Exam 3 --141 Flashcards
Pressure
force spread out over an area
What is the unit of pressure?
a Pascal (Pa)
N/m2
What happens to pressure you exert on ground if you stand on one foot?
pressure increases since area decrease
area has an inverse relationship with pressure
What does pressure directly depend on in tank of water?
depth
Why do we not feel atmospheric pressure even though it is so large?
our bodies are not air tight
air inside our bodies pushes out just as hard as the atmosphere pushes in
Magdeberg sphere example
when a sphere has air inside of it you can easily split the sphere in half
when the air is removed from the sphere, you have to overcome atmospheric pressure to open and it becomes hard
Does pressure have direction?
no
pressure is a scalar
pressure can give a force perpendicular to any surface
What is true of static fluids?
pressures at the same horizontal level are equal
since not moving, all pressure cancels out and are the same on a horizontal level
Pascal’s principle
an increase in pressure is transmitted uniformly all through a fluid
How do you use a barometer to measure atmospheric pressure?
pressure measured in the barometer equals the atmospheric pressure
Buoyant force
contact force when fluids come into contact with an object
buoyant force always points upwards
Archimede’s principle
an object feels an upward bouyant force equal to thee weight of the fluid that is displaced
How to quantify the amount submerged of an object?
volume of displaced fluid equals the volume of the object submerged
What is true of force when an object is stably floating?
force in the y equals 0
What are two ways to quantify how fast a fluid flows?
1) focus on one molecule and see how many m/s the molecule covers
2) look at a cross-sectional gate and see what volume of a fluid passed through the cross-section over time (known as the flow rate)
Flow rate
volume / time that crosses through a cross-section
Why does syrup start thicker and thins as it falls?
flow rate remains constant and flow rate = Av
since v increases as you fall, A has to decrease to make up for increase in v and remain constant
Bernoulli’s equation
energy conservation for fluids
allows us to predict velocities of fluids
Where can energy hide in fluids?
pressurization, motion and gravity
Do faster moving fluids exist at higher or lower pressures?
faster moving fluids exist at lower pressures
Simple harmonic oscillators
a subgroup of oscillators that follow the mathematical form:
accleration = -w^2x
mirrors the fact that we have a linear restoring force
What is w^2 in oscillation?
k/m
Amplitude
distance from equilibrium to max/min (A)
Period
time for one complete back and forth motion (T)
goes through all points
Frequency
how many oscillations occur per time (f)
How are frequency and period related?
they are reciprocals
Spring force
a restoring force that acts towards equilibrium
force is linear (gets larger further away from equilibrium)
What type of forces are all simple harmonic oscillators governed by?
linear restoring force
What is acceleration at equilibrium of a simple harmonic oscillator?
a=0
since F=ma and F=-kx and x=0
What do you get when you compile many individual oscillations?
a wave
no single oscillator makes progress but the wave/disturbance does travel
2 broad classes of waves
transverse waves
longitudinal waves
Transverse waves
when any one oscillator moves it’s motion is perpendicular to the wave’s travel direction
ex: stadium wave
Longitudal waves
when any one oscillator moves it’s motion is parallel to the wave’s travel direction
ex: slinky
What is the compression of a longitudal wave represented by graphically?
it is like the max/crest of the transverse wave
What is the rarefaction (expanding) of a longitudal wave represented by graphically?
it is like the min/trough of the transverse wave
What happens if you shake your hand faster and faster for a string wave?
the velocity is medium dependent so it does not change
the frequency is source dependent so it will increase
have to decrease wavelength as frequency increases to make velocity remain constant
v=wavelength * frequency
Constructive interference
add together amplitudes to get bigger wave
Destructive interference
subtract amplitudes to get smaller wave
What type of waves are sound waves?
longitudal vibration
Rank the velocity of sound in different mediums
solid > liquid > air
in a solid molecules are close together so easy to transfer the vibration of the wave
How loud is sound?
picture-wise: loudness corresponds to the amplitude of the wave
quantitatively: loudness depends on source’s strength and the observer’s distance from the source
Source’s strength
how much energy per time the source puts out
measured in J/s which is known as a Watt
power
Why does loudness decrease further from the source?
