hello scholars doctor a bear here coming up with our second video installment for week 2 on forces in motion so you should be reading along in your chapter to a physical universe and as you read make sure you answer the questions that I've put on Moodle to see if it kind of helps you process what you're reading in addition to your reading assignment I've got this lesson here that we're about to go through note that I'm gonna separate chapter two into two smaller videos one focusing on measuring motion and the second video would be more specifically related to the forces causing motion so let's go ahead and get started the core concept behind this lesson is that a net force is going to be required for any change in motion so the only reason why an object might be in motion is because a force has acted on it it could be gravitational force magnetic force it could be a push or a pole so something that has acted on an object it's gonna cause it to move and that's what we're gonna understand in Chapter two so there are three basic concepts in order to fully describe an object's motion first you need to describe the object's motion relative to its position you'll know it's moving if it's displacement or distance is changing relative to a reference point we're gonna talk about speed and velocity and the difference between these they are different terms a lot of people use them interchangeably but that's incorrect speed is a scalar value and velocity is a vector we'll talk more about that later in this video and it's also gonna be important that you understand how acceleration works and all of these things ultimately apply themselves to your daily life it could be driving in the car riding a bike jogging down a sidewalk or it could be the satellites used to navigate so we're gonna apply that to horizontal motion vertical motion of and falling objects so let's go ahead and talk about motion I've got a quick question for you and picture one versus picture two was did motion take place and some of you might be confused you're like well he's sitting still in this vid and he's sitting still in this picture so there's no motion that we were able to witness but you do know that motion has taken place because of a reference point so a reference point is going to be just a framework for describing motion and it can be really helpful especially if you can get distances from that reference point so the only thing we have yet to measure now is how fast did the boy move so we don't know the distant it took for the boy to move from here to here we just have a still image and a still image we might call that stop motion so if we were to reproduce these pictures back a number of times then we could make a flip book and and actually appear to have a movie of the boy just moving back and forth very quickly so if you understand two motion you'll need to understand that it's from a position change and then we can measure the time of that position change to capture the speed so in measuring motion there are two fundamental components the first one is going to be the change in position as illustrated in the previous slide and measuring that change in position with some sort of ruler or meter stick or mileage marker those are gonna be things that are gonna be helpful in having us understand the change in position next would be the change in time we're gonna need a clock or a stopwatch and once we know the change in position and we change the change in time then we can calculate the speed in which they they moved and so that's how we'll measure motion so here we have a car that started at an initial position we'll call that zero and then at this position we'll call that one my 1 mile and two miles three miles and four miles so a very obvious question might be how fast was the car traveling what was the speed or the rate of travel and so it's very easy to calculate because if you just look at the total time elapsed four minutes and the total distance traveled four miles well then you can calculate the speed very easily if they're going four miles in four minutes then they're basically going one mile a minute or sixty miles an hour so that's the same thing you just convert minutes 60 minutes and one hour and then you're left minutes cancel out and you're left with 60 miles per hour so what we're gonna see is the three important combinations of length and time speed velocity and acceleration so we're gonna talk about these three important combinations of length and time right now we've just been talking about speed we haven't talked about velocity or acceleration so speed can be calculated relative to an object's position and respect to an object and the time it took for it to change that position usually when we talk about speed we're talking about the average speed for example if it takes me 30 minutes to go 24 miles to work and so basically it would take you know what is my average speed well my average speed would be 48 miles per hour and that is an average speed obviously my instantaneous speed is not gonna be 48 miles per hour I don't pull out of the garage at 48 miles an hour and drive down the residential streets at 48 miles an hour what it means is that I'm gonna be going a lot slower there so there's probably some periods in time where I'm going a lot faster in other areas like 60 miles an hour down the highway or 55 and so when you average all that'll slow vilasa beats with the higher speeds I get an average speed of about 40 miles per hour so this is called a motion graph and what it does is it shows the position on the y-axis and meters and the time on the x-axis in this case it's in minutes the red line here is going to represent the average speed or velocity depending on the direction of the traveller so you can see from beginning to end you can see that they've gone from time zero position zero they've gone 500 meters in 10 minutes so their speed is 500 meters and 10 minutes and of course we would simplify this to 50 meters a minute now in reality that's not exactly what's happening because you don't just travel non-stop at the same speed and in reality you're going to be speeding up slowing down so you can look at the slope of the line in this motion graph and you can see the speed of the traveller and so for example the fastest that they're ever going would be in this section right here and that's because they're traveling the farthest distance in the Y direction over the smallest amount of time in the X direction and of course where the slope is very shallow then they're going to be going much slower now notice at this point their position actually comes closer back to the start so their backtracking here faster than they were here so it might be that they're they're driving slowly along and then they get frustrated so they make a turn very quickly to get back on track and then they speed along this way and of course they finally slow down before they get to their final destination so this is kind of a motion graph and it tells us a little bit about the travel these kinds of things would be useful for companies like lyft or uber or GPS map come apps that are trying to look at congestion and areas and offer alternative route so as we saw in the last slide we take we took a look at a motion graph and the motion graph plotted distance and time so the resulting slope on that motion graph would be the traveller speed so let's do a quick calculation for average speed the first thing you should know is you're not gonna need all of the details details in the middle you simply just need to pay closely pay close attention to the initial and the final and so if we look at the at the time the final time in the final distance we can get an average speed so in this case we've got starting in one hour and going to three the difference would be two hours and if we go start at fifty kilometers and we travel 150 kilometers then the difference would be 100 meters kilometers excuse me and so we can simply