Transcript Projectile Motion

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
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 motion
What 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:
V=d
t
This is because gravity does
not effect horizontal motion
If you are solving for
vertical motion (y
direction), only use:
V = gt or d = 1/2 gt2
This is because gravity is the
only thing that affects vertical
motion
The amount of time that an object falls will be the same for
both horizontal and vertical motion
- This is because no matter if an object is falling or if it has
been thrown, it will fall towards the earth with the same
acceleration (10 m/s/s) due to gravity
A projectile Motion problem:
• A rock is thrown off a cliff with a horizontal speed
(Vx) of 20 m/s. It lands 5 seconds later.
Determine how high the cliff is and how far the
rock fell horizontally
Horiz.
Vert.
V
D
T
Create a
chart to
help
figure out
your data
What is the actual speed of the ball
before it hits the ground?
We found the vertical and horizontal speed of the ball so we can combine
the two vectors to find the actual speed
20 m/s
5 m/s
Another problem:
• A baseball is thrown out of a window with a horizontal
speed (Vx) of 45 m/s. It landed 9s later.
▫
▫
▫
▫
How high is the window?
What was the vertical velocity when it hit the ground?
How far away did the ball land?
What is the resulting speed of the baseball just before it
landed?