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Cornell University

Rutgers, The State University of New Jersey

Hope College

University of Winnipeg

02:55

Keshav S.

(II) A person has a reasonable chance of surviving an automobile crash if the deceleration is no more than 30 $g$'s. Calculate the force on a 65-kg person accelerating at this rate.What distance is traveled if brought to rest at this rate from 95 km/h?

03:27

Kai C.

(I) A constant friction force of 25 N acts on a 65-kg skier for 15 s on level snow.What is the skier's change in velocity?

03:38

04:39

Muhammed S.

(I) A 7150-kg railroad car travels alone on a level frictionless track with a constant speed of 15.0 m/s. A 3350-kg load, initially at rest, is dropped onto the car. What will be the car's new speed?

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welcome to the next section in traveling waves in this section, we're going to talk about the Doppler effect. The Doppler effect is what occurs when you have a source that is moving relative to you. So, for example, if we have a source here that is moving towards us with some velocity, we know that every time, every moment it's emitting sound. And at this moment it's emitting sound that ISS centered at this original position. Well, sometime later, when it has moved forward and produces another sound wave, this sound wave will have grown at the rate of the speed of sound. It will be centered here and it will look something like this. Notice that the leading edge, the two has much closer. And what this means is that the observed wavelength between these two waves is much shorter than the observed wavelength between thes two waves. So what does that mean for what we will hear? Well, since we're on the side where the wave fronts air squished together, we will have a shorter wavelength. Remember that wavelength B is equal to F times. Lambda and F is equal to v overland. So even for a fixed speed of sound. If we have a lambda that gets smaller, that means our frequency gets larger so well here. Ah, higher frequency over here. If, on the other hand, we had been to standing behind it, where the wavelength appears to be spaced out more because of the motion of the source than Lambda would get larger and frequency would get smaller. So this basic idea governs all of the physics of the Doppler effect, and it's really just relative motion. Now. We could go through and do quite a bit of algebra to come up with many different equations to describe what happens in the Doppler effect. But you would need a unique equation for every set up one for when it's approaching you, one for when it's moving away, one for when you're approaching it, one for when you're moving away. And there's a lot of algebra involved here. Um, I find it much simpler to write one equation that describes all of the motion. It goes something like this. The frequency that is observed So that's the frequency over here is equal to the frequency at the source. So that's the frequency over here in multiplied by V, which is the speed of sound minus speed of the observer divided by the speed of sound. Nice speed of the source. Okay, it's unfortunate that sounded source. Start with an s. So we've got V source and the sound now the minus here actually isn't guaranteed. It actually is a plus or minus. In order to write the most generic equation we can we could have assigned the signs, but then again, we'd have to assign four different sets of signs for the four different situations we have in this case. What? All we're going to do is use some logic to figure out what the sign ought to be, and you only have to remember a single equation. I find this to be a lot more elegant. Okay, So if this is what we need to do, let's consider our first situation where it is moving towards us. If it's moving towards us and we have no motion, then our velocity is zero. So we don't even have to worry about that sign. And the source is attempting to increase the frequency so that f not is greater than f s. Okay, so we want f not greater than f s. Okay, if that's true, then it wants the denominator to be smaller than the numerator. In order to do that, it wants to subtract. Okay, so it wants a subtract sign in the denominator. So we need V s minus V source. Okay, if, on the other hand, it is moving away from us, then it wants to decrease the frequency f not who wants f? Not less than f s, which means that it wants this ratio here to be greater or sorry to be less than one in order to be less than one. It wants the denominator to be greater than the numerator, which means we need to add. So we're gonna have V s plus V source, okay. And then you can do the same trick for yourself. If you are moving towards the source, you are attempting to increase the frequency so you have f not greater than fs up here. If you want to have a larger frequency than you need a large numerator and a small denominator. So we're going to add so we'll have B s plus V not, and with removing away will want f not smaller than f s. And then we'll need RV not to be creating as small a denominator as possible to make this ratio as small as possible, Which means we need VVS minus V not. Okay, So these are all the signs that we need to have and all you have to do is remember this one equation and then remember the physics of the situation. If you're trying to shorten the distance between you and the source, that means you're trying to increase the frequency. If you're trying to increase the distance between you and the source will be trying to decrease the frequency. Okay, so we're going to try applying this in a number of different examples and hopefully you'll be able to get the hang of it by the end of

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