A powerpoint presentation demonstrating two problems with solutions that explore applications of power and intensity in real life. It provides steps on how to derive equations to solve questions.
2. Question Part 1
You are home trying to study for your
physics midterm. However, your brother is
playing loud music in his room. So you
move away from your original distance to
twice the original distance from his room.
How many decibels did you decrease his
music by?
Your
brother’s
room
Where you
were
originally
Where you
moved to
d
2d
3. Hint 1: think of it as increasing the
distance by a factor of 2, what happens to
the intensity?
What formula(s)
can you use?
4. Hint 1: think of it as increasing the
distance by a factor of 2, what happens to
the intensity?
Hint 2: How do you relate intensity to
decibels?
5. Solution to Question Part 1
O
Initial
(15-30) page 252 of textbook
(15-28) page 251 of textbook
6. Does your solution make sense?
This solution means that as you moved
further away from his room, the intensity
of his music decreased, and therefore the
decibels you hear has decreased. So yes!
This solution makes logical sense.
7. Question Part 2
You go into your very quiet sound studio
to get away from your brother’s music.
Inside the studio, the pressure amplitude
of sound waves is 30.0 μPa. What is the
intensity of sound you hear in the studio?
Is this sound detectable?
(assuming its 20°C in your house)
8. How to Approach These Types of
Questions
1) When a question has too many unknowns, we
can start by listing the values we do know,
either from what the question tells us and/or
set values in data tables.
2) We can then write down the main formula that
will calculate what the question is ultimately
asking for (in this case intensity).
3) Then, list out all the formulas that are related to
the variables in the main formula.
4) Substitute in the formulas to possibly cancel
out unknowns.
5) Plug in values and solve!
9. Hint 1: The bulk modulus, speed of sound,
and density of air at 20 °C is given in
Table 15-1 of the textbook Physics for
Scientists and Engineers – An Interactive
Approach
10. Hint 1: The bulk modulus, speed of sound,
and density of air at 20 °C is given in
Table 15-1 of the textbook Physics for
Scientists and Engineers – An Interactive
Approach
Hint 2: Think about how to cancel out
frequency in the formula since it was not
given in the question.
11. Solution to Question Part 2
Continued
Next Slide
Main Formula (15-13) page 242 of textbook
(15-11) page 241 of textbook
(13-9) page 161 of textbook
(15-11) page 241 of textbook
13. Does Your Solution Make Sense?
• An intensity of 2.14 x 10^(-12) W/m^2 is
BARELY noticeable to the human ear
because it is still slightly higher than the
lowest intensity detectable by the human
ear, which is 10^(-12) W/m^2. As you can
see, the two values are very similar. This
is an amazing sound studio!
13
14. Work Cited
Physics Textbook(for equations):
Physics for scientist and Engineers – An
Interactive Approach
