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  1. #1
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    tis question..?

    can anyone help mi wif tis..?
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  2. #2
    MHF Contributor Quick's Avatar
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    Quote Originally Posted by xiaoz
    can anyone help mi wif tis..?
    Quote Originally Posted by some stupid assignment
    The time,  T minutes, taken by the moon to eclipse the sun totally is given by the forumula T=\frac{2}{v}\left(\frac{Rd}{D}-r\right) where R and r are the radii, in km, of the sun and moon respectively; D and d are the distances , in km, of the sun and moon respectively from the earth and v is the speed of the moon in km/min.

    (i) Express d in terms of D, R, r and v
    This is Simple Algebra, all you need to do is solve for d...
    T=\frac{2}{v}\left(\frac{Rd}{D}-r\right) divide both sides by \frac{2}{v}

    T\frac{v}{2}=\left(\frac{Rd}{D}-r\right) get rid of paranthesis..

    T\frac{v}{2}=\frac{Rd}{D}-r Add r to both sides..

    T\frac{v}{2}+r=\frac{Rd}{D} multiply both sides by D

    DT\frac{v}{2}+Dr=Rd finally, divide both sides by R

    \frac{DT\frac{v}{2}+Dr}{R}=d simplify

    \frac{DTv}{2R}+\frac{Dr}{R}=d and that is what d is.

    Quote Originally Posted by some stupid assignment
    (ii) Calculate the distance from the earth to the moon...
    All you need to do is substitute the given numbers into the equation that I gave you, and then you'll know!
    Last edited by Quick; June 21st 2006 at 04:03 AM. Reason: Finishing
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  3. #3
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    Hello, xiaoz1

    Part (a) cn b solvd wif just algebra . . .


    We hav: . \frac{2}{v}\left(\frac{Rd}{D} - r\right)\;=\;T

    Mult by v:\;\;2\left(\frac{Rd}{D} - r\right)\;=\;vT

    Div by 2:\;\;\frac{Rd}{D} - r\;=\;\frac{1}{2}vT

    Add r:\;\;\frac{Rd}{D}\;=\;\frac{1}{2}vT + r

    Mult by D:\;\;Rd\;=\;D\left(\frac{1}{2}vT + r\right)

    Div by R:\;\;d\;=\;\frac{D}{R}\left(\frac{1}{2}vT + r\right)


    an (b) is arithmetic, can b done wif yr calculator
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  4. #4
    MHF Contributor Quick's Avatar
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    Question 3

    Quote Originally Posted by Quick Rocks :)
    (b) The wheel of a lorry travelling at 90km/h makes 8 revolutions per second. What is the diameter of the wheel in metres?
    The wheel makes 8 revolutions per second, therefore it makes 28,800 revolutions per hour. Those 28,800 revolutions push it 90km, so each revolution pushes it  90\div28,800=0.003125

    That means the circumference of the wheel is 0.003125, and the diameter is of course the circumference divided by pi

     d=c\div \pi so I'm trusting you can find it from here.
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  5. #5
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    omg.. i cannot figure out 11(a) ii
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  6. #6
    MHF Contributor Quick's Avatar
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    Quote Originally Posted by xiaoz
    may i no 11(aii) how to do ?
    xiaoz, maybe it will help if I write out what the variables mean,

    \frac{DTv}{2R}+\frac{Dr}{R}=\text{The Distance From the Moon to the Earth}

     D=\text{The Distance Between the Sun and the Earth}

     T=\text{The Time it Takes to Totally Eclipse the Sun}

     v=\text{The Speed of the Moon}

     R=\text{The Radius of the Sun}

     r=\text{The Radius of the Moon}

    The book gave you all the numbers, so all you need to do is substitute them into the equation above.
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