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Need Help

  1. Oct 9, 2006 #1
    Hey everyone,

    Just wondering if anyone could help me with my physics homework. It's my first time ever doing physics and I was always terrible at maths. It needs to be in soon so I'm kind of freaking out :frown: If anyone could either show me how to answer these questions or even pm I'd be very grateful.

    1. [a] The radius of a circle is 2mm. Write down an expression for its area A in scientific notation of the form A = a x 10 to the power of -b m to the power of -2, where a and b are constants.
    A sphere of lead has a mass of 0.1 kg. If the density of lead is 11.3 x 10 to the power of 3 kg m to the power of -3, find the radius of the sphere [remembering that density = mass over volume]

    2. [a] What is the speed in kilometres per second of a car travelling at a constant speed of 60 miles hr to the power of minus 1? [1 mile = 1.609 km]

    What is the speed of the car in metres per second? [1km = 10 to the power of 3 m]

    [c] What is the average speed for a trip of 157 km that requires 2.45 hr?
    [d] How far in km will a car go in 3.5 hr at a constant speed of 95 km hr to the power of -1?

    I know this might seem really easy to people who are into physics but to me this is really hard. :blushing:
     
  2. jcsd
  3. Oct 9, 2006 #2
    well for parts 2 a) b) and c) you could use speed = distance
    --------
    time
    and then just rearrange the formula for d), that is how i would do that anyway! Hope it helps!
     
  4. Oct 9, 2006 #3

    Hurkyl

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    What have you tried on these? Have you at least had some ideas, even if they didn't work?

    P.S. the typical way to write

    [tex]a^b[/tex]

    in a text format is as a^b. (Make sure to use parentheses if your base or exponent are expressions, and not just a single symbol)
     
  5. Oct 9, 2006 #4
    I can't even attempt them. Speed = distance?? :confused:

    I haven't got the slightest clue what you're trying to say. I've always been very slow when it comes to anything involving maths. I took a year out before college so even basic maths is dusty. Really going to be stuck if this is like a different language.
     
  6. Oct 9, 2006 #5

    radou

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    Hurkyl meant that you can write 'a^b' instead 'a to the power of b'.
     
  7. Oct 9, 2006 #6
    Oh okay :redface: thanks
     
  8. Oct 9, 2006 #7
    and I think staceyw meant to write [tex]speed = \frac{distance}{time}[/tex]
     
  9. Oct 9, 2006 #8
    Actually #2 parts a and b are giving you a value in one type of units and asking you to convert it into another type of units. (This is sometimes called dimensional analysis.)

    You want to set up a multiplication problem to cancel units, like so:
    to get 3 weeks into days
    [tex] \frac{3weeks}{1}\times\frac{7days}{1week}=21days [/tex]

    Do you see what happened? Take your value (in this case 3 weeks) and multiply it by a convenient form of 1(in this case 7days/1week). The weeks canceled, and you're left with days.
     
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