Download Newton`s Laws

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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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Inertia
INERTIA
• 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
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Newton’s First Law
FIRST 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
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Inertia
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
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Inertia
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
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Inertia
. . . 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
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Mass vs. Weight
MASS 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
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Weight
CALCULATING 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
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Weight
WEIGHT
• Varies according to the force of gravity pulling
on you
• a smaller mass means a smaller gravitational
pull
Where would you weigh
more? Why?
Earth
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Moon
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Mass
MASS
• Remains constant
• You have the same amount of mass
regardless of your location
Would your mass change?
Why or why not?
Earth
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Moon
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Newton’s Second Law
NEWTON’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
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Newton’s Second Law
Mathematically Speaking...
• Force = mass x acceleration
F = ma
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Newton’s Second Law
F = ma
• If the same force is applied to a bowling
ball and a tennis ball, which ball will have
the greater acceleration?
WHY?
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Newton’s Second Law
Force Measured in Newtons
• 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
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Newton’s Second Law
Better Gas Mileage?
Remember:
F = ma
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
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Newton’s Second Law
Not 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
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Acceleration
REMEMBER:
• Acceleration does not only mean to go faster!
• Acceleration can mean:
–
–
–
–
–
direction of motion changed
motion started
motion stopped
speed increased
speed decreased
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Acceleration
a = 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
decreases
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remains constant
F
a =
m increases
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Acceleration
a = F/m
• If the snowball begins to melt (mass
decreases), the same force applied to the
snowball will cause it to accelerate more
increases
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remains constant
F
a =
m decreases
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Newton’s Third Law
NEWTON’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
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Newton’s Third Law
EQUAL AND OPPOSITE
FORCES
• The ground pushes up against
your feet with an equal and
opposite force
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Equal and opposite forces
• You move forward and the
earth moves in the opposite
direction!
– Since the mass of the
earth is so large, its
motion is unobservable
• As you walk, your
feet push against
the ground
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Newton’s Third Law
Equal and opposite forces
Space Shuttle
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• Various fuels are burned in
the engine, producing hot
gases
• As the gases move
downward, the rocket moves
in the opposite direction, or
upward
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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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Bibliography
• K. Batista notes
• Physical Science (1988). HBJ, pp.313316; pp.322-323
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