0014 Force, Mass and Motion: 1. distinguish between mass and weight of an object.

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0014 Force, Mass and Motion 1. distinguish
between mass and weight of an object.
2
Mass vs Weight

• Quantity of matter in an object
• The measurement of inertia
• Brick 1kg

• The gravitational force exerted on an object by
  the nearest, most massive body (Earth)
• Brick 2.2 pounds

3
The Newton (metric unit)

• In the metric system, the unit of weight, or any
  other force, is the newton, which is equal to a
  little less than a quarter pound.
• Newton force needed to accelerate 1 kg 1 m/s2
• 1 kg brick weighs about 10 N
• Or a baseball 1 N

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0014 Force, Mass and Motion 2. identify
characteristics of forces that act on objects
(e.g. frictional, gravitational)
5
0014 Force, Mass and Motion 3. determine the
relationship between the velocity and
acceleration of an object.
6
Acceleration

• Acceleration is the amount of change in velocity
  divided by the time it takes the change to occur.
• Acceleration (m/s2)
• final velocity initial velocity (m/s) / time
  (s)
• A (vf - vi) / t

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A car traveling at a rate of 10 m/s accelerates
to 90 m/s in 12 seconds. Calculate its
acceleration.

• A (vf - vi) / t
• A 90 m/s 10 m/s / 12 s
• 80 m/s / 12 s
• 6.67 m/s/s
• or 6.67 m/s2

8
3 devices in your car make it accelerate

• Accelerator pedal
• Brake pedal
• Steering wheel
• Whenever an object changes speed or direction it
  accelerates.

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Figure 2-8 Galileos falling-ball apparatus with
a table of measurements and a graph of distance
versus time.
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Galileo found the following

• a ball rolling down a ramp moves with constant
  acceleration
• a ball attains a greater acceleration from
  steeper inclines
• regardless of weight, when air resistance is
  negligible, all objects fall with the same
  acceleration

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Free-Fall Velocity

• The velocity of a falling object is proportional
  to the length of time it has been falling.
• Velocity (m/s) constant g (m/s2) x time (s)
• V g x t
• Galileo found g 9.8 m/s2

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Acceleration due to Gravity

• Suppose a falling rock is equipped with a
  speedometer
• In each succeeding second of fall, the rocks
  speed increases by the same amount 10 m/s
• Time of Fall (s) Instantaneous Speed (m/s)
• 1                                                 
             10
• 2                                                 
             20
• 3                                                 
             30
• 4                                                 
             40
• 5 50

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Gravity

• Suppose a falling rock is equipped with an
  odometer
• The readings would indicate that the distance
  fallen increases with time according to the
  relationship d ½ gt2
• Time of Fall (s) Distance of Fall (m)
• 1 5
• 2 20
• 3 45
• 4 80

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Free Fall and Air Resistance

• In free-fall, force of air resistance counters
  force of gravity.
• As skydiver falls, air resistance increases til
  it approaches the magnitude of the force of
  gravity. Once the force of air resistance is as
  large as the force of gravity, skydiver is said
  to have reached a terminal velocity.
• Skydiving

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0014 Force, Mass and Motion 4. solve
quantitative problems involving force, mass, and
motion of objects.
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0014 Force, Mass and Motion 5. demonstrate
knowledge of Newtons 3 laws of motion and their
application to everyday situations.
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Isaac Newton and the Universal Laws of Motion

• English scientist (1642-1727)
• Synthesized the work of Galileo and others
• 3 laws describe all motion

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First Law Inertia (matter resists change)

• A moving object will continue moving in a
  straight line at a constant speed, and a
  stationary object will remain at rest, unless
  acted upon by an unbalanced force.
• animation

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Second Law F m x a

• The acceleration produced by a force on an object
  is proportional to the magnitude of the force,
  and inversely proportional to the mass of the
  object.
• tutorial

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Third Law action / reaction

• For every action there is an equal and opposite
  reaction.
• See some examples

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calculate the force needed to produce a given
acceleration on a given mass (F ma)

• A 20 kg mass has an acceleration of 3 m/s2.
  Calculate the force acting on the mass.
• F (20 kg) (3 m/s2)
• F 60 kg m/s2 60 N

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What force is needed to accelerate a 75 kg
sprinter from rest to a speed of 10 meters per
second in half a second?

• First find acceleration.
• Accel final vel initial vel (m/s) / time (s)
• 10 m/s 0 m/s / .5 s 20 m/s/s
• Force (N) mass (kg) x accel (m/s2)
• F 75 kg x 20 m/s2
• F 1500 N

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Newtons Law of Universal Gravitation

• Between any two objects in the universe there is
  an attractive force proportional to the masses of
  the objects and inversely proportional to the
  square of the distance between them.
• F (G x m1 x m2) / d2
• The more massive 2 objects are, the greater the
  force between them.
• The farther apart 2 objects are, the less the
  force between them.

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Figure 2-13 An apple falling, a ball being
thrown, a space shuttle orbiting the Earth, and
the orbiting Moon, all display the influence of
the force of gravity.
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0014 Force, Mass and Motion 6. apply knowledge
of the concepts of work and power to the analysis
of everyday activities.

• Work is done when a force is exerted over a
  distance.

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Work

• is equal to the force that is exerted times the
  distance over which it is exerted.
• W F x d
• The unit of work combines the unit of force (N)
  with the unit of distance (m)
• Newton-meter (N-m) aka Joule.

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You carry a 20 kg suitcase upstairs, a distance
of 4m. How much work did you do?

• W F x d
• F ma
• (20 kg) (10m/s2) 200 N
• W F x d
• (200 N) (4m)
• 800 J

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Power

• measures rate at which work is done.
• Power is the amount of work done, divided by the
  time it takes to do it.
• Power (watts) work (joules) / time (sec)
• P W/t

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Power

• Since work performed equals energy expended,
• Power (watts) energy (joules) / time (sec)
• The watt is defined as the expenditure of
• 1 joule of energy in 1 second.
• (75 watt light bulb consumes 75 J/sec)

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0014 Force, Mass and Motion 7. demonstrate
knowledge of types and characteristics of simple
machines and their effect on work.

• Simple Machine
• device for
• multiplying or
• changing the
• direction of force.