Physics of Motion We will look at: Position Distance
Displacement Speed Velocity Acceleration First you need to realize
that motion is relative
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Motion is Relative Motion is relative to the observers position
and their reference point Sometimes called a frame of reference
Consider the picture If this man is driving at 15 mph, how fast is
his coffee cup moving? Does the man feel like the cup is moving?
Why?
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Motion is relative An object is in motion if it changes
position relative to a stationary reference point. Use this to
establish a COORDINATE SYSTEM pick an origin or 0 point decide
which direction is positive
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Draw an Example:
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Direction We use North, South, East, West or left & right
to describe the direction of movement. We can also use POSITIVE and
NEGATIVE to describe direction.
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POSITION A location with respect to the origin or zero
point.
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Finding Position 4 miles 6 miles 0 6-4 104 0 0-6-10 Position
depends on where you put 0.
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Distance and Displacement Distance measures the actual path an
object takes Displacement measures your overall distance from the
initial position to final position in a STRAIGHT LINE. DISPLACEMENT
values must include a DIRECTION! Which color line represents
distance? Displacement?
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Scalar vs Vector Quantities All measured quantities can be
classified as being either a scalar or a vector. Scalar _________
only (size of the quantity .a number) Vector _________ and
_________ Magnitude Direction
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Slide 12
Distance vs. Displacement
http://physics.info/displacement/
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Use the diagram to determine the resulting displacement and the
distance traveled by the skier during these three minutes.
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Answer distance The skier covers a distance of displacement
(180 m + 140 m + 100 m) = 420 m and has a displacement of 140 m,
right or east.
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Write in your own words- what is the difference between
distance and displacement? Distance- Displacement-
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Formula for Displacement X = Xf-Xi Xf= final position Xi=
initial position Remember- displacement can be positive or
negative! Why?
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What is the coach's resulting displacement and distance of
travel?
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Answer The coach covers a distance of (35 yds + 20 yds + 40
yds) = 95 yards and has a displacement of 55 yards, left or -55
yards.
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Speed Speed is the distance traveled in an amount of time We
know some things move faster than othersbut how do we measure
it?
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What two quantities must you know to determine speed? Speed=
distance/time units- miles/hr, km/hr m/s
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There are three types of speed you must know Constant speed
Instantaneous speed Average speed
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Constant Speed When an object covers equal distances in equal
amounts of time Ex- if a race car travels at a CONSTANT SPEED of
96m/s, it will travel a DISTANCE of 96 meters EVERY SECOND.
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But most objects do not travel at a constant speed. The speed
of an object can change from one minute to another. So we can use
AVERAGE SPEED to describe its motion-overall distance moved over
the entire time Use this equation Average Speed = total Distance /
total Time
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Instantaneous Speed Instantaneous speed- measures speed at an
instant in time
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Does a speedometer of a car read instantaneous or average
speed?
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What 2 controls on a car enable a change in speed?
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What if I want to describe speed AND direction? For examplewhat
if you wanted to find a plane. Knowing the speed would only tell
you how far away to look but not in what direction. For that we
need VELOCITY
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Lets get back to the car example Name another control that
enables a change in velocity.
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SoWhat is the difference between speed and velocity? SPEED-
reports the magnitude of distance over time (just the number).
VELOCITY- reports the speed AND direction of motion.
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Are speed and velocity always going to be the same? NO! Only if
the object is moving in the SAME direction the whole time. What is
an example of a motion where avg speed is not equal to average
velocity? (demo)
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Constant Velocity Same displacement per unit of time
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So how do you calculate speed vs velocity? Mathematically:
Velocity = displacement / time (Includes a direction !)
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Lets look closer Average velocity is calculated by the
equation: V avg = (x f -x i ) / (t f -t i )
Slide 34
Use the diagram to determine the average speed and average
velocity between the following points. A. Going from A and B B.
Going from A to B and ending at C
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Answers: A. Between A and B, speed and velocity will be the
same because they are in the same direction the whole time. Speed =
180 m/min Velocity = 180 m/min east B. When the motion continues to
point C, the speed and velocity are different because the distance
traveled and displacement are different. Speed= 320 m/2 min= 160
m/min Velocity= 40 m/2 min= 20 m/min east
Slide 36
Acceleration Chapter 10.2-3
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Acceleration is a change in velocity in a period of time. SoIs
it a vector or a scalar quantity? VECTOR It must include a
direction (like velocity) This also means that you can change
acceleration by changing what? Speed or Direction
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Does travel at a constant speed mean you are not accelerating?
NO!!! Remember that you can change velocity by changing direction,
thus CHANGING ACCELERATION!
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Causes of acceleration Increasing velocity Example: Car speeds
up at green light Decreasing velocity Example: Car slows down at
stop light Changing Direction Example: Car takes turn (can be at
constant speed) screeeeech
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What do the numbers mean? Small acceleration speed is
increasing slowly Large acceleration- speed is increasing
rapidly
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Positive vs. Negative Acceleration Acceleration and velocity
are in the same direction when speeding up and opposite directions
when slowing down
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Calculating Acceleration If an object is moving in a straight
line V= change in velocity (Final Velocity Initial Velocity) (m/s)
A= acceleration T= time (s) (final time- initial time) So what are
the units of acceleration? If A= V/T Acceleration is in m/s 2
V
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Calculating Acceleration 0 s 1 s2 s3 s4 s 0 m/s 4 m/s 8 m/s 12
m/s16 m/s Acceleration = (V f -V i ) t Lets look at the picture
belowwhat is this cars avg acceleration?
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Question A skydiver accelerates from 20 m/s to 40 m/s in 2
seconds. What is the skydivers average acceleration?
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Constant velocity
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Now consider a car moving with a rightward (+), changing
velocity - that is, a car that is moving rightward but speeding up
or accelerating.
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Acceleration
Slide 48
Distance time graphs Draw 2 graphs One showing a slow constant
speed One showing a faster constant speed
Consider a car moving with a constant, rightward (+) velocity -
say of +10 m/s. A car moving with a constant velocity is a car with
zero acceleration. Draw a graph!
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Constant velocity = zero acceleration
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Positive Acceleration Now consider a car moving with a
rightward (+), changing velocity - that is, a car that is moving
rightward but speeding up or accelerating. Since the car is moving
in the positive direction and speeding up, the car is said to have
a positive acceleration.acceleratingpositive acceleration
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Positive acceleration
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Describe this graph!
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Does the velocity of the wind affect such things as a sprinters
speed or an airplanes flight time?
A small airplane heads east with a speed of 200 mph with
respect to the air (the air speed). This would be the planes speed
if the air was NOT moving no wind) If the wind/jet stream is moving
east at 50 mph, what is the planes resulting velocity with respect
to the ground (the ground speed)? Adding Vectors Example: with the
wind 20050 250 mph, east
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If, later, the airplane is flying west into the 50 mph wind
with an air speed of 200 mph, now what is the planes resulting
velocity with respect to the ground (the ground speed)? against the
wind 50 200 150 mph, west
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1. Find the velocity in m/s of a swimmer who swims 110 m toward
the shore in 72 s.
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1.5 m/s toward the shore
Slide 62
1. Imagine that you could ride a baseball that is hit high
enough and far enough for a home run. Using the baseball as a
reference frame, what does the Earth appear to do?
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1. Calculate the displacement in meters a cyclist would travel
in 5.00 h at an average velocity of 12.0 km/h to the
southwest.
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But first lets look at some graphs If I wanted to graph speed,
what should I label my axes??? So the slope of the line=SPEED
Distance Time Speed
Slide 65
Constant Speed What would a position-time graph look like for a
constant speed?