# Introduction to Motion | Physics

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Hello there! Now we’ll start the topic kinematics. So in kinematics what we’ll basically study is the ways to define motion. So the basic modules that we’ll be studying here in the next few topics is mechanics. So mechanics is divided into two main categories. The first one is the kinematics which we’ll start now. The second is dynamics. So both are involved in study of motion but in dynamic, we’ll study about cause of motion and kinematics, we basically look at the parameters which define motion. So you could say measurement, measurements of motion. So in kinematics, we find ways different ways in which you can quantifyably define motion, right? You may have heard the definition. So if you talk about motion, so a particle will be said to be in motion, any given particle may be said to be in motion, if it changes, the particle changes its position with the respect to its surroundings, the surroundings, right. So a particle will be said to be in motion. If the particle changes its position with respect to the surroundings and if it does not change in its position the particle is said to be at rest, right? So now we’ll briefly discuss the significance of surroundings. So the two key words here, the first one is position, the second surroundings. So let’s first talk about surroundings because this is something that we’ll briefly discuss now and this will become significant as we move to the later chapters, right. So a simple example is imagine two people who are above the same platform and this platform is moving with a constant speed, let’s say five meters per second. So this is a platform on which there are two people. Let’s call them A and B and both of them are moving at five meters per second and there’s a third person C, who is standing on ground, right. So now if you ask C, if you ask C, so according to C, the speeds of A and B both will be five meters per second, right? If you ask this person C about the speed of person A and B, he will report the speeds to be 5 m/s, right? But what will A report, if you ask him about the speed of B? If you think about it, A is moving with the same speed as B, right? They’re both moving on with the same speed. They’re on the same platform. So A will find that B is stationary. To help you figure this thing, imagine sitting inside a car but someone sitting next to you. So when you look at that person you don’t feel like that person is moving, right? In fact you feel that you and the person sitting next to you, both of you are stationary, right? Similarly when you ask A, he will say that speed of B is 0 m/s and similarly if you ask A about the speed of C, he will say that speed C is moving in the opposite direction five meters per second, right? We put a minus sign here. So this example was simply to give you an idea that whenever we talk about motion, whenever we talk about motion there’s always an observer, right? So whenever you’re talking about motion they will the entire definition or entire description will be with respect to somebody, right? Somebody must be watching the entire event and then reporting, right? Like I told you this speed is five meters per second. Somebody is measuring that speed, right? This is not a general value, five meter per second. This is the speed with respect to somebody because if we change the observer like you went from C to A, the speed changed, right? So that is why the concept of observer is important when you study mechanics. So for now we will be dealing with what we call as a ground observer. So you come across this term, so we will for now be dealing with the ground observer and a ground observer is an observer like C who’s at rest, right? So we will be defining all the definitions that you see in this particular topic. They’ll all be with respect to the ground observer meaning an observer who is at rest, right? So this is the significance of surroundings. So this is why motion depends on surroundings because surroundings is basically our way of including the observer into this definition. Because you saw that if you ask A, he will say B is at rest, right? So motion itself depends on observer and for now we will only talk about a ground observer, right. So rest is obviously when a particle does not move. So in the same definition it does not change its position. So in that case the particle will be said to be at rest. The second term that we saw is position. Now you’ve started vector. So position will be measured with the measurement will be done through what is called the position vector, right? So if you’re not aware, let me tell you so if we have a three axis, the x-axis, the y-axis and the z axis. So if this is the x, y and z. So whenever you do that you have to make sure that it satisfies the right hand rule, the x cross y should point towards z, right? i cross j has to be k. Now if you have any particle located at any particular point, for instance if you have a particle somewhere here, then to measure its position, to find its position vector you simply draw a line from origin up to this point, right? And the vector that is formed this vector r will be the position vector. This vector r is the position vector. Now to write it in Cartesian coordinate forms, what you have to do is draw a perpendicular on the XY plane. So that will be something like this and from this you can draw perpendicular to the y axis. Then draw a perpendicular to the x-axis. So this distance, this distance