[Music] well greetings and welcome everyone uh this is a phrase i never thought i would ever say in my entire teaching career but welcome to physics oh for those of you who know me you'll know that that's going to be a good running joke throughout this entire series here my name is joe lamb jlan bio make sure you like subscribe leave a comment down below just kidding you can't because i turned the comments off i think oh crap maybe i didn't do that i'll need to check anyways um so today for our very first video we're gonna break into physics just a little bit by describing motion speed and velocity should be pretty straightforward i'm assuming you're taking physics you're probably pretty good with math already to begin with so so a little about our student learning objectives for this video by the end of this video you should be able to describe and analyze the concept of motion and be able to calculate and understand concepts of velocity and speed so again this is a physics course so you can anticipate probably having quite a few math problems to be able to go through and solve but we're on a very similar journey together my first time teaching it to your first time learning it so hopefully we'll have a good time let's get started so when we describe motion you know an object that's moving uh typically we need a frame of reference and typically we use a reference note that's known as a coordinate system you can think of this like the cartesian planes that you use when you're graphing things in math class calculating slope and whatnot we'll be doing a heck of a lot of that in here so some frames of reference here the soccer ball rolled 25 meters in two seconds we drove 35 miles per hour we need some sort of quantifiable information that is we need numerical information to understand a little bit more about what that motion looked like whether it was fast or slow you know we need numbers to be able to calculate that and think about that so we're going to use numbers obviously and we're going to use a coordinate system that's very very similar to what you've seen in math class so i want to talk about two key vocabulary terms here the first one being distance and so let's say you walk from your house to your friend's house and then you walk to good old udf the pride and joy of southeastern indiana and southwestern ohio if it's 2.1 kilometers to your friend's house and 4.3 kilometers to udf what's the total distance traveled well you went from your house to your friend's house and that's 2.1 kilometers and then you went another 4.3 kilometers to udf so the distance being the total length that is traveled would be 2.1 kilometers plus 4.3 kilometers which yields a total of 6.4 kilometers please make sure you include your units so distance is the total length traveled now that would include let's say you're making a round trip to the grocery store and let's say that it's 10 kilometers to the grocery store well if you make a round trip you travel a total of 20 kilometers 10 kilometers to get to the grocery store and 10 kilometers to get back from the grocery store to your house displacement however is a little bit different displacement is the net change in position this can be the same as distance if the movement is in the same direction so for example in the previous problem the distance and the displacement are the same it's going to be 6.4 kilometers because distance would be how far you traveled and then the displacement is how far you are from where you started so those would be the same values but let's take a look at the example here let's say we're going from the dorm and we go to our physics class and then we are hungry after physics of course because we used our brains so much it just made us absolutely starving and we decided to go to the cafeteria well if i take a look going from the dorm to physics class is five blocks and then going from physics to the cafeteria is six blocks so the total distance is 11 blocks right five blocks going from the dorm to physics and then 11 blocks going from physics to the cafeteria the displacement though is the net change in position so even though i went five blocks to physics and then six blocks to the cafeteria my total displacement is negative one block because my final position is one block behind my initial position so that's a little bit different there remember the distance and displacement are not the same thing distance is the total distance traveled displacement is the net change in position so even though you traveled 11 blocks and is a distance you only have a displacement of negative one because of the movement distance is always a positive value displacement can be positive or negative based on where you are located based on your original position so let's take a look at the sample problem what is the distance traveled and the displacement if you travel from house b to udf and then back to house a well let's stick house b our starting point and we'll say that that is 0. it's 4.3 kilometers to udf and 2.1 back to house a so the distance remember is always a positive value so for the terms of distance we're going to go i'm going to change my color here for distance so we'll use red for distance we go from house b to udf and then all the way back to house a so in this instance we walked 4.3 miles i'm sorry 4.3 kilometers to go from 0 to 4.3 we then walked another 4.3 kilometers to get back and then another 2.1 kilometers to go from house b to house a so we just add all three of those values together and what we should get is a total of 10.7 kilometers and that is our distance now remember that displacement is just the change from where you start to where you finish so here i started here and i'm going to finish