Intro to Analysis of Motion

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Intro to Analysis of Motion

• By Mike Maloney

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What is Kinematics?
Or in normal human words, it is describing how a
s!t load of stuff moves in as many ways as
possible so we can understand the world better.

• Kinematics is the science of describing the
  motion of objects using words, diagrams, numbers,
  graphs, and equations. The goal of any study of
  kinematics is to develop sophisticated mental
  models which serve to describe (and ultimately,
  explain) the motion of real-world objects.

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How can we describe Motion?

• Lets say Homer is sitting on my desk. Everyone
  but you closes their eyes and when they open them
  Homer is now on a different desk.
• How could you describe to the rest of the class
  the motion of Homer you observed?
• What terms could you use?
• What different ways could you describe it?
• Lets use InterLace to discuss it.

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Distance vs. Displacement

• We usually call the movement of an object its
  distance traveled.
• In the physics world, we have two words that
  describe the amount of movement of an object
• Distance
• Displacement
• We will learn how they are different next
  chapter.
• For now, we will use displacement, which is your
  change in position.
• Now lets try to describe Homers motion.

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Displacement (change in position)

• Lets call your position s.
• How might you write change in position?
• Hint think to chemistry or lines in math class?
• We can use delta (?), which means change
• Displacement (change in position) is written as
  ?s
• How can you change your position?
• Go forward
• Go backward
• Go Sideways, or up, or down,
• How can we tell the difference?

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Displacement (?x)

• That is what and is for.
• ?s means you have moved in the positive
  direction.
• - ?s means you have moved in the opposite
  direction.

SPECIAL NOTE In real life what you call forward
or backward or positive or negative direction is
completely arbitrary. As long as you are
consistent, and forward is opposite backward, and
positive is opposite negative direction it does
not matter.
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Velocity (v)

• Another way to describe motion is to combine
  displacement and time.
• If we use time to determine how fast an object
  can change position, and fine the time rate of
  change in position, we get something called speed
  or more accurately velocity.
• Rate is something that comes up again and again
  in physics. So what does it mean?
• It is just how much something changes something
  else does. For example
• How much money you earn per week
• How many shots you make per game
• How many times I roll my eyes per class

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Velocity (v)

• Since you can move forward or backward, we need
  a way to describe this in terms of velocity. Any
  ideas?
• Right, we use and again.
• velocity (v) means you are moving in whatever
  you called the positive direction, or you have a
  positive change in position over time.
• - velocity (-v) means you are moving in the
  negative direction or you have a negative change
  in position over time.

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Beyond Velocity

• So now we know that position can change
  (displacement) and the time rate at which it
  changes is called velocity.
• Is this enough to completely describe Homer?
• Velocity is not always constant.
• When you step on the gas pedal in a car the car
  speeds up, the velocity increases.
• Stepping on the brake does the opposite, the
  velocity decreases, the car slows down.
• So we need a way to describe this too.

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Acceleration (a)

• Just as velocity is the time rate of change of
  displacement, acceleration a is the time rate
  of change of velocity.
• Acceleration can be positive or negative -.
• A positive acceleration means increasing
  velocity
• A negative - means decreasing velocity
• Hmmm. Is there any other way we could get a
  acceleration?
• Ahh! It is a little tricky, but we could also use
  a decreasing () velocity. Anyone know how or
  why?

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How can we diagram motion

• Now that we know some terms that describe motion,
  how can we diagram it or graphically describe it?
• Motion Diagrams (Interlace Java)
• Graphs (with Usain Bolt)
• Graphing Challenge (doc game)

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The 4 ways to describe Motion
Motion Diagrams
Words
Variables and Equations
Graphs