- #1
jrd007
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Okay here goes. I think I am doing the problems right but not the correct answers... :(
(1) A stone is dropped from the top of a cliff. The splash it makes when striking the water below is heard 3.5 s later. How high is the cliff? Answer: 55 m
My approach: What I did was used the key equation: d = vt since I know both water and air's v. (Water = 1440 m/s & air = 343 m/s) And since I know the change in t = 3.5 s. So I set the equation like so:
{air}v(t - 3.5s) = {water}v(t)
343(t - 3.5s) = 1400(t)
t - 3.5s = (1440/343)(t)
I get the time = -1.09 s
then I use distance(water) = (1440 m/s)(-1.09s) = -1576 m
and for air = -1574 m... what am I doing wrong?
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(2) A person is standing a certain distance from an air plane with four equally noisy jet engines is experiencing a sound level bordering on pain, 120 dB. What sound level would this person experience if the captian shut down all by one engine? (Hint: Add intesties; not dBs) Answer: 114 dB
My approach: Fine the instestiny of all four then divide by three to get what ONE I would be.
So... I = I(intial) = 10^(sound level(B)/(10)
I = (1.0 x 10^ -12)^(120/10)
I = (1.0 x 10^ -12)^(12)
I = 1 and 1/3 = .33... Again what did I do wrong?
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(3) Expensive amplifier A is rated @ 250 W, while the most modest Amp B is rated @ 40 W. Estimate the sound level in decibels you would expect at a point 3.5 m from a loudspeaker connected in turn to each Amp. Answers: 122 dB, 114 dB
No idea how to get the answers. I know that we have Watts but not how to do it.
Thanks to anyone who can help.
(1) A stone is dropped from the top of a cliff. The splash it makes when striking the water below is heard 3.5 s later. How high is the cliff? Answer: 55 m
My approach: What I did was used the key equation: d = vt since I know both water and air's v. (Water = 1440 m/s & air = 343 m/s) And since I know the change in t = 3.5 s. So I set the equation like so:
{air}v(t - 3.5s) = {water}v(t)
343(t - 3.5s) = 1400(t)
t - 3.5s = (1440/343)(t)
I get the time = -1.09 s
then I use distance(water) = (1440 m/s)(-1.09s) = -1576 m
and for air = -1574 m... what am I doing wrong?
------------------
(2) A person is standing a certain distance from an air plane with four equally noisy jet engines is experiencing a sound level bordering on pain, 120 dB. What sound level would this person experience if the captian shut down all by one engine? (Hint: Add intesties; not dBs) Answer: 114 dB
My approach: Fine the instestiny of all four then divide by three to get what ONE I would be.
So... I = I(intial) = 10^(sound level(B)/(10)
I = (1.0 x 10^ -12)^(120/10)
I = (1.0 x 10^ -12)^(12)
I = 1 and 1/3 = .33... Again what did I do wrong?
-----------------
(3) Expensive amplifier A is rated @ 250 W, while the most modest Amp B is rated @ 40 W. Estimate the sound level in decibels you would expect at a point 3.5 m from a loudspeaker connected in turn to each Amp. Answers: 122 dB, 114 dB
No idea how to get the answers. I know that we have Watts but not how to do it.
Thanks to anyone who can help.