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- [Instructor] What do
vector components mean?
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Vector components are a
way of breaking any vector
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into two perpendicular pieces.
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For convenience we typically
choose these pieces
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to lie along the X and Y directions.
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In other words to find
the vertical component
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of this total vector knowing this angle,
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since this vertical component
is opposite to this angle,
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we could write the vertical
component as the magnitude
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of the total vector
times sine of that angle.
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And since this horizontal component
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is adjacent to that angle,
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we could write the horizontal component
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as the magnitude of the total vector
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times cosine of that angle.
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And if instead we were given this angle
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and we wanted to determine
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the vertical component
of the total vector,
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since this vertical component
is now adjacent to this angle
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we'd write the vertical
component as the magnitude
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of the total vector times
cosine of this angle.
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And since the horizontal component
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is now opposite to this angle
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we'd write the horizontal component
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as the magnitude of the total vector
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times sine of this angle.
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So remember, to find the
opposite side you use sine,
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and to find the adjacent
side, you use cosine.
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So what would an example problem
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involving vector components look like?
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Say you had this question
and you wanted to determine
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the X and Y components
of this velocity vector.
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Since the horizontal component is adjacent
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to the angle that we're given,
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we're gonna write the horizontal component
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as the magnitude of the total
vector 20 meters per second
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times cosine of the angle,
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which gives us 10 meters per second.
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And since the vertical component
is opposite to this angle,
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we can write the magnitude
of the vertical component
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as 20 meters per second
times sine of the angle
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which gives us 17.3 meters per second.
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But since this vertical
component is directed downward,
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technically this vertical component
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would be negative 17.3 meters per second.
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So using sine and cosine
will give you the magnitude
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of the components, but you have to add
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the negative signs accordingly.
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So if the vector points right,
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the horizontal component will be positive.
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If the vector points left,
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the horizontal component will be negative.
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If the vector points up,
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the vertical component will be positive.
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And if the vector points down,
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the vertical component will be negative.
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What does tail to tip or head
to tail vector addition mean?
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This is a graphical way to add
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or subtract vectors from each other.
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And the way it works is by taking the tail
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of the next vector and
placing it at the tip
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or the head of the previous vector.
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And once you're done doing
this for all your vectors,
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you draw the total vector
from the first tail
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to the last head.
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In other words, if you were
adding up vectors A, B, and C,
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I would place the tail of vector
B to the head of vector A,
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and then I'd place the tail of vector C
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to the head of vector B.
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And once I'm done I would
draw the total vector
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going from the first
tail to the last head.
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And that total vector would
represent the vector sum
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of all three vectors.
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And if you had to subtract the vector,
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you could still use vector addition.
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Simply add the negative of that vector.
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In other words, if you had some vector B
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and you wanted to subtract vector A,
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instead of thinking of it
as subtracting vector A,
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think of it as adding negative vector A.
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And the way you find vector negative A
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is by taking vector A and
simply placing the arrow head
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on the other end of the vector.
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So what would an example
involving tail to tip
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vector addition look like?
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Imagine we have this question,
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and we have these four vectors,
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and we were asked to
determine what direction
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is the sum off all of those vectors?
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So we'll use tail to tip vector addition.
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I'll take vector A
preserving its direction.
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I'm not allowed to rotate
it or change its size.
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And I'll add to that vector B.
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The way I do that is
putting the tail of vector B
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to the tip of vector
A, and we add vector C.
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And the way we do that is
put the tail of vector C
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to the tip of vector B.
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And finally we'll add the vector D
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by putting the tail of vector
D to the tip of vector C.
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And now that we've drawn
all of our vectors,
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our total vector will go from
the tail of the first vector,
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to the tip or head of the last vector,
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which means this is the
direction and magnitude
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of the total vector, A
plus B plus C plus D.
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Another more mathematical
way of adding vectors
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is by simply adding up
their individual components.
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So to find the total vector A plus B,
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instead of graphing them tail to tip,
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you can find the horizontal
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and vertical components separately
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by adding up the individual components.
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In other words, to get the
total horizontal component
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of the total vector A plus B,
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I could just add up the
horizontal component of A,
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which is negative 20, and
the horizontal component of B
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which is negative five to get negative 25.
