An Introduction To Physics .

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An Introduction To Physics.

• Physics is the study of the relationships between
  matter and energy.

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Physics deals with the real world
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Versus
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Physics deals with the interactions of matter and
energy, both PE vs KE!!!!
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• Beginning Physics is called Newtonian Physics.

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Newtonian Physics

• Named for Sir Isaac Newton, it deals mainly with
  the interactions of visible objects around us
• Physics explains the universe in mathematical
  terms.

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Motion

• MOTION
• a change in position, measured by distance (also
  called DISPLACEMENT) and time.

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REFERENCE POINT

• Reference point
• the point from which movement is determined, and
  which is assumed to be nonmoving itself.

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What is motion?

• If you are standing in one place, and your friend
  walks by you, are you moving relative to your
  friend?
• Is your friend moving relative to you?
• Is either of you moving relative to the earth?

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Answer

• You are moving relative to your friend, and your
  friend is moving relative to you!
• You (the Joker) are not moving relative to the
  earth, but your friend is. You are both moving
  relative to the sun!
• Who is moving
• relative to the
• computer
• screen?

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What is motion?

• If you and your friend are walking down the hall
  together at the same speed, in the same
  direction, are you moving relative to your
  friend?
• Is your friend moving relative
• to you?
• Are either of you moving
• relative to the earth?

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Answer

• You are NOT moving relative to your friend, and
  your friend is NOT moving relative to you. You
  both are moving relative to the earth.

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Frame of REFERENCE

• EXAMPLE Tossing a ball
• Describe the motion of the ball."
• "If we were going down the road in a school bus,
  me standing in the isle and you sitting in the
  seats, would the motion of the ball be
  different?"
• "If someone was standing beside the road as we
  passed, what would they see the ball doing?"

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Frames of Reference

• To measure movement, some point must be
  considered as nonmoving.
• Most common is the
• earth
• Motion is a change
• in position, relative to
• a frame of reference

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• Hey Bob!!!
• Did I scare Ya?

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• Earth is the most common frame of reference,
  however
• Earth rotates on its axis at almost 1100
  miles/hour.
• Earth moves around the sun at over 68,000
  miles/hour.
• The whole galaxy is rotating at about 490,000
  miles/hour.
• Is there a universal frame of reference we can
  use to define the motions of all things?

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Heres a problem (or two)..

• If the earth spins on its axis at 1100
  miles/hour, what is the speed of the Earth's
  rotation in feet per second?
• The earth travels at 68,000 miles/hour as it
  moves around the sun. How many miles does the
  earth travel in one trip around the sun?

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Speed

• Speed Distance Time
• S D/T
• Example A car travels 300km in 6 hours. What
  is the speed of the car?

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SPEED!!!!

• Instantaneous Speed the rate of speed at any
  particular moment in time
• Average speed the speed of moving objects is not
  always constant
• Average speed total distance / total time

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MORE SPEED!!!!

• Average Speed Practice Problem You drive 300
  kilometers in 3 hours before stopping for 30
  minutes for lunch and gas. After lunch you travel
  150 kilometers in an hour and a half. What was
  your average speed for the trip?

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Answer

• Speed distance time
• Speed 450 Km 5 hours
• Speed 90km/hr

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More practice

• 1. How far can a plane travel if it flies
  800km/hr for 9 hours?
• 2. How long does it take a ship to go 500 km if
  it travels at a speed of 50km/hr?

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Answer

• If S D/T
• Then D S x T
• 800km ? 9hrs 7200km
• hr

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Answer

• And then (since S D/T)
• T D/S
• 500km 50km 10 hrs
• hr

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• RANDOM
• FAR SIDE!!

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Velocity

• Speed in a given direction.
• What is the velocity of a boat that travels from
  St. Peter to Mankato (10 miles) in 15 minutes?

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Answer

• Speed distance time
• Speed 10 miles 15 minutes
• Speed 0.67 mi/min
• Velocity 0.67 mi/min South

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Change your answer to mi/hr!

• 0.67mi/min x 60min/hr
• 40 mi/hr

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Distance-time graphs

• On your paper, graph the following
• D (m) T (sec)
• 0 0
• 5 7
• 10 14
• 15 21

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Distance (m)
time (sec)
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Distance - Time Graph

• Time is the independent variable and always
  plotted on the horizontal axis.?Distance is the
  dependent variable and always plotted on the
  vertical axis.?The slope of this line indicates
  the speed.

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Was your graph a straight line?

• A distance-time graph which is a straight line
  indicates constant speed.
• In constant speed, the object does not speed up
  or slow down. The acceleration is zero.

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SPEED!!!! (one more thing)

• Instantaneous Speed the rate of speed at any
  particular moment in time
• What device measures instantaneous speed on you
  car???

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VELOCITY

• -- speed in a given direction.
• Speed only gives distance and time.
• Velocity gives distance, time, and the direction
  of travel.

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Velocity is known as a vector quantity because it
has both speed and direction.
Consider the flight of an airplane The first
arrow (A arrow) shows the speed and heading of
the plane. The second arrow (Another arrow)
shows the speed and direction the wind is
blowing. Since the wind is changing the speed and
direction of the plane, the two vectors are
added, head to tail, to determine the actual
speed and direction traveled. If the length of
the blue vectors is drawn to scale, the length of
the resultant vector will indicate the actual
velocity of the plane.
When you go around a curve at a Constant speed,
are you changing Direction???
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More VELOCITY NOTES

• ?Constant Velocity constant speed and constant
  direction ( the object is moving in a straight
  path)
• ?Changing velocity means either a decrease or
  increase in speed, motion along a curved path, or
  both
• ?Cars have 3 controls to change velocity
• 1) gas pedal 2) brake 3) steering wheel

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ACCELERATION

• Acceleration The rate of change in velocity.
• Acceleration (Final Velocity) - (Original
  Velocity) / Time
• a (Vf Vi) / t

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ACCELERATION!!!

