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Homework Help: Help in sound question and reflection question

  1. Jan 25, 2005 #1

    :uhh: i have these questions that i don't know how or what to do :uhh:

    :confused: Matt drops a water ballon from a balcony 15m up. As it falls, someone calls out 2.0 s after release to warn the bystanders below. If the air temperature is 20 C and the people can move instantly, will they be able to avoid being hit? :confused:

    :yuck: While at the local swimming hole (pool) you fall off your flutterboard, and begin sinking to the bottom exactly 4.5m from the edge. Calculate how far you must sink below the surface in order for you to observe total internl reflection while staring at the edge. :confused:

    if you can show me how or the steps or any formulas involved, it would be very helpful, thanks alot!
  2. jcsd
  3. Jan 25, 2005 #2


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    Imagine you are one such bystander. What are the events that must occur for you to avoid being hit ?

    Under what condition do you have total internal reflection ? What does your text say ?
  4. Jan 25, 2005 #3
    in the text there is a very similar question to the first one i have posted but it doesn't show you how to get it just the answer

    and the second question

    in the text i can only find information on the total internal reflection

    the two conditions required for total internal reflection to occur are as follows

    -the light must travel from an optically more dense medium into an optically less dense medium

    -the angle of incidence must exceed the critical angle, theata c, associated with the material

    (theata c or the critical angle)

    than it has a model question on finding the critical angle for diamond which uses Snell's law
    Last edited: Jan 25, 2005
  5. Jan 25, 2005 #4
    also i dont know how the distance to the pool (4.5) will help me find how deep i must be to find or see the total internal reflection

    another thing is that i tried doing the first question and i tried using the
    Speed of Sound in Air formula

    v= 331+ 0.59 Tc
    Last edited: Jan 25, 2005
  6. Jan 26, 2005 #5


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    You won't learn the skill of thinking if you only try and mimic problems solved in your text.

    What needs to happen for you to be able to avoid the water balloon ?

    Answer this, and the equations will follow automatically.

    Is this condition satisfied ? Are you inside the denser medium ?

    So, what is the relavant material here, for which you need to find [itex]\theta _c [/itex] ? Can you calculate it's critical angle using Snell's just, just as was done for diamond ?

    Draw a picture. You are underwater. You are 4.5m from the edge of the pool. Your eyes are some depth d below the water. You are looking towards the edge of the pool (this is where light comes to you from). Label all the distances and agles. Having determined the required critical angle, you can use simple trig to find 'd'.
  7. Jan 26, 2005 #6
    yes i am in a denser medium, water looking to a less dense, air
    i calculated and got 48.6 degrees
    and i think the distance is 5.1 metres

    for the first question i need to hear the guy warning me of the water balloon in order to avoid it

    but then i need to know if the guy's warning can reach me before the water balloon does

    and he gave the warning 2 seconds after the releasing of the water balloon
    Last edited: Jan 26, 2005
  8. Jan 26, 2005 #7


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    Perfect !

    In other words, the sound must reach you before the balloon. Agreed ?

    How long will it take the balloon to fall ? And how much time will sound take to cover this distance ? You have the equation for the temperature dependence. Just plug in Tc = 20 and you get the required speed.
  9. Jan 26, 2005 #8
    i got the speed of 342.8 m/s
  10. Jan 26, 2005 #9
    but what do i do with this speed, this is where i got stuck

    the guy started screaming 2 seconds after the balloon release
    Last edited: Jan 26, 2005
  11. Jan 26, 2005 #10
    thank you for the help provided, i will try to answer the rest on my own.
    if i have a question again, i will seek your guidance.

    i personal question, which sounds better? MECHATRONICS OR NANO TECHNOLOGY?
  12. Jan 26, 2005 #11


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    Has the balloon already hit you, before these 2 seconds elapsed ? If it has, the warning is futile. If not, you may still have time to react.

    So how long does it take for the water balloon to fall through 15m ? Is this time greater than 2s, or less ?

    As for the other question : "Which sounds better ?". Do you literally mean "sounds better" as in, "has a nice ring to it", or are you asking about academic/career possibilities ?
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