Transcript Newton`s Laws - Petoskey Public Schools

Newton’s Laws
Describing motion
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Newton’s three laws describe how
things move and how this motion can
be changed by other forces/objects
Newton’s laws lead to the formulas that
lets us express motion with math
Newton’s 1st Law
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This is also known as the Law of Inertia
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An object at rest will stay at rest
An object in motion will stay in motion – in
a straight line
An object will remain at rest or in linear
motion unless acted on by a force
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A car parked in the parking lot does not
move until the engine is started or
another car pushes it
A soccer ball keeps rolling across a
parking lot in a straight line
A soccer ball slows down as it crosses
the field because the force of friction
caused by the grass pushes against the
ball
Mass & Inertia
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Mass is a measure of the amount of matter in
an object
Ultimately, this is a measure of the amount of
atoms in an object
The more mass an object has, the more
inertia is has
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It is harder to push a big suburban than a little
sports car because the suburban has more mass
It is harder to catch a thrown bowling ball than a
softball
Resistance to Change
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Inertia is a resistance to change in
motion
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This can be seen with the car examples, or
anything else either in motion or sitting still
(motion of zero)
Whether in motion or at rest, inertia is
resistance to change
Inertia is also a resistance to changing the
direction of motion
Reorder the following based
on their inertia (lowest to highest)
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A speeding
motorcycle
A slow moving semi
truck
A five story
skyscraper
A speeding bullet
from a rifle
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A fast pitch softball
A classroom metal
chair
The tire from a farm
tractor
A golf ball hit with a
driving iron
Reorder the following based
on their inertia (lowest to highest)
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A speeding bullet
from a rifle
A golf ball hit with a
driving iron
A fast pitch softball
A classroom metal
chair
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The tire from a farm
tractor
A speeding
motorcycle
A slow moving semi
truck
A five story
skyscraper
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Weight and mass are not the same
Weight depends on gravity
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If you weigh 150 lbs on earth, you would
weight 25 lbs on the moon and would not
weigh anything on the International Space
Station
SI Units
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Grams and kilograms are used to
measure mass
A newton is used to measure weight,
just like a pound measures weight
1 kg on earth weighs 9.8 newtons
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Newton’s 2nd law will help us calculate this!
Newton’s 2nd Law
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The acceleration of an object is directly
proportional to the force pushing on it
and inversely proportional to the mass
of the object
Force
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A force is a push or pull on an object
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A force can be caused by contact
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The force of your arm pushes the door
open
A force can be caused over a distance
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The force of the pull from the magnetic
north pole pulls the needle in a compass to
point towards north
Newton’s 2nd Law Formula
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F=m*a
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F = force, measured in newtons
m = mass, measured in kilograms
a = acceleration, in m/s2
Using F=m*a to find force (weight) of
gravity for a 1 kg block
F = 5 kg * 9.8m/s2
F = 49 N (N is used for newtons)
Acceleration and Force
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If the force increases, the acceleration
increases
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If two people push a car, is might move,
but it will move slowly and increase speed
slowly
If ten football players push the same car, it
will accelerate faster
Acceleration and Mass
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Acceleration and Mass are inversely
related
This means that as mass increases, the
other acceleration decreases
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Ten football players push a small sports
car, it accelerates at a moderate rate
Ten football players push a suburban full of
golfers with their clubs, the suburban
accelerates much slower because it has
more mass
Find the Force
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How much force do the football players
need to push the car with to make it
accelerate at .2 m/s2 if the car is 780
kg?
Find the Force
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How much force do the football players
need to push the car with to make it
accelerate at .2 m/s2 if the car is 780
kg?
F= .2 m/s2 * 780 kg
F = 156 N
Finding the Acceleration
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If a car engine generates 350 N, and
the car has a mass of 700 kg, what will
its acceleration be?
Finding the Acceleration
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If a car engine generates 350 N, and
the car has a mass of 700 kg, what will
its acceleration be?
F = m*a or a = f/m
A = 350 N/700 kg
A = .5 m/s2
Newton’s 3rd Law
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Every action has an equal and opposite
reaction
Every force has a corresponding force in the
opposite direction
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Try it – tape the latch on a door so it can not latch
and stay closed, then push as hard as you can on
the door
Instead of pushing real hard on the door, you tend
to fall through the doorway
The door did not “push back” to keep you from
falling
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What happens if you try to throw a
basketball while on a skateboard?
If you throw the ball in the same
direction the skateboard is facing, you
will start to roll backwards
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You pushed the ball when you threw it,
and the ball pushed you backwards!!
What are the two balanced
forces pushing on you?
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The force of gravity is pulling you down
. . . So why don’t you keep going
down?
The chair you are sitting in is pushing
you up at with the same amount of
force as gravity, just in the opposite
direction
Find the opposite force
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A propeller on an airplane pushes the
plane forward, how? What is the
opposite force?
Find the opposite force
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A propeller on an airplane pushes the
plane forward, how? What is the
opposite force?
The airplane moves forward because
the propeller pushes air backwards
The opposite force is the air pushes the
propeller forwards (and the plane since
it is attached to the propeller)
Find the opposite force
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What are the two forces in action when
you use a pull-up bar to do a chin-up?
Find the opposite force
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What are the two forces in action when
you use a pull-up bar to do a chin-up?
You pull down on the bar
The bar pulls up on you