• Problem Suppose a car moving in a straight line
  steadily increases its speed each second. First
  from 35 to 40 km/hr, then from 45 to 50 km/hr.
  What is its acceleration?

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ACCELERATION!!!

• Problem too In 5 seconds a car moving in a
  straight line increases its speed from 50 km/h to
  65 km/h while a truck goes from rest to 15 km/h
  in a straight line ? Which undergoes greater
  acceleration? ? What is the acceleration of each
  vehicle?

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ACCELERATION!!!

• Acceleration also applies to changes in direction
• EX going around a curve at a constant speed of
  50 km/hr, what do you notice or feel?
• motion is changing every instant because velocity
  is changing every instant
• YOU ARE ACCELERATING!!!!

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ACCELERATION

• Deceleration ( negative acceleration)
• A term commonly used to mean a decrease in speed.
• Acceleration in a direction opposite to the
  direction of travel.
• Deceleration is negative acceleration and has a
  negative value to indicate direction.

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Graph the following on a distance-time graph

• D (m) T (s)
• 0 0
• 5 1
• 20 2
• 45 3
• 80 4
• 125 5

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Distance (m)
0 1 2 3 4 5
time (sec)
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Was your graph a curve?

• A graph that curves on a distance-time graph
  shows that the object is accelerating
• A graph of acceleration, always has shape!!

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Distance-time graphs

• Describe the motion of the object as shown in the
  
• graph.

From 0-8 sec, constant speed (25 m/sec) From
8-12 sec, no motion From 12-16 sec,
acceleration From 16-20 sec, constant speed
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Speed-time graphs

• Using the distance-time graph from the last
  frame, draw a speed time graph. First fill in
  the table below
• Average Speed (m/s) Time (sec)
• ____ 0 to 8
• ____ 8 to 12
• ____ 12 to 20

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0
37.5
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What does your graph look like?

• Constant speed will be a horizontal line on a
  speed time graph.
• If the speed decreases, the line will slant down.
• If the speed increases, the line will slant up.

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What do the following speed-time graphs depict?
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Acceleration problem

• A roller coasters velocity at the top of a hill
  is 10m/s. Two seconds later it reaches the
  bottom of the hill with a velocity of 26m/s.
  What is the acceleration of the roller coaster?

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Answer

• Acceleration ?V/ ?T
• a 26m/s 10m/s
• 2 s
• a 16m/s
• 2s
• a 8m/s/s or 8m/s2

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More acceleration problems

• 1. A car accelerates at a rate of 20mi/hr/s.
  How long does it take to reach a speed of 80
  mi/hr?
• 2. A car travels at 60 miles per hour around a
  curve. Is the car accelerating?
• 3. A car travels in a straight line at 60mi/hr.
  Is the car accelerating?

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Answers

• 4sec t
• 2. yes! Because its changing direction!
• 3. no! Its not changing speed or direction!

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Answer

• Acceleration ?V/ ?T
• Acceleration Vf Vi
• t
• a 20mi/hr 60mi/hr
• 4 s
• a -40mi/hr
• 4s
• a -10mi/hr/s

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Review Distance-time graph of acceleration
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Distance-time graph of deceleration
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ReviewSpeed-time graph of acceleration
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Review Speed-time graph of deceleration
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Review Distance-time graph of constant speed
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GO.
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Momentum

• Momentum Mass x Velocity
• Which has more momentum a 300lb football player
  moving at 5m/s or a 200lb quarterback moving at
  10m/s?

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Answer

• Momentum of the 300lb player is
• 300lbs x 5m/s 1500lb-m/s
• Momentum of the quarterback is
• 200lbs x 10m/s 2000lb-m/s
• The quarterback has a greater momentum!

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Momentum problems

• 2 cars are heading east, car A is traveling
  30mi/hr, car B is traveling 60mi/hr. Each car
  weighs 2000lbs.
• What is the momentum of car A?
• What is the momentum of car B?
• If car B crashes into car A, what is the total
  momentum?

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Answers

• Pmv
• Car As momentum 30mi/hr x 2000lbs
• PA 60,000 mi-lbs/hr east
• Car Bs momentum 60mi/hr x 2000lbs
• PB 120,000 mi-lbs/hr east
• Total momentum PA PB
• 60,000 120,000
• 180,000 mi-lbs/hr east

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inelastic collisions
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Inelastic collision
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elastic collisions
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Elastic collision
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Two dimensional collision
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Another momentum problem!

• Car X is traveling 30mi/hr east, car Y is
  traveling 60mi/hr west. Each car weighs
  2000lbs.
• What is the momentum of car X?
• What is the momentum of car Y?
• If car X crashes into car Y, what is the total
  momentum?

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Answers

• Pmv
• Car Xs momentum 30mi/hr x 2000lbs
• PA 60,000 mi-lbs/hr east
• Car Ys momentum 60mi/hr x 2000lbs
• PY 120,000 mi-lbs/hr west
• Total momentum PY - PX
• 120,000 - 60,000
• 60,000 mi-lbs/hr west

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Which has more momentum?
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