on it.

    *It would help if someone could show me their working on this question.
     
    Last edited: Jul 14, 2015
  7. Jul 14, 2015 #6

    billy_joule

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    You don't need velocity as the resistive force is constant.
    The work done by the resisting force must be equal to the initial energy, so:

    mgh = Fd

    where d = 120m + 40m, and F is the only unknown
     
  8. Jul 14, 2015 #7

    haruspex

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    You didn't mention that in the OP.
    why did you switch from sin to cos?
     
  9. Jul 15, 2015 #8

    CWatters

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    As haruspex said, you don't actually need the angle but it would be a lot simpler to just do

    Angle = Sin-1(1/30) = 1.9 degrees.
     
  10. Jul 15, 2015 #9
    Billy joule sorted it for me, thanks guys, I've been struggling on this one for a long time.
     
  11. Jul 15, 2015 #10

    Because I'm stupid and it was late!
    Yeah I should have been more clear but I actually wasnt sure about the grades either(obviously) but you guys cleared both up.
    You have no idea how much I got stuck on this question, I've never had to ask for help on a physics question before I just couldnt figure out what I was supposed to do despite having done many work energy equations before! I think it was the fact I read 'constant resistance' but I didn't take it into account, I kept trying to figure out the varied resistance like an idiot.
     
    Last edited: Jul 15, 2015
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