Introduction to motion Video

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Introduction to motion

Hello. Welcome, this would be the first in the series of lectures on Physics and my goal is really to give you an intuitive feeling of what Physics is all about because especially on the mechanic side of things and the projectile motion and force and moments it’s actually pretty intuitive so let me know if I’m not giving you the intuitive sense. Well let just start with probably the most basic formula in Physics and then a lot of you, you’ve already seen this. Distance is equal to velocity times time (d=vt) and you might seen this in different forms. You might have seen in witness, distance is equal to rate times time or distance is equal to speed times time or you know instead of a d if you’re doing a Math class, sometimes you write an s for distance because they used d for derivatives but it’s all the same thing. It just says the distance you travel is equal to rate of the speed or the velocity you traveled times time. Now before I do a couple of quick just example problems to hit this point anomaly I want to make a slight distinction between velocity and speed. So when people used an everyday language it tends to be used in changeably. That’s kind how fast are you going, right? Velocity versus speed but technically there's the difference. Velocity is a vector. It’s a vector on measurement and speed is a scalar. I probably have already confused you and all you have to know is a velocity of a vector has a magnitude and the direction, right? So if I were to give a velocity, I really shouldn’t just say five miles per hour. I should shay 5 miles per hour north or instead of saying five miles per hour North, I can say negative five miles per hour south, right. Those would be the same thing while speed or scalar only has magnitude. So speed would say if five miles per hour but I do not know what direction I’m going in, I can point forward, backward, left, right, north, south, up down, who knows. So that’s the technical difference between a vector and scalar, and between velocity and speed. It might not seem so obvious that probably on this few example problems that we’re doing right now the distinction probably could be use interchangeably. But later on as we progress to fancier problems, I think you’ll see with the velocity is more useful motion because those are the idea of a negative velocity, right? We can go in one direction which is positive and then you can turn around and go negative and with speed it’s only additional direction. So with that said, I do not want to dwell on that all too much because I do not want to make you think that this is difficult. Let’s do a couple of really good problem. Let’s say I went 50 meters, so 50 meters, so distance is equal to 50 meters. So I go 50 meters and this should really be well I guess you could say distance so we could actually change in distance for the problems would do it does not make a difference. Let’s say the time is ten seconds. So if we use that formula, we have 50 meters is equal to velocity times ten seconds. So this is a pretty simple algebraic equation. If you want to solve for velocity, we just divide both sides of this equation by ten seconds. I’m doing this because actually I want to show you that when you divide the number you should also divide the units with them, and then you always end up with the right units. So let’s multiply both sides of this equation by one over ten seconds. So when I get 1/10sec x 50 m=velocity x 10sec1/10sec, this obviously cancels out. That’s what we did in the first place and then the 50 and the ten cancel and this becomes five. So we’re left with five, meter in the numerator and the seconds in the denominator. So five meters per second is our velocity. You could have done that. But the one thing I just want to highlight is that the units--you can manipulate with numbers and then you’ll always get the right answer. And it might have been obvious in this case. You didn’t have to do at this way, but once again later on when we start doing power and work and energy which is actually the same thing as work. But once when we start doing those things then the units might not seem so obvious. So it’s good to be able to deal with the units this way. So anyway we could solve. If I say that the velocity is equal to seven meters per second and that time is equal to five seconds, how far did I go? Well I could use that formula again distance is equal to velocity, seven meters per second times time which is five seconds. And once again not only can we multiply the numbers, seven times five is 35 but we can multiply the units, so we have meters over second times second. You can almost do them like variables but they’re not, they’re units. So meters over seconds times seconds. Well then the seconds and the numerator on the second and denominator cancel out. So you're left with 35 meters. And there, not only do you have the right number, you have the right units and actually this going to be super useful when if you have to convert from centimeters to meters, hours to seconds and all that and maybe we’ll do a couple of examples so actually I already made a separate video on unit conversion and that’s going to come in really handy when we do the Physics. With that our way, let’s make things a little bit more complicated. So most of what you’ve probably experience so far, you know distance is equal to velocity times time or rate times time is where velocity is constant, right? You’re going 30 meters per second, you're always going the 30 meters per seconds and you stay going to 30 meters per second. But we know from moving generally that your velocity isn’t. I mean sometimes just stationary and then sometimes you're moving in order to start stationary and then get moving your velocity had to changed, right? So how could we describe a change in velocity? And once again I’m teaching you anything fundamentally new. You know what it is, it’s acceleration. So the velocity is acceleration times time and there is a pretty good analogy here, right? Just to ask distance is velocity times time, velocity is acceleration times time or you could view it as the change and distance over the change and time is velocity while the change in velocity versus the change in time is equal to acceleration. Let me answer that a little bit later because once again you are more important. I was saying are they very similar, right? There’s kind of an analogy here. So what can we do with this notion? Well, I think you’ll find us pretty useful. They are called projectile problems and projectile problems involve the acceleration of gravity but we could other acceleration problems involving in acceleration car etcetera and actually a problem will do that. But the acceleration of gravity is actually 9.8 meters per second. It get downwards and once again its acceleration is actually not down, it’s toward the center of the Earth. Acceleration is also a vector quantity but for the sake of our competition let’s say ten meters per second. So acceleration is equal to g, g is normally they are variably used when people talk about acceleration of gravity and let’s say that equals 10/ms2. I knew you're thinking this is kind of strange set of units, meters per second squared is hard to visualize. Well that’s acceleration and I think once you see it used in some of this formulas, it will start to make a little sense in terms of how these units work out. So let’s start with a fairly simple problem. Let’s say I drop a rock from and I do not recommend you to do this. You could kill somebody. Let’s say I drop a penny from the Empire State Building and I’m assuming no rate or just distance. Actually I just set at the clock and I realize that I’m running out of time. So I’m going to actually start this problem with the next presentation. I’ll see you soon.