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And to find the total vertical component
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of the total vector A plus B,
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I can simply add up the
vertical component of A
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which is negative 15 to
the vertical component of B
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which is 10, to get negative five.
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This technique lets you quickly determine
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what the individual components
are of that total vector.
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And again, if you need
to subtract a vector
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you can still add the components,
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except the components of a negative vector
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all get multiplied by negative one.
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In other words, if vector A
has components negative 20
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and negative 15, then vector negative A
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would have components
positive 20 and positive 15.
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So what would an example
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of vector component addition look like?
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Let's say you had this question.
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You had vectors A and B
with these components,
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and you wanted to know the components
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of the total vector A plus B.
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So to find the horizontal component
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of the total vector A plus B,
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I can add up the individual
components of A and B.
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So the horizontal component of A is four,
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plus the horizontal component
of B is negative one,
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gives me a total horizontal
component of positive three.
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And I could do the same thing
for the vertical component.
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I could add up the component
of A plus the component of B,
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which would be five plus negative four,
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gives me positive one.
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So since my horizontal component
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of the total vector is positive,
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I know it points to the right three units.
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And since the vertical
component of the total vector
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is positive, I know it points up one unit.
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That means my total vector A plus B
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points up and to the right.
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One unit up, and three units to the right.
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How do you deal with
2D kinematics problems?
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2D kinemtatics are projectile problems
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describe objects flying
through the air at angles.
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For these objects, if there's
nothing acting on them
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besides gravity, their
vertical acceleration
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is gonna be negative 9.8.
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And they will have no
horizontal acceleration
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since gravity doesn't pull sideways.
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Also, the X and Y components
behave independently.
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That means you'll use different equations
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to solve for vertical components
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than you will for horizontal components.
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Since the vertical
acceleration is constant
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you can use the kinematic formulas
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to solve for quantities
in the vertical direction.
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But you can only plug
in vertical quantities
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into these equations.
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Similarly, since the acceleration
is zero in the X direction
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you can simply use distance
as a rate times time
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to relate the quantities
in the X direction,
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but you should only plug
in horizontal components
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into this equation.
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In other words, as a projectile
is flying through the air,
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since there's no horizontal acceleration,
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the horizontal component of the velocity
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is gonna remain the same
for the entire trip.
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Which means the rate at
which this projectile
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is moving the X direction never changes.
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But since there is acceleration
in the vertical direction,
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the vertical component of the velocity
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will get smaller and smaller
until it reaches the top,
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and that means the total
speed of the projectile
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is gonna decrease as well
as you approach the top.
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And then at the top there is zero
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vertical component of velocity
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since the projectile's not
moving up or down at that moment.
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And then on the way down
the vertical component
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of the velocity gets
more and more negative,
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which increases the
speed of the projectile.
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Keep in mind during this entire trip
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the vertical acceleration
is the same, negative 9.8
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on the way up, at the
top, and on the way down.
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The vertical acceleration never changes.
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So what would an example problem
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involving 2D kinematics look like?
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Let's say a meatball rolls horizontally
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off of a dinner table of
height H, with a speed V.
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And we want to know how far
horizontally does the meatball
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travel before striking the floor.
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So the first thing we
should do is draw a diagram.
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So the height of the table is H.
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The initial speed of the meatball is V,
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and we want to determine how
far horizontally it makes it
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from the edge of the table.
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But note this problem's symbolic.
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We're not given any numbers
so we're gonna have to give
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our answer in terms of given quantities
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and fundamental constants.
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The given quantities
are things like H and V,
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and fundamental constants
are things like little G.
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So this quantity we want to find
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is the displacement in the X direction,
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which is gonna be the
speed in the X direction
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times the time of flight.
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We know the speed in the X direction,
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it's gonna remain constant.
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So this V is gonna be the horizontal speed
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for the entire trip.
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So we can plug in V for
the speed, but I don't know
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what the time of flight is gonna be.
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To get the time of flight
we'll do another equation
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for the vertical direction.
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The vertical displacement
is not gonna be H,
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it's gonna be negative H since
this meatball fell downward.
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And the initial velocity
in the Y direction
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is not gonna be V, it's gonna be zero,
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since this meatball had
no vertical velocity
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right when it left the table.