here so the displacement is just the distance between these two points and in that instance because we are going backwards from our frame of reference negative 2.1 kilometers is our displacement okay remember that when we're looking in terms of directions we need to consider what is forward and what is backwards okay just like in a cartesian plane um you know this the the direction to the right is going to be forwards and the direction to the left is going to be the reverse okay pretty straightforward there let's talk a little bit about speed speed is simply the average rate of motion of an object and this is given by the following formula where the average speed is equal to distance over time keep in mind distance not displacement it's really important that we're able to keep those two things separate from one another the units for this is meters per second or m over s and speed is always a positive value because we don't care about which direction we're going if we're going north south east west speed is always a positive value because we're looking at distance versus time a kingfisher is a bird that catches fish by plunging into the water from a height of several meters if a kingfisher dies from a height of 7.0 meters with an average speed of 4.00 meters per second how long does it take to take to reach the water well remember the speed is equal to distance over time i'm given the speed and i'm given the distance so i can calculate for time so 4 equals 7.0 meters over time we'll just leave it seconds okay 4.00 meters per second we're going to rearrange this equation so we're going to multiply by s to get it out of the denominator we're then going to divide both sides by four we're going to divide those two values to get 1.75 seconds simple enough against speed distance over time always a positive value and real simple to do some basic calculations with that let's move on now velocity is different velocity is defined as displacement over time not distance over time and that is given by the following formula where the average velocity is equal to displacement over elapsed time so the only difference here is that we're looking at displacement in our numerator versus distance velocity tells us how fast an object is moving and can be both positive or negative so this is when we start including um the vector that's associated with it whereas we looked at speed it was positive in all directions in this case depending on which direction the car is going and your frame of reference velocity can be both positive and negative an athlete sprints 50 meters in six seconds stops and then walks slowly back to the starting line in 40 seconds if the direction of the sprint is taken to be positive what are a the average sprint velocity b the average walking velocity and c the average velocity of the complete round trip okay so we have three different problems we're going to calculate velocity for but we can kind of imagine this and i always find it helpful to draw a picture a little guy here okay he sprints 50 meters stops this takes six seconds we go this way stops this takes 40 seconds so we want to know the average sprint velocity and keep in mind that this is 50 meters away okay so remember that we're looking for velocity and that is displacement over time so for the sprint velocity all we're going to look for the difference between this distance here and this here so that is where our starting and ending point is so our displacement for part a is 50 meters and it took 6 seconds so we just simply plug those numbers in and solve 50 divided by 6 is equal to 8.33 meters per second let's look at the next part of the problem the next instance i wonder the average walking velocity well he walked back yes so walking backwards the displacement because we're going backwards is negative 50 meters because he's going from here to here that took a total of 40 seconds so we're going to divide 50 by 40 seconds to get an average velocity of negative 1.25 meters per second the very last bit is asking for the average velocity of the complete round trip well we started at this point and we ended at the exact same point so the total displacement for part c is zero zero meters over 46 seconds which means that the total velocity for the complete round trip is zero meters per second okay so it's important to distinguish between these concepts and it's weird to think velocity can be zero even though he actually moved but remember that displacement specifically is the init final position minus the initial position it's the change in position not the total difference traveled the position is the same for the total round trip and so therefore his total displacement is zero and zero divided by anything is well zero so just to recap speed versus velocity speed is rate of motion it is always positive and gives absolutely no information about the direction whatsoever the greater the speed the faster an object moves velocity however is rate of motion and direction the sine of velocity gives the direction of motion remember we can have positive velocity and negative velocity based on where we start and our frame of reference magnitude of velocity is the speed of motion okay so just kind of keep those things in mind that um distance versus displacement and speed versus velocity and that will cover pretty much what you need to know for the first week so hopefully by the end of this video you can describe and analyze the concept of motion by thinking of displacement versus distance and also calculate and understand the concepts of velocity and speed hey hope you guys have a great day thanks again for watching make sure you like comment and subscribe and we will talk to you guys later have a great day everyone bye