Average speed & velocity (with examples)

this is my house and my good friend named SATRA lives close by about two kilometers away so I call him up and say hey you want to come over and he says yeah of course I'll leave right now and so he gets on his bike and starts riding towards my house but guess what he doesn't stop at my place you see Sutter loves ice cream and so he decides to first visit an ice-cream shop which is about eight kilometers away from my house and he doesn't tell me about this and so this entire journey of ten kilometer takes him about one hour so he reaches the ice-cream shop after 1/r gets his ice cream and then starts riding back towards my place and this takes him another one are so he reaches my place after two hours let's say two hours I have a big question then we want to try an answer in this video is how fast was satra or this entire journey now it turns out that there are two ways to answer this question one way is to talk about his speed you kind of know what speed is it's a measure of how fast somebody is moving right in other words it's a measure of how much distance something is traveling in unit time unit time means in one hour or one second or one minute etc so let's go ahead and calculate his speed over the entire journey to do that we need to know how much distance he traveled over the entire journey so the distance he traveled was ten kilometers initially and then another eight kilometers so ten plus eight 18 kilometers and how much time he took to travel that well he took two hours to do that right because he he finished his journey at two hours so 18 kilometers in two hours so what is his speed to calculate speed we need to know how much kilometers is traveling per hour in one our unit time so all we have to do is divide 18 kilometers by two R's and that gives us nine kilometers 9 kilometers per hour and so we'll say satara's speed was 9 kilometers per hour but now comes a very important detail is he really traveling 9 kilometers every our no because we saw that in the first one our he traveled 10 kilometers and in the next one our he traveled 8 kilometers so what does this mean what does 9 kilometer per hour mean well this is called the average value what does that mean well notice that in the first hour he was actually faster because he traveled 10 kilometers in one hour and in the next stretch he was a little slower because he only traveled 8 kilometers in one are so over his entire journey he was not traveling with a constant speed he was faster initially and then he slowed down but now we are asked to calculate the speed over the entire two hours whenever we do that notice when we divided 18 by 2 we ended up assuming that he traveled equal distances every R and when we assume that we call it the average value this is a very similar to how we usually marks are calculated in schools for example let's say you get 10 out of 10 in physics and I get only 8 marks in physics then our average marks would be 8 plus 10 divided by 2 which is 18 by 2 which is 9 now notice neither of us got 9 you got a little more I got a little less and that's why this is called the average value because we divided our total marks equally amongst each other in a similar manner we divided that total distance traveled equally into ours and as a result this is called an average speed so he didn't literally travel nine kilometers every R and so in general to calculate average speed notice we have to divide the distance by time and the unit of speed is kilometers per hour but although this is a very familiar unit it is not a standard unit and that's because the standard unit of distance is not kilometers but meters and the standard unit of time is seconds and so if you look at the standard unit which we often call SI standard international we say and the standard unit of speed becomes meters per second all right now there's a second way to think about how fast he was the second way is in terms of velocity there's a small difference between velocity and speed velocity talks about how much his position changes in a unit time so when we talk about velocity you don't look at the distance but you only keep track of his position changes so let's take our friends example to understand better where was satra in the beginning well he was he were here this was his initial position initial location at the end of two R's where was he well he has now reached my house my place at the end of tours so in two hours how much did his location change how much did his position change can you see it only changed by two kilometers now I know that he has traveled 18 kilometers the distance that he has traveled is yes 18 kilometers but as far as his position is concerned it has only changed by two kilometers and velocity keeps track of how much that is changing in a unit time so let's write that down to calculate it so his position change position change which is also called displacement by the way the position change in short is called displacement of our friend is only two kilometers and we must always mention direction when we talk about position changes because if I just say two kilometers it doesn't tell me in which direction very ended up it could mean two kilometers to the left it could mean two kilometers upwards although you can't ride your bicycle upwards it doesn't tell me so - to make sure that we tell that he has ended up in my house it's important to mention the direction and in our case the direction is two kilometers to the right so we'll say his displacement was two kilometers to the right and how much time did he take well just like before the total time is just two hours and now we just have to figure out how many kilometers he's being displaced per hour that would be the velocity so we just have to divide it so velocity would be two kilometers divided by two hours which is equal to one kilometer per hour and of course we should mention the direction because you're talking about position changes and just like with speed he is not literally changing his position 1 kilometer every R is not literally doing that that's why again this is an average value so we will call this the average velocity and so in general to calculate average velocity we have to take displacement and divide it by time and just like speed the standard unit for velocity is also meters per second so long story short if you care about how quickly someone is covering distance then you're thinking about speed on the other hand if you are like me who's waiting for his friend and who's wondering why does it take two hours just to change the location by two kilometres and you're pretty mad at him then you're thinking about velocity because velocity only talks about how quickly your position is changing it doesn't care about how much distance is traveled and another big difference between the two is that velocity has direction whereas speed has no direction also I couldn't stay too mad at him because it turns out he brought some ice cream for me as well