Phys 1240: Sound and Music
www.colorado.edu/physics/phys1240
LAST: waves and sound and SHM
TODAY: resonance, work, energy,
instruments
NEXT: more instruments
READ: 3.4
Help session Wed evening
Have you noticed the back flyleaf of
the text?
1st midterm: Thurs 22 (!) See Web
C 1.5.xxx
A bell is ringing.
What happens to the sound when you
1) put a solid, thick glass jar over it and
2) pump the air out from the jar.
A) 1 => hardly any difference
2 => hardly any difference
B) 1=> hardly any difference
2 => much quieter
C) 1=> noticeably quieter
2 => hardly any MORE quiet
D) 1=> noticeably quieter
2=> much quieter still (near silence)
E) None of these/something else/??
CT 2.4.5
Freq. = (constant) * √ ( K/m)
A mass on a spring oscillates with a
certain amplitude and a certain
frequency. If the mass is replaced
with one 4 times LIGHTER, what
happens to the frequency…
A doubles
B: increases by 4
C: halves
D: decreases by 4
E: None of these/not sure
What happens to the period?
CT 2.4.5b
F = (constant) * √ ( K/m)
A mass on a spring oscillates with a
certain amplitude and a certain
period T. If the mass is doubled, the
spring constant of the spring is
doubled, and the amplitude of
motion is doubled,
… then, the period …
A: increases
B: decreases
C: stays the same.
D: Not enough information to decide
Resonance
Essentially all objects act (sort
of) like a mass on a spring!
Simple objects tend to have a
natural frequency or
resonant frequency
Depends on the shape, materials,
inertia…. But if you don’t change
those, it’s got some definite value.
Complex objects may have
multiple resonant frequencies
CT 2.4.6.1
F = (constant) * √ ( K/m)
A vibrating string is different
from a a mass on a spring, but
what would you expect happens
to the resonant frequency of a
vibrating string if you make it of
more massive material?
A) Goes up
B) Goes down
C) Stays the same.
If you increase the tension
of the string?
Work and energy
Work => pushing on something,
making it move
(Work = force applied * dist. moved)
Energy = ability to do work!
KINETIC Energy:
energy of a moving object
POTENTIAL Energy:
stored, “hidden” energy
e.g.: energy in a stretched spring
or gravitational energy
or chemical energy…
CT 2.4.4
You are going skydiving. While you are free
falling what is happening to the energy in the
system?
a)
b)
c)
d)
e)
Potential energy is being transferred (or
“transformed”) to kinetic energy
Kinetic energy is being transferred (or
“transformed”) to Potential energy
The total energy of the system is
decreasing
The total energy of the system is
increasing
??
A spring sits on the table, fixed at
one end. A mass (with wheels, for
low friction!) is bobbing back and
forth on the other end.
E
The relaxed position is labeled “E”
for “Equilibrium”.
M
E
M’
“M” shows you the maximum extent
of motion
CT 2.4.6.2
M
E
M’
When the cart is at point “M’”
(Maximum extent), in what form is
the energy of the system?
A) Kinetic Energy
B) Potential energy
C) A mix of both
D) There is no energy stored at this
instant.
E) Not sure/something else
CT 2.4.6.2b
M
E
M’
When the cart is at point “E’”
(equilibrium) on its way out, as
shown, in what form is the energy of
the system?
A) Kinetic Energy
B) Potential energy
C) A mix of both
D) There is no energy stored at this
instant.
E) Not sure/something else
CT 2.4.6.3
Picture a vibrating string
Standing wave
At the instant when the string is
maximally stretched, how is the
energy stored?
A) Kinetic energy
B) Potential energy
C) A mix of both
D) There is no energy stored at
this instant.
E) Not sure/something else
CT 2.4.6.3b
Picture a vibrating string
Standing wave
At the instant when the string is
instantaneously flat, how is the
energy stored?
A) Kinetic energy
B) Potential energy
C) A mix of both
D) There is no energy stored at
this instant.
E) Not sure/something else
Waves carry energy
A slinky sends a transverse
wave down its length. The end
jiggles, it could do work!
You have transmitted energy
from one place to another
A singer put energy into the
sound wave at their mouth,
(some of this energy does
work on your eardrum, later!)
Energy was transmitted via the
sound wave through the room