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Rutgers, The State University of New Jersey

University of Washington

University of Sheffield

05:29

Kathleen T.

(I) What is the weight of a 68-kg astronaut ($a$) on Earth, ($b$) on the Moon ($g =1.7 m/s^2$) ($c$) on Mars ($g = 3.7 \,m/s^2$) ($d$) in outer space traveling with constant velocity?

01:40

Keshav S.

(II) According to a simplified model of a mammalian heart, at each pulse approximately 20 $g$ of blood is accelerated from 0.25 m/s to 0.35 m/s during a period of 0.10 s. What is the magnitude of the force exerted by the heart muscle?

03:09

Aditya P.

(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?

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?

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welcome to our fourth example video looking at sound and light. This example We're going to combine the two by considering the physics of lightning. Okay, so when you see a lightning strike, you're probably all familiar with the fact that it takes some time before you hear the thunder clap that follows. So why is that? Well, it's because of the difference in speed between the speed of light, see, and the speed of sound of the thunderclap. Okay, well, let's just figure out exactly how much that is. Let's say that we see Ah, fun. We see a lightning bolt and it takes approximately three seconds for us to hear the thunder afterwards. How far away was the lightning bolt? Now we actually have to do a little bit here to remember that if we're a distance d away when this happens, it's actually going to take some time for the speed of light for light toe reach our eyes as well. Now that time will be negligible. For example, if we are 1000 m away, we can say All right, well, I know my distance. My delta T is going to be 1000 m divided by three times 10 to the 8 m per second. Clearly, this is going to be an extremely small amount of time, but it still is some amount of time and the larger the distance, the larger this time is going to be. So we have to remember that by the time we see it, it's already happened. Um, now generally, uh, these sorts of problems will tell you to neglect this term. Andi, that's fine. But then we also want to say, Well, I know I also have a delta t between when I see it and when I hear it. So speed of sound has to travel that same distance and we can plug it in in this case instead of three times 10 to the eight, it will be approximately 343 meters per second. But if it's a different temperature, you could type that into the equation that we gave Previously. V Sound is equal to 331 m per second times the square root of temperature and Celsius plus 2 73 divided by 2 73. Okay, so you can see here that 1000 m significantly larger than 343 m per second is going to take a lot longer for it to get here. Um, so going back to our original question, how do we solve for D Now we're going to neglect the time it takes for this light to arrive. But we do have a time that it will take for the sound to arrive. So if we say all right, I know that I have three seconds before it arrives. Then I can say All right, I know that my distance is going to be equal to my velocity times. My delta t Okay, If I know my velocity is 343 m per second and I multiply it by three seconds, Then I'm done. If I hadn't neglected this, then how would I do it? Well, I would have to add on and say, Well, I see. I hear it after 33 seconds after I heard it. But I also have to add in the fact that it took me a while to see it, so I would have to increase my delta t here so my delta T would be equal to three plus, Whatever the distance is divided by three times 10 to the eight meters per second and then we have a little bit more challenging of problem we have d is equal to v times three plus Sorry. Three seconds plus de divided by three times 10 to the eight meters per second. Still not too challenging. We can quickly solve for D. In this case, we will have d is going to be equal to the times three seconds as we have before. But now it's going to be divided by one minus the over three times 10 to the eight meters per second. You can see this should be ah, very small time indeed.

Superposition

Thermal Properties of Matter

The First Law of Thermodynamics

Kinetic Theory Of Gases

The Second Law of Thermodynamics

04:25

01:50

02:50

04:23

01:27

05:49

02:12

02:35

04:09