you hear less units of sound since the units of sound spread out in an increasing radius
Intensity
a linear scale that measures power/area
W/m2
How do we rescale intensity of sound?
use a log scale of either 10^Bell or 10^Beta/10
Beta is the number of decibels
Doppler effect
when an observer recieves a different frequency (pitch) than the source is emitting because of motion of the source and/or observer
ie. moving changing the pitches of sound you hear
What happens if the source is moving forward, towards the observer?
this shortens the wavelength the observer hears
What happens if the observer is moving towards the source?
makes the observer recieve more waves per second than the should and increases frequency
What happens if the source is going faster than the speed of sound?
energy gets compressed and you get sonic boom from all the sound waves hitting at once in shock cone
beats
what you get when two waves of almost the same pitch but not quite overlap and interfere with each other
amplitude (loudness) fades in and out
bigger difference in frequency will lead to more fading in/out per second
standing waves
the phenomenon that essentially all instruments rely upon to produce one freq/note on demand without getting random frequencies
the wave does not travel
the amount of nodes matters
How do you move up in a harmonic?
you add a node to a standing wave
In a pipe where are the nodes?
each open-end of the pipe has an anti-node
What are higher harmonics good for?
higher harmonics give tone
when you pluck a string, all the harmonics are simultaneously excited but you only hear the fundamental frequency
the higher harmonics are what makes a harp sound different from a guitar
Heat
that which flows between objects of different temps
flows from hot to cold
What do we measure heat in?
calories
Temperature
measure of how quickly the objects in a molecule vibrate
What is the prerequisite for a heat transfer?
a temperature difference
why does metal feel colder then plastic even though they are at the same temperature?
metals are good heat conducters so heat leaves your hand to transfer to the metal quicker than heat leaves your hand for the plastic
What is the symbol for heat transfer?
Q
+ means that heat flows into the object
- means that heat flows out of the object
what does converting calories to joules show us?
that both heat transfers and work are transfers of Energy
leads to the 1st law of thermodynamics
what is mechanical work in a piston?
pressure * the change in volume
- when using the formula have volume units in m3
- have pressure units in N/m^2
What are two characteristics of an engine?
must operate on a reset-able cycle
must perform useful work on environment
What does a single point on a Pv diagram correspond to?
one specific state of the piston
can only have a specific temperature when P and v are determined from graph
List the steps of car engine
1) light a fire under gas and let the piston expand
2) lock piston in place and connect to a cold reservoir (energy flows out)
3) compress the piston while exposing to a cold reservior for a long time
4) lock the piston and light a fire under it (return to original state)
What should ∆U of the entire system of an engine be?
should be zero
you should end up at same temperature that you started at
What do we approximate ∆U to be?
∆T
Why can an engine not be 100% efficient?
friction and you need a non-zero Qcold due to the second law of thermodynamics
Second law of thermodynamics
in any physical process, the overall entropy of the universe must increase
Second law of thermodynamics and Qcold
When you decrease from hot to cold, you are losing entropy (Qhot)
therefore, you need a Qcold to go from cold to hot and increase entopy
Difference between positive and negative sign in work formula for piston?
positive sign in formula refers to work done by the gas
negative sign in formula refers to work done on the gas
How to calculate work if pressure is not constant?
have to take the “integral” ie. the area under the curve
What is the sign of the work done on the gas if the gas is expanding?
negative
What is the sign of the work done by the gas if the gas is expanding?
positive
*increase in energy
what is the sign of the work done on the gas if the gas is compressing?
positive
What are the units when calculating the Carnot efficiency>
need to have temperatures in kelvin
What direction do liquids flow in?
liquid flow from areas of high to low pressure
How does Archimedes’ principle work as a thought experiment?
replace the volume of the object with the correpsonding volume of water
the surrounding frame of water can hold up that volume of water
if you place an object, the same force that was holding the water must hold the object
and the force holding up the water was equal to the weight of the water so the force holding up the object also equals the weight of the water
Why does a floating object not sink due to atmospheric pressure?
the increase in atmospheric pressure is distributed equally through all depths, so there is no overall change
Pascal’s principle
What do you use to find the velocity of springs?
use energy
In transverse wave, do individual oscillators move left/right?
NO
They only move up/down. There is no movement for individual oscillators in the left/right direction