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Hello. Welcome, this would be the first in the series of lectures on Physics and my goal is really to give you an intuitive feeling of what Physics is all about because especially on the mechanic side of things and the projectile motion and force and moments it’s actually pretty intuitive so let me know if I’m not giving you the intuitive sense.

Well let just start with probably the most basic formula in Physics and then a lot of you, you’ve already seen this. Distance is equal to velocity times time (d=vt) and you might seen this in different forms. You might have seen in witness, distance is equal to rate times time or distance is equal to speed times time or you know instead of a d if you’re doing a Math class, sometimes you write an s for distance because they used d for derivatives but it’s all the same thing. It just says the distance you travel is equal to rate of the speed or the velocity you traveled times time.

Now before I do a couple of quick just example problems to hit this point anomaly I want to make a slight distinction between velocity and speed. So when people used an everyday language it tends to be used in changeably. That’s kind how fast are you going, right?

Velocity versus speed but technically there's the difference. Velocity is a vector. It’s a vector on measurement and speed is a scalar. I probably have already confused you and all you have to know is a velocity of a vector has a magnitude and the direction, right?

So if I were to give a velocity, I really shouldn’t just say five miles per hour. I should shay 5 miles per hour north or instead of saying five miles per hour North, I can say negative five miles per hour south, right. Those would be the same thing while speed or scalar only has magnitude. So speed would say if five miles per hour but I do not know what direction I’m going in, I can point forward, backward, left, right, north, south, up down, who knows. So that’s the technical difference between a vector and scalar, and between velocity and speed.

It might not seem so obvious that probably on this few example problems that we’re doing right now the distinction probably could be use interchangeably. But later on as we progress to fancier problems, I think you’ll see with the velocity is more useful motion because those are the idea of a negative velocity, right? We can go in one direction which is positive and then you can turn around and go negative and with speed it’s only additional direction. So with that said, I do not want to dwell on that all too much because I do not want to make you think that this is difficult.

