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Math Help - 2 physics problems

  1. #1
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    2 physics problems

    1.) What is the intensity of a sound that has an intensity level of (a) 50 dB or (b) 90 dB?

    2.) A person standing 4.0 m from a wall shouts such that the sound strikes the wall with an intensity of 2.5 x 10 ^-4 x 10 W/m ^2. Assuming that the wall absorbs 20% of the incident energy and reflects the rest, what is the sound intensity level just before and after the sound is reflected?
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by John 5 View Post
    1.) What is the intensity of a sound that has an intensity level of (a) 50 dB or (b) 90 dB?
    dB re what? The theshold of hearing I_0=10^-12 watts/m^2 or I_0=1 watt/m^2?

    SL=10log_10(I/I_0),

    so:

    I=I_0 10^{SL/10}.

    2.) A person standing 4.0 m from a wall shouts such that the sound strikes the wall with an intensity of 2.5 x 10 ^-4 x 10 W/m ^2. Assuming that the wall absorbs 20% of the incident energy and reflects the rest, what is the sound intensity level just before and after the sound is reflected?
    What intensity to you mean?

    I=2.5 x 10^-3 W/m^2 and I=2 x 10^-3 W/m^2

    SL=10 log_10 (2.5 x 10^-3) dB re 1 w/m^2

    and

    SL=10 log_10 (2 x 10^-3) dB re 1 w/m^2

    RonL
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  3. #3
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    What about these two?

    1) Two plane mirrors, M1 and M2, are placed together as illustrated in the figure below. a) If the angle a between the mirrors is 70 degrees and the angle incidence, θi1, of a light ray incident on M1 is 35 degrees, what is the angle of reflection, θr2, from M2? b) If a = 115 degrees and θi1 = 60 degrees, what is θr2?



    2.) A beam of light traveling in air is incident on a transparent plastic material at an agle of incidence of 50 degrees. The angle of refraction is 35 degrees. What is the index of refraction of the plastic?
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  4. #4
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    Call the point of incidence on the first mirror B.
    Call the second point of incidence on second mirror point C.

    The angle of reflection on one side of the normal to the mirror is the same as the angle of incidence on the the other side of the normal. The normal is at 90 degrees to the mirror so the angle ABC is 90-θi1 degrees.

    Since the angles inside a triangle add up 180 degrees the angle BCA is 180-ABC-a = 180-(90-θi1)-a = 90+θi1-a degrees.

    The second angle of incidence is 90-BCA with respect to the normal (and so is the final angle of reflection with respect to the normal of M2)

    90-BCA = 90-(90+θi1-a) = a-θi1 degrees

    Q1a) a-θi1 = 70-35 degrees, so the final angle of reflection is 35 degrees.
    Q1b) a-θi1 = 115-60, so the final angle of reflection is 55 degrees.

    For the second question google Snell's Law and refraction
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