b. berenger 12/99 2 summary of discussion: newton’s first law –inertia mass vs. weight...
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B. Berenger 12/99
2B. Berenger 12/99
Summary of Discussion:Summary of Discussion:
• Newton’s First Law– inertia
• Mass vs. Weight• Newton’s Second Law
– F=ma– a=f/a
• Newton’s Third Law
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INERTIAINERTIA
• The tendency of objects to remain in motion or stay at rest
• If it were not for friction, an object set in motion would continue to move forever
Inertia
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FIRST LAW OF MOTIONFIRST LAW OF MOTION
• An object at rest remains at rest, or if in a state of motion, continues at constant velocity (same direction & speed) unless acted upon by an outside force
Newton’s First Law
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Riding In A Car . . .Riding In A Car . . .• When you are riding in a car and it stops
suddenly, you keep moving forward
• if you did not have a safety belt to stop you, your inertia could send you through the windshield
Inertia
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Standing On A Bus . . .Standing On A Bus . . .
• When the bus starts to move forward you are thrown off balance and fall backward
• Your body has inertia– it is at rest and tends to stay at rest,
even though the bus is moving
Inertia
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. . . On The Same Bus. . . On The Same Bus
• When the moving bus stops, you fall forward
• Even though the bus stops, you do not– You are an object in motion
Inertia
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MASS VS. WEIGHTMASS VS. WEIGHT
• Weight– a measure of the force of gravity on an
object– weight = mass x acceleration due to gravity
w = m x g– measured in Newtons
• Mass– a constant value; does not change– amount of matter– measured in Kilograms
Mass vs. Weight
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CALCULATING WEIGHTCALCULATING WEIGHT
• measured in Newtons• weight = mass x acceleration due to gravity
w = m x g – acceleration due to gravity on Earth = 9.8 N/kg
• Example: m = 100 kg g = 9.8 N/kg
• Weight =100 kg x 9.8 N/kg = 980 N
Weight
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WEIGHTWEIGHT• Varies according to the force of gravity
pulling on you• a smaller mass means a smaller
gravitational pull
Weight
Earth Moon
Where would you weigh more? Why?
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MASSMASS• Remains constant• You have the same amount of mass
regardless of your location
Mass
Earth Moon
Would your mass change? Why or why
not?
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NEWTON’S SECOND LAWNEWTON’S SECOND LAW
• A force acting on an object will produce an acceleration of the object proportional to the force and in the direction of the applied force– if you double the force that
you throw a ball, you will double its acceleration
Newton’s Second Law
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Mathematically Mathematically Speaking...Speaking...
• Force = mass x acceleration
F = ma
Newton’s Second Law
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F = maF = ma
• If the same force is applied to a bowling ball and a tennis ball, which ball will have the greater acceleration?
Newton’s Second Law
WHY?
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Force Measured in Force Measured in NewtonsNewtons
• 1 N = 1 kg x 1 m/sec/sec = kg m
s2
• One newton equals the force required to accelerate one kilogram of mass at one meter/second/second
Newton’s Second Law
Remember:F = ma
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Better Gas Mileage?Better Gas Mileage?
Small Car• mass = 750 kg• acceleration = 2 m/sec/sec• Force required to accelerate car = 750 kg x 2 m/sec/sec = 1500 N
• Large Car• mass = 1000kg• acceleration = 2 m/sec/sec• Force required to accelerate car = 1000 kg x 2 m/sec/sec
= 2000 N
Remember:F = ma
Newton’s Second Law
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Not Just: F = maNot Just: F = ma
• Formula can also be written as:a = F/m
• Acceleration is directly proportional to the force
• Acceleration is indirectly proportional to the mass
Newton’s Second Law
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REMEMBER:REMEMBER:• Acceleration does not only mean to go
faster!
• Acceleration can mean:– direction of motion changed– motion started– motion stopped– speed increased– speed decreased
Acceleration
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a = F/ma = F/m• If a person keeps adding snow to the snowball
(increasing its mass), yet each time throws it with the same force, the snowball will accelerate less each time
Fm
a = remains constantdecreases
increases
Acceleration
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a = F/ma = F/m• If the snowball begins to melt (mass
decreases), the same force applied to the snowball will cause it to accelerate more
Fm
a = remains constantincreases
decreases
Acceleration
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NEWTON’S THIRD LAWNEWTON’S THIRD LAW
• For every action, there is an equal and opposite reaction
• Every force must have an equal and opposite force– all forces come in pairs
Newton’s Third Law
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• As you walk, your feet push against the ground• You move forward and the
earth moves in the opposite direction!– Since the mass of the
earth is so large, its motion is unobservable
• The ground pushes up against your feet with an equal and opposite force
EQUAL AND OPPOSITEEQUAL AND OPPOSITEFORCESFORCES
Eq
ual a
nd
op
posite
force
s
Newton’s Third Law
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Space ShuttleSpace Shuttle
• Various fuels are burned in the engine, producing hot gases
• As the gases move downward, the rocket moves in the opposite direction, or upward
Newton’s Third Law
Eq
ual an
d o
pp
osi
te f
orc
es
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TOPICS COVERED:TOPICS COVERED:
• Newton’s First Law– inertia
• Mass vs. Weight• Newton’s Second Law
– F=ma– a=f/a
• Newton’s Third Law
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BibliographyBibliography
• K. Batista notes• Physical Science (1988). HBJ,
pp.313-316; pp.322-323