Let’s do a couple of really good problem. Let’s say I went 50 meters, so 50 meters, so distance is equal to 50 meters. So I go 50 meters and this should really be well I guess you could say distance so we could actually change in distance for the problems would do it does not make a difference. Let’s say the time is ten seconds.

So if we use that formula, we have 50 meters is equal to velocity times ten seconds. So this is a pretty simple algebraic equation. If you want to solve for velocity, we just divide both sides of this equation by ten seconds. I’m doing this because actually I want to show you that when you divide the number you should also divide the units with them, and then you always end up with the right units.

So let’s multiply both sides of this equation by one over ten seconds. So when I get 1/10sec x 50 m=velocity x 10sec1/10sec, this obviously cancels out. That’s what we did in the first place and then the 50 and the ten cancel and this becomes five. So we’re left with five, meter in the numerator and the seconds in the denominator. So five meters per second is our velocity. You could have done that.

But the one thing I just want to highlight is that the units--you can manipulate with numbers and then you’ll always get the right answer. And it might have been obvious in this case. You didn’t have to do at this way, but once again later on when we start doing power and work and energy which is actually the same thing as work. But once when we start doing those things then the units might not seem so obvious. So it’s good to be able to deal with the units this way.

So anyway we could solve. If I say that the velocity is equal to seven meters per second and that time is equal to five seconds, how far did I go? Well I could use that formula again distance is equal to velocity, seven meters per second times time which is five seconds.

And once again not only can we multiply the numbers, seven times five is 35 but we can multiply the units, so we have meters over second times second. You can almost do them like variables but they’re not, they’re units. So meters over seconds times seconds. Well then the seconds and the numerator on the second and denominator cancel out. So you're left with 35 meters.

And there, not only do you have the right number, you have the right units and actually this going to be super useful when if you have to convert from centimeters to meters, hours to seconds and all that and maybe we’ll do a couple of examples so actually I already made a separate video on unit conversion and that’s going to come in really handy when we do the Physics.

With that our way, let’s make things a little bit more complicated. So most of what you’ve probably experience so far, you know distance is equal to velocity times time or rate times time is where velocity is constant, right? You’re going 30 meters per second, you're always going the 30 meters per seconds and you stay going to 30 meters per second.

But we know from moving generally that your velocity isn’t. I mean sometimes just stationary and then sometimes you're moving in order to start stationary and then get moving your velocity had to changed, right? So how could we describe a change in velocity? And once again I’m teaching you anything fundamentally new.

You know what it is, it’s acceleration. So the velocity is acceleration times time and there is a pretty good analogy here, right? Just to ask distance is velocity times time, velocity is acceleration times time or you could view it as the change and distance over the change and time is velocity while the change in velocity versus the change in time is equal to acceleration. Let me answer that a little bit later because once again you are more important. I was saying are they very similar, right? There’s kind of an analogy here.

So what can we do with this notion? Well, I think you’ll find us pretty useful. They are called projectile problems and projectile problems involve the acceleration of gravity but we could other acceleration problems involving in acceleration car etcetera and actually a problem will do that.

But the acceleration of gravity is actually 9.8 meters per second. It get downwards and once again its acceleration is actually not down, it’s toward the center of the Earth. Acceleration is also a vector quantity but for the sake of our competition let’s say ten meters per second.

So acceleration is equal to g, g is normally they are variably used when people talk about acceleration of gravity and let’s say that equals 10/ms2. I knew you're thinking this is kind of strange set of units, meters per second squared is hard to visualize. Well that’s acceleration and I think once you see it used in some of this formulas, it will start to make a little sense in terms of how these units work out.

So let’s start with a fairly simple problem. Let’s say I drop a rock from and I do not recommend you to do this. You could kill somebody. Let’s say I drop a penny from the Empire State Building and I’m assuming no rate or just distance. Actually I just set at the clock and I realize that I’m running out of time. So I’m going to actually start this problem with the next presentation. I’ll see you soon.

Transcription by: Scribe4you Transcription Services