hey everyone in this video we're going to introduce projectile motion we'll start with some examples and definitions then we'll talk about the difference between one-dimensional and two-dimensional projectile motion after that we'll do a quick overview of the kinematic equations and finally we'll cover some important things to know when working with projectile motion so what are we talking about when we say projectile motion basically it's when an object moves through the air and is only affected by gravity the object could be moving sideways or falling straight down the object's initial speed and angle determine its path through the air this shows up a lot in sports sometimes the path of the ball has to be precise other times the ball can have a wide range of motions we usually focus on a single object but projectile motion can help us describe other things like the shape of a stream of water those are some examples of projectile motion but what does the term really mean in physics projectile motion is the motion of an object while it's only being affected by gravity and nothing else projectile motion only refers to the period of time when the object is moving through the air and it's not touching anything else so this cannon ball is not in projectile motion while it's accelerating inside the cannon or when it's rolling on the ground it's only in projectile motion when it's in the air the ball on the right is only in projectile motion between the moment it's released and the moment it hits the ground we call the object a projectile and we call the path of the projectile motion the trajectory projectile motion is also referred to as freefall but the terms are used in different ways and we'll come back to that later if we say an object is in projectile motion then air resistance or the drag force is ignored basically air resistance makes the calculations a lot more complicated and we can still predict the motion pretty accurately By ignoring air resistance finally in projectile motion the vertical acceleration of the object is 9.8 m/s squar downwards represented with the constant lowercase G this is the acceleration due to Gra gravity if there's no air resistance then gravity causes all objects to fall down at the same rate 9.8 m/ squared the horizontal acceleration is zero which means the horizontal velocity is constant these might be the most important things to remember about projectile motion because they determine which kinematic equations and values we're going to use now let's talk about one-dimensional versus two-dimensional projectile motion here's a few different scenarios that we might see an object could be dropped from rest meaning it has no initial velocity and it falls straight down it could have an initial upwards velocity so it moves up and then falls down it can have an initial downwards velocity so it falls down it can have an initial horizontal velocity and Falls sideways and it can also have an initial velocity at an angle so it has an initial horizontal velocity and an initial Vertical Velocity in this course we're going to call all of these scenarios projectile motion or we might say all of these things are in freef fall in every case when the object is in the air the only thing affecting the motion is gravity so the vertical acceleration is not 9.8 m/s s downwards we're going to say the examples on the left are one-dimensional projectile motion because they only move vertically and we only use the kinematic equations in the y direction the examples on the right are two-dimensional projectile motion because they move in the vertical and horizontal directions so we use the X and Y kinematic equations and two dimensional vectors in other places the term projectile motion might only refer to the two-dimensional projectile motion and the term freefall might refer to one-dimensional projectile motion so that's something to be aware of but again we're going to call all of these projectile motion and split them up as one-dimensional and two-dimensional so how do we apply the kinematic equations that we already know to projectile motion for 1D projectile motion we only need the equations in the y direction if we choose up to be the positive y direction then the acceleration would be negative because it points down so when we use these equations we plug in negative G or 9.8 m/s squ for the acceleration a y and since there's acceleration the Y velocity is not constant and we don't really use the equation for average velocity for 2D projectile motion the equations for the y direction will be the same but since there's no acceleration in the X Direction then the x velocity is constant and we won't use the X equations with acceleration 1D and 2D projectile motion are really just specific cases of 1D and 2D motion where we know the acceleration is 9.8 m/s downwards but there are a few things about projectile motion which we need to remember when solving problems this will just be a summary and the next few videos we'll cover these in more detail first every problem is different but there are some key points in the trajectory that are important to think about the initial point the point of maximum height and the point when the object hits the ground or the motion ends usually we're given the velocity at the initial point if it's 1D projectile motion that will just be the initial y velocity if it's 2D projectile motion we might be given the magnitude and angle of the initial velocity Vector then we have to find the components of that velocity Vector which are the initial x velocity and the initial y velocity then we can use those velocities in the X and Y equations when the object reaches its maximum height the vertical velocity VY is zero this applies to 1D and 2D projectile motion also if the initial and final points are at the same height the trajectory is symmetric the amount of time it takes to reach the maximum height is half the time it takes to reach the final point it's half the duration of the full trajectory again this is only true if the initial and final points are at the same height if the object starts and ends at different heights the trajectory is not symmetric and the time at the maximum height is not half of the total time at the point when the object hits the ground or anything else the projectile motion ends we call the horizontal distance between the initial and Final points the range of the projectile motion another important concept is that the duration of the motion the amount of time the object is in the air is only determined by the Y motion and doesn't depend on the X motion next the vertical velocity VY has the same magnitude at the same height on both sides of the trajectory but they have opposite directions if the initial and final points are at the same height the final y velocity is the same as the initial y velocity but it has the opposite sign finally we learned in the lesson on 2D motion that the X and Y motions are completely independent of each other so for 2D projectile motion we're going to study the X and Y motions separately since we might be dealing with a lot of variables and equations it'll definitely help to keep the X and Y motions organized all right that's it for this video thanks for watching and I'll see you in the next one