Transcript Projectile Motion …it’s as easy as rolling off a cliff… Prediction  Before you participated in the PhET simulation, you made a prediction and explained.

Projectile
Motion
…it’s as easy as rolling off a
cliff…
Prediction
 Before you participated in the PhET simulation,
you made a prediction and explained your
reasoning to me via email.
A Little History…
A few years ago,
researchers went to
elementary, middle,
and high schools as
well as universities
and showed students
this image and asked
them,
“Ignoring air resistance, which of the following correctly
shows what an object would do if it rolled off a cliff?”
The Results
 The
breakdown of answers they got was almost exactly
the same at all ages.
 About 60% said “A” was correct. The
object will stop in
midair, and then start to fall straight down. Because some
people referred to the coyote in cartoons, the researchers
called it the Wile E. Coyote Effect.
 About 25% said “B” was correct. The
object will move
forward at first, but will eventually just fall straight down.
 About 15% answered
“C”. The object will continue to
move forwards the entire time it is falling.
So What’s the Correct Answer?
 I’m not going to tell you. We’ll revisit the question
at the end of these notes. 
Observations from the Simulation

As we saw with the simulation, the projectile that fell
straight down and the one that was shot from a
cannon horizontally - hit the ground in the same
amount of time.

So what effect did horizontal velocity have on the time
it took (the downward motion of) the projectile to hit
the ground?


None!
The best conclusion we can make from this is that the
horizontal motion of a projectile does not affect
downward motion of the projectile.
Observations from the lab
 Intuition will tell you that the horizontally
launched object will “hang” in the air.
 But…

YOUR INTUITION (at least in this case) IS WRONG!
 Here’s a video
 Here’s another
video
Horizontal and Vertical Motion
 The
most important thing you can remember
about projectile motion is this:
 Horizontal and Vertical motion are completely,
100% INDEPENDENT of each other – even when
they are happening at the same time.
 Your
lab question was, “How does initial velocity
affect the amount of time it takes a horizontallylaunched object to reach the ground?”
Does Horizontal Affect Vertical?
 The

short answer to the lab question is:
It Doesn’t! The horizontal motion of the projectile is
unaffected by the downward (vertical) force of
gravity.
 What does
affect how long it takes an object to hit
the ground (ignoring air resistance)?


One thing and one thing only…
The height it is launched from!
Looking at velocity vectors
 On the next slide, we’re going to look at the paths
two projectiles follow. One projectile is shot out of
a cannon, the other is dropped at exactly the
same moment.
 yes, just like the lab

 When going through the slide, remember:



velocity is a vector (has magnitude & direction)
gravity is a constant force
constant forces cause acceleration
two cannon balls – one is
given a horizontal force,
the other is just dropped
gravity acts
downward on
both
horizonta
l motion
continues
gravity acts on all
objects equally
gravity is a
constant force
that
accelerates all
falling objects
(ignoring air
resistance)
While clicking
through this
slide, keep in
mind that the
law of inertia
tells us that
objects in
motion stay in
motion at a
constant speed
and in a straight
line. Since
horizontal
motion is
independent of
vertical motion,
the horizontal
vector never
changes!
Back to the cliff…
So…which
path will
the red
ball
travel?
It will follow path C – because its horizontal motion will
continue at the same speed and direction (law of inertia)
while gravity exerts a downward force at the same time!
Reflection
 As we saw with the ball and
the cliff question, many
people – even highly educated people – have
misconceptions about falling objects versus objects
with a high horizontal velocity.
 Ask five
of your friends or family members which
bullet will stay in the air longer: one shot from a pistol
or one dropped from pistol height. (Obviously, don’t
ask students in this class or physics majors )
Reflection
 On the
discussion board page, join the discussion
about why you think most people have the
misconception that a bullet fired from a gun will
“hang” in the air. What is it about a bullet from a gun
(or an arrow from a bow and arrow, or a cannon ball
out of a cannon, etc.) that makes it so hard to believe
that gravity acts on it exactly the same way gravity
acts on an object with no horizontal velocity (ie, one
that is dropped)?
 Make one original post and
other student’s posts.
respond to at least two