calculate the average speed as we saw from that red line in the previous slide the motion graph that 100 kilometers divided by 2 hours would be 50 kilometers per hour so you can check yourself with you know the math here pause the video and see if you can follow along it's a little neater down below so as we've seen a little bit about speed and calculating average speeds it's worth noting another term that you'll come across in your reading is velocity and velocity is not the same thing as speed speed as a scalar quantity meaning it just has a value but with velocity it's got a magnitude and a direction so we call velocity of vector quantity so the major thing that you should know about velocity is it describes the speed how fast it's going and it has direction so the difference would be an automobile going 50 miles per hour is a speed but another car going 50 miles per hour north is a velocity so it's just a subtle difference but worth noting because they are different terms sometimes are indicated with arrow heads they'll have the direction compass direction next to these magnitude so you can tell that it's a vector quantity so this is just a visual showing you the same thing essentially they have different speeds same direction so they would have different velocities even though they're going in the same direction they have the same speed but different directions so although their speed is the same they have different velocities because they're going in different directions so if you're talking about traveling a distance 50 miles and then you travel another distance 50 miles the total distance traveled would be a hundred miles but that works for scalar quantities but for vector quantities if you're going 50 miles north and then you go 50 miles east it is not summative you cannot say 100 miles northeast that is not going to be correct it's not you can't add them because they're different velocities so instead you would have to do represent 50 miles north 50 miles east and actually get that vector quantity here using the Pythagorean theorem a squared plus B squared equals C squared you can calculate the hypotenuse of that right triangle so acceleration is another concept that we see when describing motion and acceleration is basically how fast and time does a velocity change so if you recall the units for velocity were meters per second and to describe the velocities change in time that's going to be meters per second per second so that's how fast the velocity changes in an amount of time and that's going to be equal to acceleration another way of writing that if you remember if you remember your math and your fractions is going to be meters per second square so these are the units for acceleration it's how fast your velocity changes over time the speed can change your acceleration the Direction can change your acceleration ambo speed and direction can change your acceleration let me give you an example a car going 30 to 60 miles an hour is going to be accelerating because over some period of time it might be two seconds we can look at their acceleration this is common stand this is a common statistic and a lot of high-performance vehicles sports cars where you're looking at how fast it'll go zero to 60 miles per hour and so that you're looking at the acceleration if someone says it'll go zero to 60 and 4.5 seconds then you can calculate the acceleration by taking 60 miles per hour divided by seconds of course miles per hour is a different set of units you can actually accelerate by slowing down because remember it's just how fast your velocity is changing over time it doesn't necessarily mean that it's always got to be positive in a car you might say it's decelerating or you're braking but in physics it's still in acceleration it's just negative and then you could be going the same speed but change your direction remember velocity is about changing directions so you could be going 30 miles an hour and take this turn while keeping a constant speed of 30 miles per hour but because you changed your direction you have also accelerated so this is acceleration as well and of course if you do both you change your speed and your direction you're definitely accelerating so on a quick example if you're going from rest to 5 meters per second and you do that in five seconds how fast is the cyclist accelerating and so you can do the simple math zero to five in five seconds so it's five meters per second divided by five seconds and so you're gonna get one meter per second per second which is also equal to one meter per se squared so that's a quick example of how to calculate and there's the solution if you'd like to see it pause your video and the last thing I'd like to talk about is gravitational acceleration so we actually can measure how fast an object changes its velocity over time when dropped from an elevated position for example if you took a ball and dropped it out of a second-story window you will notice that if you time it at certain points along the way in its fall that it's actually speeding up its velocity increases as its falling so that's represented here with position M that's in meters and time and seconds down here on the x axis so if you're looking at meters per second then the slope of this line is your velocity and so you can see as you drop an object its velocity actually increases as it falls however if you plot the velocity over time you'll notice that it is constant and the slope of this line at this constant this is acceleration meters per second per second is going to be equal to nine point eight meters per second and so this 9.8 excuse me 9.8 meters per second squared is the acceleration that's the slope of this line and so we've calculated the acceleration of gravity on earth at 9.8 meters per second squared and that's a constant it's always gonna be true on on planet earth it's gonna be different on the moon as a different mass and so it has different gravity and so the acceleration due to gravity is gonna be different on different celestial bodies for example however that as you calculate the drop of a ball and time and you calculate its distance you'll find that it has a constant acceleration at 9.8 meters per second squared even though it's continuing to increase its velocity time is also increasing you can calculate the change in velocity over time is always going to be 9.8 meters per second squared and lastly if we look at say motion in an upward direction or a in a horizontal direction you could see that by throwing the football of course a 45-degree angle is going to be the best way to do this you have to consider the vertical velocity as well as the horizontal velocity and figuring out how far is gonna be and how long it's gonna be in the air we're not gonna talk about angular velocity and things like that and get into the geometry of it but that would be a more advanced examination of what's happening if you took a ball and tossed it up into the air at the point where it starts to come back down you can if you're able to time that take the time that it takes from here to here you can calculate how high it went if you know the time it took before it started to drop back down and so by this equation here it's just half of gravity times the time square and that's gonna get you the distance so if you think about this mathematically height would be measured in meters gravities and meters per second squared so the second squared the time has to be in seconds they'll cancel out and so you will calculate how high it was conversely you can use this equation to figure out how long it'll take something to hit the ground if you drop it from up here and you know the height to the ground you couldn't calculate based on gravity how long it's gonna take to get there by rearranging the equation so that's all I wanted to talk about in the first part of chapter two we'll get into forces and Newton's laws of motion and the second part of check chapter two you thanks