is the x-coordinate. So this is the x-coordinate. This the y coordinate and this height, this height of the point, meaning basically the height of the perpendicular, this height is the z coordinate, right? So just to visualize this more clearly, imagine a cuboid. So in this diagram the vector basically forms a cuboid with the axis. So now the point, this point is the origin. This of course is the x-axis. This is the y axis and this is tha z axis, right? And your vector. So this is your point and the vector is the body diagonal of this cuboid. So this goes from here up to this point. So this body diagonal, i hope you are able to visualize this, it joins the opposite vertices, right. Right. So this is the perpendicular that we drop and hence the z coordinate. So from the foot of the perpendicular, we drew a line perpendicular y axis and this length the x-coordinate and similarly this other line that is drawn this is the y coordinate, right? So in this particular case, the position vector r becomes x i cap plus y j cap plus z k cap, right? So position vector is the vector that is used to identify the position of a particle, right? So when we talk about motion when we said that it changes its position. So we can say that when we turn all the quantities into vectors then we can talk about motion being any change in the position vector with respect to the surroundings, right? So whenever a particle changes its position vector then it’ll be said to be in motion, right? So now if the particle moves such that only the x-coordinate change, if only x, only y or only z changes. If only one, only one of x, y and z is changing, if only one coordinate is changing then the motion is one-dimensional motion, right? And if two of them change then you have 2-D motion and similarly 3-D motion will imply that all three x, y and z are changing, right? Right. So so far we’ve talked about the two main branches, the first one being kinematics which we are going to study here. So in kinematics which we are more interested in measurement of motion, right? So here we’re gona talk about things like speed, acceleration, distance, right? So these are the quantities we’ll be interested in. We’ll be interested in quantifying motion, right? To be able to measure different aspects of motion and in dynamics we’ll talk about cause of motion where we’ll talk about the force which is acting then go on to study torque which causes rotation, right? So in dynamics will study about the things that cause motion, right. Then we talk about the definition of motion. We saw how motion is a particle will be set it in motion when it changes its position, right? And we saw how this change in position essentially depends on what is called an observer. Because as you change the observer, the definition or the description of motion will change. Then we saw the basic vector, the position vector which is used to define the position of a particle, right? So this was a general basic discussion about motion. In the next session, we’ll talk about the basic quantities which are distance, displacement, speed and velocity but before we go there we should understand that these quantities distance and speed they are scalar quantities. So you have started this in vectors, just revising these basics. Distance and speed are scalar quantities, meaning distance and speed will not have direction. Whereas displacement and velocity, these are vector quantities, right? So there’ll be a difference about in these definitions and the simple difference would be that distance and speed are scalars and these are the corresponding quantities which are vectors, right? Then we’ll make a distinction between instantaneous quantities, instantaneous values and average values. So the distinction between these two is that instantaneous is measured at a given time instants. So instantaneous values are measured for a time instance and average values are measured for a time interval. So let’s understand the difference. Now say a particle moves from a given point A to a point B and why it was moving? We were recording time. So we know that it was at A at t equal to five seconds. So the time at point A was five seconds. So it was 0.5 seconds. So by the time it reaches point B that time was let’s say 15 seconds, right? On the clock you saw that you started your stopwatch and when it reached point A on your stopwatch that time was 5 seconds and by the time it reached point B, the time became 15 second, right? Now these two are time instances like t is equal to five seconds and t is equal to 15 second and these are time instance. It is a specific point of time, right? When the clock read five-second that is a specific instance, right. Similarly, when the clock read 15 seconds, it was a specific time instance. Interval is this total time which is 10 seconds or which the particle was moving, right? So we can say that particle where it went from A to B it was moving for 10 seconds. So this 10-second is a time interval and you will see that whenever we measure something over an interval that quantity is an average quantity and whenever you measure speed at a specific time that time is called a time instance and the quantity measured will be instantaneous quantity, right? So these are the basic definitions that you should know. Understanding these we’ll start the basics of kinematics in the next session. As I said we’ll talk about the simple quantities like distance, displacement, speed and velocity in the next session. That is it for now. Thank you!