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Help with periods and oscilation *and* wavelengths

  1. Mar 29, 2015 #1
    1. The problem statement, all variables and given/known data

    Water waves in a shallow dish are 6.0 cm long. At one point the water oscillates up and down at a rate of 4.8 oscillations per second.
    a. What is the speed of the water waves?
    b. What is the period of the water waves?

    2. Relevant equations
    frequency = 1/period
    Speed = wavelength x frequency

    3. The attempt at a solution
    Guessing 6.0 cm is thr wavelength but that wouldn't help much since I'm looking for the period and I don't even know what oscillations are.
     
    Last edited by a moderator: Mar 29, 2015
  2. jcsd
  3. Mar 29, 2015 #2

    berkeman

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    Staff: Mentor

    You show a relevant equation for calculating the speed from the wavelength and frequency. You are given both in the problem statement.

    Be sure to be careful with your units. You should convert everything into MKS units for this problem.
     
  4. Mar 29, 2015 #3
    Do I convert the 6.0 cm to meters because its wavelength? I don't understand what 4.8 oscillations a second translates into frequency or speed
     
  5. Mar 29, 2015 #4

    berkeman

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    Yes, convert 6cm into meters, so you can calculate the speed.

    When they say 4.8 oscillations per second, that is the same as saying that the frequency is 4.8Hz. Hz is "Hertz", and is equal to one oscillation per second. You write the units of Hz as 1/s. Try to always carry along the units for each quantity in your calculations, and cancel them when you have the same units in the numerator and denominator of a fraction. That helps you to use units to check your calculations.
     
  6. Mar 29, 2015 #5
    So it would be 0.288m/s?
    And for b) is the answer 4.8s?
     
  7. Mar 29, 2015 #6

    berkeman

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    Yes on the first one -- v = 0.06m * 4.8Hz = 0.288m/s.

    On the 2nd one, they have given you the frequency as 4.8Hz. Use your other equation to figure out the period that is associated with that frequency...
     
  8. Mar 29, 2015 #7
    But I'm looking for period and I only have frequency and the number "1".
     
  9. Mar 29, 2015 #8

    berkeman

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    What does that relevant equation mean? What does it mean to take 1/anything? What is 1/2? What is 1/10? :smile:
     
  10. Mar 29, 2015 #9
    Are you asking me to divide 1 by so and so? I don't know what that so and so is.
     
  11. Mar 29, 2015 #10

    berkeman

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    If frequency = 1/period, and you have the frequency and want to find the period, what simple algebraic manipulation do you use to get the equation for period in terms of frequency?
     
  12. Mar 29, 2015 #11
    Okay just realized where you were going with the last hint. It's probably incredible simple but I draw a blank as to what 1 can be divided to get a number as big as 4.8
     
  13. Mar 29, 2015 #12

    berkeman

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    Just to review some algebra...

    If y = 1/x, and you are given y, how do you solve for x?
     
  14. Mar 29, 2015 #13
    I would've thought it was to multily y and 1 to get 4.8 but 1 divided by 4.8 clearly isn't 4.8
     
  15. Mar 29, 2015 #14

    berkeman

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    Stick with my more general algebra example for a bit. In algebra, we manipulate equations to change there form in order to get to our answer. As long as we do the same thing to both sides of an equation, the equality still holds. We can add the same number to both sides, for example. Or we can multiply both sides by the same constant, or by the same variable.

    What can you do to this equation: y = 1/x

    To get it into the forum of x = f(y), where (y) is some function of y that you got to by a couple manipulations of the original equation?
     
  16. Mar 29, 2015 #15
    Okay so I was messing with the numbers and divided 1 by 4.8. Then I got 0.2083 to which I divided it to 1 and got 4.8
     
  17. Mar 29, 2015 #16

    berkeman

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    Please answer my question in Post #14. Just blindly manipulating equations is pointless. Please use real algebra, and write out each step...
     
  18. Mar 29, 2015 #17
    Well it works with 2 as well. 1/2 is .5 and .5 goes into one 2 times.
     
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