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It only had horizontal velocity.
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The acceleration is negative 9.8,
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but we're gonna write that in
terms of fundamental constant,
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so we'll write that as a negative G.
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This lets us solve for T.
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We get the square root of two H over G
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which we can now bring over to here
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to get the horizontal
displacement of this meatball
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before it hits the ground.
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Something else you'll
definitely have to know
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for the AP exam is how to
graph data to a linear fit.
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And what I mean by that is
that when you graph data
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it doesn't always come out linear.
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And when you don't get a linear graph
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it's hard to find the
slope of that curved graph.
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However you can force
your data to be linear
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if you write down the expression
that gives the relationship
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between your data in the
form of a straight line.
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So the form of a straight
line is Y equals M X plus B.
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Y would be the vertical axis.
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X would be the horizontal axis.
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M is the slope, and B
would be the Y-intercept.
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In other words, if you had the expression
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P equals 1/2 D squared,
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if you just plot P versus D
you're gonna get a parabola,
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and that means you got problems
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'cause finding the slope
of a parabola is hard.
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But if you instead choose
to plot P versus D squared,
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you will get a straight line
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because now you've required
P to be your vertical axis,
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you've required D squared
to be your horizontal axis,
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and the slope that's
multiplying what you called X
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is just a constant, and that means
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your slope's gonna be constant.
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So this lets us predict
what the slope would be
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if I plot P versus D
squared since the slope
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is always what's multiplying
my horizontal axis,
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the slope in this case should just be 1/2.
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So in other words, if
you force your expression
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to take the form of a straight line,
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now only will you get a linear fit,
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but you can predict what the slope is
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by looking at everything
that's multiplying
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what you called X.
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But a lot of people find
this confusing and strange,
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so what would an example problem
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where you have to graph data
to a linear fit look like?
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Say you were given this question.
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You repeatedly roll a sphere off a table
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with varying speeds V,
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and then you measure how
far they travel delta X
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before they strike the floor.
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If the table has a
fixed and known height H
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what could we plot to
determine an experimental value
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for the magnitude of the
acceleration due to gravity?
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So we repeatedly change the speed
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and measured how far the ball went.
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That lets us know these are the
quantities that are varying,
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so these are gonna be
involved in the X and Y axes.
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But to figure out what
to plot we need to find
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some sort of relationship
between these two variables
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so that we could put that relationship
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in the form of a straight line.
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Now in the 2D kinematics
section right before this,
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we derived a formula
for how far a ball goes
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rolling off a table in terms
of the height of the table
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and the acceleration due to gravity.
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So this is the expression that relates
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how fast it was going to how far it went.
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And we need to put this in
the form of a straight line
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so it's easiest to just
make this left-hand side
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since it's already solved for Y.
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So our Y quantity on our vertical
axis would just be delta X
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and that's okay 'cause
that's one of the quantities
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that we're varying over here.
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Similarly, the other
quantity that's varying is V,
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so I'll just call V X.
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That means the horizontal
axis is gonna be V.
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I was able to do that
since this was just V.
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If this had been V squared, I
would've had to have plotted
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V squared on the horizontal axis.
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And if this was square root of V,
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I would have to plot square
root of V on the X axis.
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But since it was just V, I can
get away with just plotting V
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on my horizontal axis
and now we can figure out
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what our slope would represent.
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We've got Y equals M times X.
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Everything that multiplies
what we called X
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is gonna be our slope.
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The way it's written here
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the M is multiplied on
the right-hand side,
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but it doesn't matter 'cause M times X
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is the same as X times M.
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That means this entire
term, root two H over G
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is the slope of this graph.
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In other words, we're
gonna get a linear fit,
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and the number we find for the slope
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is gonna equal the square
root of two H over G.
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And since there's no added B term,
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there's no Y-intercept
to worry about over here.
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So how do we actually determine
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the experimental value for G?
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We take our data, we plot them,
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we draw a best fit line through the data.
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We would use points on our line
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to determine the slope of this line
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by taking the rise over the run.
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That rise over run would
be equal to the slope,
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and we know that that number's gonna equal
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the square root of two H over G.
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So if we know this number for the slope,
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and the table has a
known and fixed value H,
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the only unknown is G
which we can now solve for.