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    heyyalliee's Avatar
    heyyalliee Posts: 37, Reputation: 1
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    #1

    Nov 17, 2010, 10:55 AM
    Friction and Pressure
    To prevent damage to floors (and to increase friction), a crutch will often have a rubber tip attached to its end.
    If the end of the crutch is a circle of radius 1.3cm without the tip, and the tip is a circle of radius 2.3cm, by what factor does the tip reduce the pressure exerted by the crutch?

    How do I go about doing this problem. I know P=F/A
    A=Pi(r)^2
    So I can find both Areas. But what do I do after that? Also Should I change the radius to meters instead of cm?

    Thanks:)
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Nov 17, 2010, 12:09 PM

    The same force is applied on the crutch and the tip. So, you can find the pressure in terms of the force applied, let's say force F.

    In this case, you can leave the units in centimetres since the ratio between the two pressures you are looking for will cancel out the units.
    heyyalliee's Avatar
    heyyalliee Posts: 37, Reputation: 1
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    #3

    Nov 17, 2010, 12:42 PM
    Comment on Unknown008's post
    How do I know the force applied? Gravity?
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #4

    Nov 17, 2010, 12:51 PM

    We don't know the force applied, but since the ratio cancels out the force as well, we don't need to worry about it!









    Ratio:

    Can you complete it? :)
    heyyalliee's Avatar
    heyyalliee Posts: 37, Reputation: 1
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    #5

    Nov 18, 2010, 08:18 AM
    Comment on Unknown008's post
    haha maybe. So the F cancels and I have A1/A2.
    Above formulas indicate that both A's are areas of crutch. Area of Crutch is Pi(1.3cm)^2. Which I get to be 5.31. So 5.31/ 5.31 is 1, but its saying that's not the right answer.
    heyyalliee's Avatar
    heyyalliee Posts: 37, Reputation: 1
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    #6

    Nov 18, 2010, 08:20 AM
    Comment on Unknown008's post
    oh oh,I got it. I took the Area of the tip and divided by area of the crutch( circle) and got 3.1!
    Pi(2.3)^2/Pi(1.3)^2
    Thanks for your help!
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #7

    Nov 18, 2010, 08:22 AM

    Yes, that's it (^^,)b

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