Projectile Motion

Ignoring air resistance, which of the following correctly shows what an object would do if it rolled off a cliff?

#1: the object will stop in midair and then fall straight down (like Wile E Coyote)

#2: the object will move forward at first, but will eventually fall straight down

#3: the object will continue to move forwards the entire time it is falling

The correct answer is…..•#3 – the object will continue to move

forward the entire time it is falling▫Why does this happen?▫Because the object is in projectile motion

When an object is moving in 2 dimensions (vertically and horizontally) but is only affected by gravity This means that any object in projectile motion is

also in free fall▫The two movements, horizontal and vertical,

act independently of each other One does not effect the other We will calculate them completely separately

Projectile Motion, an illustration

Vertical Motion

Horizontal Motion

By the way – if you drop a ball and project a ball straight out (from the same height), which will land first??

•Make your prediction in your notebook now!!

First – we will observe the Newton’s Apparatus…

•From 330 feet, the Mythbusters guys fire a gun horizontally and drop a bullet▫Simultaneously AND from the same height▫Which do you think hits the ground

first?

•https://www.youtube.com/watch?v=D9wQVIEdKh8

Horizontal motion (X)•As the object leaves the cliff, it does so with

an initial horizontal velocity

▫The only way to change this velocity is to put some sort of new force (push or pull) on the object

▫Ignoring air resistance, there is no other force affecting the object so it’s horizontal velocity does not change So the object continues to move forward at the

same speed the entire time it falls

Vertical motion (X)•Remember that in projectile motion, the

object is in free fall▫So as soon as it leaves the cliff, gravity is

pulling the object down Acceleration due to gravity affects vertical

speed causing it to increase by 10 m/s/s

Horizontal and vertical motionWhat do you notice about the horizontal velocity vector?

What do you notice about the vertical velocity vector?

The shape that an object in projectile motion takes is known as a parabola (an arc)

When you are solving projectile motion problems…

If you are solving for horizontal motion (x direction), only use:

If you are solving for vertical motion (y direction), only use:

V = d t

V = gt or d = 1/2 gt2

This is because gravity does not effect horizontal motion

This is because gravity is the only thing that affects vertical motion

So what is the only thing both the horizontal and vertical components of the motion have in common??

TIME!!

Jordan walks by a cat sitting on a 3m tall fence. He pushes it off with a horizontal velocity of 2m/s (and runs away laughing). How far away from the fence does the kitty land?

Priscila pushes Josiah’s backpack off his desk (.5m high). It lands .4m away. With what initial velocity did she push it off?

Jariana spits off the balcony of her hotel room in San Antonio (she’s trying to hit Ti’Ara) with a velocity of 4m/s. The spit wad lands 3.5 m away on the ground from the building. How high is Jariana’s room?