Transcript Newton`s Laws of Motion - Lancaster City Schools

Mass vs. Weight
Newton’s Laws of Motion
8th Grade Science
-Mass is defined as the amount of matter an
object has.
-does NOT change no matter where
you are in universe!
-The weight of an object on earth depends
on the force of attraction (gravity) between
the object and earth
-DOES change depending on where
you are
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Formula to calculate weight:
Weight = Mass × Gravity
W = mg
Mass: measured in Kg
Gravity on Earth: 9.8 m/s/s
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Practice
-Calculate the weight of a car with a mass
of 1500 kg.
-14700N
-Calculate the weight of an object with a
mass of 550 kg.
-5390N
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1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.
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1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
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Newton’s First Law is also called
the Law of Inertia
Inertia: the tendency of an object to
resist changes in its state of motion
The First Law states that all objects
have inertia. The more mass an object
has, the more inertia it has (and the
harder it is to change its motion).
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Examples from Real Life
A powerful locomotive begins to pull a
long line of boxcars that were sitting at
rest. Since the boxcars are so massive,
they have a great deal of inertia and it
takes a large force to change their
motion. Once they are moving, it takes
a large force to stop them.
On your way to school, a bug
flies into your windshield. Since
the bug is so small, it has very
little inertia and exerts a very
small force on your car (so small
that you don’t even feel it).
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If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
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Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist
changes in their motion. When the car going 80
miles/hour is stopped by the brick wall, your body
keeps moving at 80 miles/hour.
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2nd Law
Force = mass x acceleration
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What does F = m x a mean?
F = m x a basically means that the force of an
object comes from its mass and its acceleration.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
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2nd Law (F = m x a)
• How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
• Write the formula
• F=mxa
• Fill in given numbers and units
• F = 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800
N
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3rd Law
• For every action, there is an
equal and opposite reaction.
• The baseball forces
the bat to the left (an
action); the bat forces
the ball to the right
(the reaction).
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