course PHY122 œ~§©ÇPßš™Òv“ªñyx’Ò¸ÁÈòÉïœûØassignment #019
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14:58:27 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
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RESPONSE --> The frequency gives us how many sine waves travel past a point in 1 second, and if we know the length of each sine wave then multiplying the two will give us the velocity.
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14:58:37 ** 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 **
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RESPONSE --> ok
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15:05:26 explain how we can reason out that the period of a periodic wave is equal to its wavelength divided by its velocity
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RESPONSE --> If we know the length of the wave, and we know how many lengths of the wave go by in a second we can determine the period by dividing the wavelongth by the velocity, and get the period of the wave.
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15:05:57 ** 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. **
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RESPONSE --> I think I've got it
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15:06:46 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)
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RESPONSE --> I have no idea.
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15:11:24 ** 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. **
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RESPONSE --> I think I see what you are saying. x/v is the time that the wave takes to travel a certain distance down the string.
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15:14:34 Query introductory set six, problems 11-14 given the length of a string how do we determine the wavelengths of the first few harmonics?
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RESPONSE --> This one was'nt in the assigned problem sets for PHY122. I'm not sure.
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15:16:03 ** As wavelength decreases you can fit more half-waves onto the string. You can fit one half-wave, or 2 half-waves, or 3, etc.. So you get 1 half-wavelength = string length, or wavelength = 2 * string length; using `lambda to stand for wavelength and L for string length this would be 1 * 1/2 `lambda = L so `lambda = 2 L. For 2 wavelengths fit into the string you get 2 * 1/2 `lambda = L so `lambda = L. For 3 wavelengths you get 3 * 1/2 `lambda = L so `lambda = 2/3 L; etc. } Your wavelengths are therefore 2L, L, 2/3 L, 1/2 L, etc.. **
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RESPONSE --> It makes sense that if the wavelength decreases you can fit more halfwaves on the string, but I was'nt sure what you were asking on that one.
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15:18:58 Given the wavelengths of the first few harmonics and the velocity of a wave disturbance in the string, how do we determine the frequencies of the first few harmonics?
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RESPONSE --> Take the velocity of the wave and divide it by the length of the wave and you get the frequency.
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15:19:07 ** The frequency is the number of crests passing per unit of time. We can imagine a 1-second chunk of the wave divided into segments each equal to the wavelength. The number of peaks is equal to the length of the entire chunk divided by the length of a 1-wavelength segment. This is the number of peaks passing per second. So frequency is equal to the wave velocity divided by the wavelength. **
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RESPONSE --> ok
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15:20:47 Given the tension and mass density of a string how do we determine the velocity of the wave in the string?
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RESPONSE --> I know that the more tension on the string the faster the velocity of the wave.
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15:21:39 ** We divide tension by mass per unit length and take the square root: v = sqrt ( tension / (mass/length) ). **
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RESPONSE --> sounds simple enough
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15:25:27 gen phy explain in your own words the meaning of the principal of superposition
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RESPONSE --> When two waves are traveling along in opposite directions and meet the result is a displacement that is the sum of the two displacements.
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15:25:40 ** the principle of superposition tells us that when two different waveforms meet, or are present in a medium, the displacements of the two waveforms are added at each point to create the waveform that will be seen. **
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RESPONSE --> ok
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15:31:44 gen phy what does it mean to say that the angle of reflection is equal to the angle of incidence?
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RESPONSE --> The angle of incidince is the angle that the ray hits the surface and the angle of reflection the corresponding perpendicular angle.
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15:31:52 ** angle of incidence with a surface is the angle with the perpendicular to that surface; when a ray comes in at a given angle of incidence it reflects at an equal angle on the other side of that perpendicular **
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RESPONSE --> ok
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