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Learning Goal:
To understand the basic concepts of projectile motion.
Projectile motion may seem rather complex at first. However, by breaking it down into
components
, you
will find that it is really no different than the one-dimensional motions that you have already studied.
One of the most often used techniques in physics is to divide two- and three-dimensional quantities into
components. For instance, in projectile motion, a particle has some initial velocity
. In general, this
velocity can point in any direction on the
xy
plane and can have any magnitude. To make a problem
more managable, it is common to break up such a quantity into its
x
component
and its
y
component
.
Consider a particle with initial velocity
that has magnitude 12.0
and is directed 60.0
above the negative
x
axis.
Part A
What is the
x
component
of
?
Express your answer in meters per second.
ANSWER:
=
-6.00
Correct
Part B
What is the
y
component
of
?
Express your answer in meters per second.
ANSWER:
10.4
Correct
Breaking up the velocities into components is particularly useful when the components do not affect
each other. Eventually, you will learn about situations in which the components of velocity do affect one
another, but for now you will only be looking at problems where they do not. So, if there is acceleration

Introduction to Projectile Motion
in the
x
direction but not in the
y
direction, then the
x
component of the velocity will change, but the
y
component of the velocity will not.
Part C
Look at
this applet
. The motion diagram for a projectile is displayed, as are the motion diagrams for
each component. The
x
-component motion diagram is what you would get if you shined a spotlight
down on the particle as it moved and recorded the motion of its shadow. Similarly, if you shined a
spotlight to the left and recorded the particle's shadow, you would get the motion diagram for its
y
component. How would you describe the two motion diagrams for the components?
ANSWER:
Both the vertical and horizontal components exhibit
motion with constant nonzero acceleration.
The vertical component exhibits motion with
constant nonzero acceleration, whereas the
horizontal component exhibits constant-velocity
motion.
The vertical component exhibits constant-velocity
motion, whereas the horizontal component exhibits
motion with constant nonzero acceleration.
Both the vertical and horizontal components exhibit
motion with constant velocity.
Correct
As you can see, the two components of the motion obey their own independent kinematic laws. For
the vertical component, there is an acceleration downward with magnitude
. Thus, you
can calculate the vertical position of the particle at any time using the standard kinematic equation
. Similarly, there is no acceleration in the horizontal direction, so the
horizontal position of the particle is given by the standard kinematic equation
.
Now, consider

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