Description of Motion: Describing Motion with an Apple

by Jared Rovny

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    00:01 We're going to start with translational motion, and the first thing we'll do is cover one dimensional motion so things just moving in a simple straight line, we won't do anything with two dimensions or anything yet, after covering one dimensional motion, we'll move to two-dimensional motion and once we understand how things moving in more than one dimension, we'll be able to start talking about forces and what happens to the things if we start pushing or pulling on them and see how they react.

    00:26 The first thing we're going to do is really put ourselves in the shoes of somebody who's first coming up with all these ideas in the first place, you're walking around or you're sitting at your own desk right now and you see an object in front of you and we're gonna ask a question about a simple object like this one, you just have it and you do something and why did that happened, and you really have to wonder if you look at the things in front of you.

    00:49 Again your books, or your computer and you ask why is it doing the things that it's doing.

    00:53 And we're gonna really ask two questions, first when something does what this apple just did and it does something you can ask what are the laws that govern how that object moved and behaved and why did they do that, but secondly, and very key and crucial to physics and the idea of developing physics is using the ideas of what I just saw happened and measuring what I just saw happened.

    01:15 Can I somehow predict how it will move and behave in the future if I know what happened right now? Using this two questions, we can really motivate a few things, we can measure about a simple object like the apple I just showed you, so first let look at this apple that we have on the screen here and let's ask what things could we ask about this apple, what things could I ask you.

    01:35 The first thing is where is it, we can ask where is our object.

    01:40 What we'll do to defined a position of an object is first we pick an origin some place to call home really, a zero point and then to define a position we say how far is the apple from my origin.

    01:53 You can see on this graph here, we'll define an origin that has a zero point on it and then we measure the position of the apple which we call x as how far in meters which will be our units, is the apple from the origin.

    02:05 In this case, if I ask you where this apple is, you could look at it and say well, it looks like it's about one meter from our origin and that would be the position of this apple.

    02:14 The units that we used to measure all of these positions are meters like I said and the abbreviation for this unit of meters would be m, so if you that m it's not minutes or anything else it's the actual meters telling you where your object is.

    02:30 Secondly, I could ask you is this object moving, so as we saw when I let go of the apple it just suddenly started to fall.

    02:36 We can ask a number of things about this position which we're going to call v for the motion of the object which stands for velocity, and this velocity is measuring for us how the position of the object is changing as time goes on.

    02:50 So, its meters per second, how many meters is the object moving every second in time.

    02:56 The units for velocity are meters per second because we're measuring how many meters per second it's moving.

    03:02 And so, when you this m, dash s, it's really m divided by s or meters per second.

    03:07 How many meters in a given second? If this apple is moving off to the right, in every second it goes from 1 meter, and then it's up 2 meters and then at 3 meters and so on.

    03:15 I could ask you if every interval is in fact, one second interval and its moving 1 meter in each second interval, what is the velocity of this apple? And you would say the velocity is, as you would expect 1 meter every second, so we would say, simply the velocity of this apple is 1 meter every second.

    03:32 But then not only can a position be changing, and we give that a name velocity, but the velocity itself can also be changing and so we call this change the acceleration.

    03:42 Is this apple accelerating? If it were, it will look something more like this, where the apple is actually moving but it's not moving in the same amount of distance in every second, so maybe in the first second it will only goes 0.5 meters, but then on the next second it goes a full meter and then in the next second after that, it goes a full, 1.5 meters. So you can see it's doing what we call speeding up, it's gaining speed. We measure this using acceleration.

    04:06 The acceleration of this object we call a, the units, a sort of as you'd expect following the trend are meters per second, per second.

    04:16 In other words, how many meters per second is this object changing its velocity every second? And so the units abbreviated are meters per second squared because those two units of second will come together and just say, second squared.

    04:28 If I look at this object or ask you to look at this object and measure, how was the velocity of this object changing, you can see that because it's going 0.5 meters and then 1 meter and then 1.5 meters as the seconds go on, its velocity is changing by 0.5 meters per second every second.

    04:46 So, in other words, the acceleration is measuring how the velocity is changing, while the velocity is measuring how the position is changing.

    04:54 And that is the key way to understand what you're seeing on this graph which is that each variable, is measuring how the variable before it is changing, and that gives it those units of seconds in the denominator.

    About the Lecture

    The lecture Description of Motion: Describing Motion with an Apple by Jared Rovny is from the course Translational Motion.

    Included Quiz Questions

    1. Meters/second^2
    2. Meters/second
    3. Meters
    4. Meters^2/second^2
    5. Meters^2/second
    1. Position, velocity, acceleration
    2. Position, energy, work
    3. Momentum, energy, acceleration
    4. Position and velocity only
    5. Force, velocity, acceleration

    Author of lecture Description of Motion: Describing Motion with an Apple

     Jared Rovny

    Jared Rovny

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