Physics - Motion and Force by hcj

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									Distance: describes how far an object has
   moved, regardless of its direction
   ex: 40km east + 25km west = 65 total km traveled


Displacement: describes both distance
   and direction
   ex: 40 km east + 25 km west = 15 km east
   ex: 40 km east + 25 km east = 65 km east
n       How fast an object is traveling
        regardless of direction
n       Unit: m or km
              sec hour

                   Described in two ways:
1. Instantaneous speed: measured at a specific instant
    n     initial speed and final speed are examples
    n     this is what speedometers measure

2. Average speed: total distance traveled per unit of
    time
    n     if an object travels at a constant speed, then the
          instantaneous and average speed will be equal
    n     if the object is traveling at varying speeds, then the
          average speed is calculated as total distance traveled over
          total time
n   Describes both the speed of an object AND
    its direction of motion

n   Units: speed + direction: 24 m north
                                 sec

               Measured in two ways:
1. Instantaneous velocity: velocity at a specific
     instant
2. Average velocity: the total displacement per unit
     of time
Velocity can change if:
    1. the object’s speed is changed (increased or decreased)
    2. the object’s direction can change
V = either velocity or speed
d = distance or displacement
                               v=d
t = time
                                 t
Units count!!!!!!!!!!
Let’s practice!!!!
              FLASHBACK
• What is the difference between distance 
  and displacement?

• What is the difference between speed 
  and velocity?

• Your average walking speed is 0.90 
  m/sec.  What is it in km/hour?
                FLASHBACK
•  EOC WORKBOOK - pg. 48 [1,2,4,5]
• Why do you think some people confuse 
  speed with velocity?

• Does a speedometer show both speed and 
  velocity?

• You travel to and from RHHS, which is 
  located 5 mi to the west of your house.  What 
  is your distance & displacement?
PS 5.3 and PS 5.4
n   The rate of change of an object’s velocity
    over the time it takes for that change to
    occur

n   Equation:   a = (v f – v i )
                         t

n   Unit: m/s/s or m/s2 (most common)

However, the unit used for velocity (m/s) and
  the unit used for time (s) do not necessarily
  need to be the same:
               km/hr per second
The change in acceleration may involve a
   change in speed or direction

Can acceleration be negative???
              Yes!!
When you are slowing down at a
  stoplight, speed is decreasing,
  so acceleration is opposite of
       velocity... = negative
            acceleration
1. constant acceleration: acceleration is zero
because the velocity does not change
     ex: walking 2 meters every 15 seconds

2. positive acceleration: the object is
speeding up
    ex: getting on the interstate in your car

3. negative acceleration: the object is
slowing down
                ex: pulling into a parking lot to
                      park your car
A.   All objects accelerate as they fall because
     the Earth exerts gravitational force on them
B.   All objects when released accelerate in the
     direction of the force (downward)
C.   At initial release, the object has an initial
     velocity of 0.0 m/sec
D.   As it falls, the object accelerates at a
     constant rate of 9.8 m/s2
E.   This means the object will travel 9.8 m/sec
     every second it is falling à as long as there
     is no air resistance
F.   The value 9.8 m/sec2 is called the
     acceleration of gravity (ag).
              a = (v f – v i )
                       t
  Positive Acceleration: (speeding up)

A jet airliner starts at rest and reached a
speed of 80 m/s in 20 seconds. What is
it’s acceleration?
              a = (v f – v i )
                       t
 Negative Acceleration: (slowing down)

A skateboarder is moving at a constant
speed of 3 m/s and comes to a stop in 2
seconds. What is his acceleration?
     Assignments!

     n Right Now:
pg 51 “Self Check” # 1-4
Marble Lab!!!
                  FLASHBACK
•  What needs to occur for an object to have 
   acceleration?
• Which of the following is a proper unit of 
   acceleration? 
      a. m/km2     b. km/h     c. m/s2     d. m2/s
3. When describing the rate of a race car going 
   around a track, should you use the term speed 
   or velocity, why? 

4.  How long does it take for the sound of thunder 
    to travel 1485 m, if sound travels at 330 m/s? 
                   FLASHBACK
•  If you ride your bike down a straight road for 
  500 m then ride back, your distance is ____ your 
  displacement.
      a. greater than     b. less than c. equal to

2. Acceleration is rate of change of ____.
   a. position    b. time    c. velocity    d. force
 
3. If you ride your bike up a hill, then ride down 
       the other side, your acceleration is ____.
   a. all positive     b. first positive, then negative
   c. all negative    d. first negative, then positive
            Tennis Ball Activity
        What changes when a tennis ball
                  bounces???
n   Draw and label for two bounces:
    n Positive acceleration
    n Negative acceleration
    n Increasing and
    decreasing speed
               FLASHBACK
• How long will it take to reach Bi-Lo 15 km away 
  if traveling at 50 km/hr?

2. How long will it take a whale to travel 200 km 
   swimming 4 m/s?


3. What is the value of 9.8 m/sec2 called…


4. What is the formula for acceleration?
PS 5.7
n       “The velocity of an object will remain constant
        unless a net force acts on it.”
    n     This is called the Law of Inertia.

n       Inertia: is the tendency of an object to remain
        at rest or in motion
    n     Inertia is dependent upon an object’s mass.

n       Force: a push or pull that one object exerts on
        another
    n     It is measured in Newtons (N).

    n     If one object has a greater force, then net force must
          be calculated
How does this relate to Newton’s First Law?????
n   “When a net force acts on an object, it will
    accelerate in the direction of the net force”

n   The larger the force, the greater the
    acceleration

n   The larger the mass, the smaller the rate of
    acceleration

n   Force = Mass (acceleration)

n   Friction and air resistance can affect net
    force; they act as opposing forces.
III. Newton’s Third Law
A.   Newton’s Third Law : When one object
     exerts a force on a second object, the
     second object exerts a force on the first that
     is equal in magnitude and opposite in
     direction.
     1. a swimmer pushes on the water, and the
     water pushes back

     2. a ball thrown on a wall, bounces off
Force, Mass, Weight, and
Gravity


        PS 5.8 – 5.10
I. The Math Behind the Force
A.  Force is a derived unit; it involves
    multiplying the mass of an object by its
    acceleration:
    F = mass * acceleration
B. For us, the mass is measured in Kilograms
    (Kg) and the acceleration in m/sec2.
C. This means the final answer (Force) is
    measured as Kg * m/sec2 or 1 Newton
D. 1 Newton is the amount of force required to
    accelerate a 1 Kg object at a rate of 1
    m/sec2
E. Look at these examples:

  What is the force required to move a 2400Kg
  car with an acceleration of 4 m/sec2?

  If a force of 4.2 Newtons is applied to an
  object with a mass of 75 Kg, what is the rate
  of acceleration?
II. Newton’s Law of Gravitation
A.   States that there is a force of attraction
     between all objects in the universe.
B.   The object with the larger mass will exert
     the greater force. This is why objects weigh
     more on the earth than on the moon. The
     earth is larger so it has a greater force of
     attraction (gravity).
C.   We do not notice the forces between most
     objects on earth because the force of the
     earth is so much greater than the force
     between the two objects.
D. The amount of force the earth exerts on
  an object is dependent upon the object’s
  mass & distance. The greater the mass,
  the greater the gravitational force.
E. Weight is a measure of an object’s
  gravitational force and it can be found
  using the formula:
  Fw = mass (acceleration of gravity)
F. The acceleration of gravity is what?
           9.8 m/sec2
G. Try these examples:

 What is the weight of an object with a
 mass of 10 Kg?

 What is the mass of an object with a
 weight of 25 N?

								
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