#$&*
course phy 122
010. `query 9
*********************************************
Question: Query introductory set 6, problems 1-10
explain how we know that the velocity of a periodic wave is equal to the product of its wavelength and frequency
YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY
Your Solution: The length of each wave length segment divided by the time required for the wave to pass this length is the velocity.
confidence rating #$&*:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.............................................
Given Solution:
** we know how many wavelength segments will pass every second, and we know the length of each, so that multiplying the two gives us the velocity with which they must be passing **
Your Self-Critique: If a 1 second section of the wave is 25 peaks, and every peak is 3 meters from the next, this 1 second section of the wave is 25peaks * (3meters/peak) = 75 meters. This is completed in 1 second so our velocity is 75meters/second.
Your Self-Critique Rating:
*********************************************
Question: explain how we can reason out that the period of a periodic wave is equal to its wavelength divided by its velocity
YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY
Your Solution: If a 1 second section of a wave is 25 meters long and has peaks 5 meters apart, each peak spans 5/25 of the 1 second section. Therefore the time between peaks will be 0.2 seconds.
Period = wavelength / velocity
0.2s = 5meters / 25meters / sec
confidence rating #$&*:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.............................................
Given Solution:
** If we know how far it is between peaks (wavelength) and how fast the wavetrain is passing (velocity) we can divide the distance between peaks by the velocity to see how much time passes between peaks at a given point. That is, period is wavelength / velocity. **
Your Self-Critique:
Your Self-Critique Rating:
*********************************************
Question: explain why the equation of motion at a position x along a sinusoidal wave is A sin( `omega (t - x / v) ) if the equation of motion at the x = 0 position is A sin(`omega t)
YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY
Your Solution:
confidence rating #$&*:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.............................................
Given Solution:
** the key is the time delay. Time for the disturbance to get from x = 0 to position x is x / v. What happens at the new position is delayed by time x/v, so what happens there at clock time t happened at x=0 when clock time was t = x/v.
In more detail: If x is the distance down the wave then x / v is the time it takes the wave to travel that distance. What happens at time t at position x is what happened at time t - x/v at position x=0.
That expression should be y = sin(`omega * (t - x / v)).
The sine function goes from -1 to 0 to 1 to 0 to -1 to 0 to 1 to 0 ..., one cycle after another. In harmonic waves the motion of a point on the wave (think of the motion of a black mark on a white rope with vertical pulses traveling down the rope) will go thru this sort of motion (down, middle, up, middle, down, etc.) as repeated pulses pass.
If I'm creating the pulses at my end, and that black mark is some distance x down in rope, then what you see at the black mark is what I did at time x/v earlier. **
STUDENT COMMENT (University Physics):
According to the Y&F book (p.553) we get the expression for a sinusoidal wave moving the the +x-direction with the equation:
Y(x,t) = A*cos[omega*(t-x/v)]
I am not sure where the sine came from in the equation in the question. The book uses the cosine function to represent the waves motion.
The choice of the cosine function is arbitrary. Either function, or a combination of both, can come out of the solution to the wave equation (that's the partial differential equation which relates the second derivative with respect to position to the second derivative with respect to time).
The sine and cosine functions differ only by a phase difference of 90 degrees, and either can be used to describe simple harmonic motion or the motion of harmonic waves. The choice simply depends on the initial conditions of the system.
We don't want to get into solving the wave equation here, but the point can be illustrated by considering simple harmonic motion, which is characterized by F_net = - k x (leading to m x '' = - k x or x '' = -k/m * x, where derivatives are with respect to time).
The general solution to the equation x '' = - k / m * x is x = B sin(omega t) + C cos(omega t), where B and C are arbitrary constants and omega = sqrt(k/m).
B sin(omega t) + C cos(omega t) = A sin(omega t + phi), where A and phi are determined by B, C and the choice to use the sine function on the right-hand side.
B sin(omega t) + C cos(omega t) = A cos(omega t + phi), where A and phi are determined by B, C and the choice to use the cosine function on the right-hand side. The value of A will be the same as if we had used the sine function on the right, and the value of phi will differ by 90 degrees or pi/2 radians.
STUDENT QUESTION
I don’t understand why we would subtract x/v from the omega*t value. Why wouldn’t it be addition?
INSTRUCTOR RESPONSE
What happens at x at clock time t is what happened at the origin at a certain earlier clock time.
The time required to for a pulse to propagate from the origin to position x is x / v.
So What happens at x at clock time t is what happened at the origin at clock time t - x / v.
At clock time t - x / v the point at the origin was at y position y = A sin (omega (t - x / v)).
Your Self-Critique: I know without a strong calculus course to back this up we’re not going to understand a lot of whats going on here, but bits and pieces stick out as making some sense. I’m reading through it and it doesn’t seem impossibly challenging, there was a time when it would have felt that way, but I’ve gotten better since then. I often enjoy attempting to comprehend things that should be well out of my range of understanding, like particle physics and differential equations, it gives me something to work towards. Every time I go back and look at something that previously would have been to complex to even bother reading, I find I understand it a little bit more. This is an exciting process I have noticed in my mathematical studies.
Your Self-Critique Rating:
"
Self-critique (if necessary):
------------------------------------------------
Self-critique rating:
"
Self-critique (if necessary):
------------------------------------------------
Self-critique rating:
#*&!
&#Your work looks good. Let me know if you have any questions. &#