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					An Introduction To Physics
                .

    Physics is the study of the
  relationships between matter
           and energy.
Physics deals with the real world
Versus………
Physics deals with the interactions of
matter and energy, both PE vs KE!!!!
   Beginning Physics is called
    Newtonian Physics.
      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.
          Motion
MOTION:
     a change in position,
measured by distance (also
called DISPLACEMENT) and
time.
            REFERENCE POINT
   Reference point:
     thepoint from which movement is
     determined, and which is assumed to be
     nonmoving itself.
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?
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?
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?
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.
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?"
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
“Hey Bob!!!
…Did I scare Ya?”
• 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?
Here’s 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?
Speed

• Speed = Distance ÷ Time

                   S = D/T

Example: A car travels 300km in 6 hours.
  What is the speed of the car?
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
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?
Answer:

• Speed = distance ÷ time
• Speed = 450 Km ÷ 5 hours
• Speed = 90km/hr
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?
Answer
   If: S = D/T

   Then: D = S x T

   800km ▪ 9hrs = 7200km
     hr
Answer

   And then: (since S = D/T)

           T = D/S

   500km ÷ 50km = 10 hrs
              hr
RANDOM
FAR SIDE!!
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?
Answer

• Speed = distance ÷ time
• Speed = 10 miles ÷ 15 minutes
• Speed = 0.67 mi/min
• Velocity = 0.67 mi/min South
Change your answer to mi/hr!

• 0.67mi/min x 60min/hr =

• 40 mi/hr
Distance-time graphs
• On your paper, graph the following:
  –   D (m)      T (sec)
       0           0
      5            7
      10          14
      15          21
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.
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.
SPEED!!!! (one more thing)
• Instantaneous Speed: the rate of
 speed at any particular moment in
 time
  – What device measures instantaneous
   speed on you car???
VELOCITY
-- speed in a given direction.

• Speed only gives distance and time.
• Velocity gives distance, time, and the
 direction of travel.
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???
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
ACCELERATION

• Acceleration: The rate of change in
 velocity.

• Acceleration = (Final Velocity) - (Original Velocity) / Time
                  a = (Vf – Vi) / t
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?
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?
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!!!!
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.
Graph the following on a distance-
time graph:
•   D (m)       T (s)
     0          0
     5          1
     20         2
    45          3
    80          4
    125         5
0 1 2 3 4 5
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!!
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
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)
               ____
                25           0 to 8
               ____
                 0           8 to 12
               ____
                37.5         12 to 20
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.
What do the following speed-time
graphs depict?
   Acceleration problem
• A roller coaster’s
  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?
Answer

• Acceleration = ∆V/ ∆T
• a = 26m/s – 10m/s
           2s
a = 16m/s
      2s
a = 8m/s/s or 8m/s2
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?
Answers:

1. 4sec = t
2. yes! Because it’s changing direction!
3. no! It’s not changing speed or direction!
Answer:
• Acceleration = ∆V/ ∆T
• Acceleration = Vf – Vi
                     t
• a = 20mi/hr – 60mi/hr
               4s
a = -40mi/hr
       4s
a = -10mi/hr/s
Review: Distance-time graph of
acceleration
Distance-time graph of deceleration
Review:Speed-time graph of
acceleration
Review: Speed-time graph of
deceleration
Review: Distance-time graph of
constant speed
GO……………….
            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?
                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!
          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?
                  Answers:
   P=mv
   Car A’s momentum = 30mi/hr x 2000lbs
                  PA = 60,000 mi-lbs/hr east
   Car B’s 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
Inelastic collision
Elastic collision
Two dimensional collision
    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?
                  Answers:
   P=mv
   Car X’s momentum = 30mi/hr x 2000lbs
                  PA = 60,000 mi-lbs/hr east
   Car Y’s 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
Which has more momentum?

				
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posted:8/9/2012
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