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Forces & Newton’s First
Law
Science
Ms. Arce
Forces
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A force is a push or pull that one object
exerts on another.
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Ex. Magnet pulling an iron nail
Ex. A person pulling a luggage cart
Ex. A tugboat pushing a barge upriver
Ex. The Sun and Earth pulling each other in a
way that keeps Earth in orbit around the Sun
Forces
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Forces change the motion of objects.
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Ex: Rocket’s engine provides strong push that makes
the rocket accelerate upward and away from the
launch pad.
Ex: Batter who hits a ball with a bat. The bat exerted
a force on the ball and has sent it flying toward the
outfield. Even though the bat pushed on the ball for
only a brief second, the ball may have gained enough
velocity to clear the fence for a home run.
Forces can change the shape of objects.
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Ex: clay or aluminum can
Gravity
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Gravity keeps the Moon going around Earth and
Earth going around the Sun.
When you throw a ball into the air, here’s what
happens: The Earth pulls on the ball, and the
ball pulls on Earth. The forces of gravity are
equal and opposite. The pull of Earth makes the
ball fall to Earth. The pull of the ball on Earth
also makes Earth move, BUT not by much
because Earth is hard to budge.
There is also a force of gravity between you and
the ball you just threw, BUT this force is so small
you don’t notice. WHY???
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Mass is what produces gravity. The mass of Earth is
much greater than your mass!
Gravity (continued…)
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The force of gravity between two objects
depends on the masses of the two objects
and the distance between the objects.
The closer the objects are to each other,
the stronger the force of gravity that they
exert on each other. The farther away the
objects are from each other, the weaker
the force is.
Weight
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The force of gravity between Earth (or any
planet, asteroid, or star) and any other
object is call the weight of the object.
All objects, no matter what their mass or
what they are made out of, fall with the
same acceleration, when only gravity acts.
If this is true, then why does a book fall
faster than a piece of paper?
Weight (continued…)
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A book falls faster than a piece of paper
because gravity is not the only force
acting. The air slows the paper down. If
you crumple the paper up, this gets rid of
the effect of air and the paper will fall
faster.
Balanced Forces
Unbalanced Forces
Newton’s First Law
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An object at rest (velocity = 0) tends to
stay at rest
An object moving in a straight line at
constant speed tends to keep moving that
way
An object’s velocity can only be changed
by applying a force to it
Example
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A bus is traveling along with constant velocity. It is
moving at 30 mph on a straight road. When the
driver suddenly slams on the brakes, the bus comes to
a stop. The passengers were all moving 30 mph.
Newton’s first law says this motion can only be
changed by applying a force. The brakes provide this
force for the bus and anything attached to it. When
the brakes are applied by the driver, the velocity of the
bus decreases. However, the passengers are not
firmly attached to the bus. As a result the passengers
continue to move ahead at 30 mph. The passengers
will go forward until it does encounter a force that
changed its velocity. They may bump the seat in front
of them.
Inertia
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Inertia is the tendency of an object to
oppose any change in its motion.
The passengers on the bus are an
example of inertia. The passengers tend
to keep moving even as the bus slows to a
stop.
Only when a force is applied to an object
does its velocity change.
Inertia
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When the driver steps on
the brakes, the passengers
in the bus continue moving
forward because of their
inertia. If the bus stops
moving gradually, the
friction between the seat
cushions and the
passengers can reduce the
velocity of the passengers
to match the slowing of the
vehicle. In a quick stop,
however, the inertia of the
passengers can make them
slip or even jolt forward.
NASA
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NASA ~ What does
NASA have to say
about inertia?
Another Example
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Have you ever gone around a curve in a
car and felt like you had to lean into the
turn? If you didn’t lean, you would tip in
the other direction. Your inertia makes
you feel this way. As a passenger in the
car that is turning, you would continue
moving straight. This makes you feel like
you are tipping in the other direction.
Inertia
Mass, Weight – Same
Difference…OR IS IT?
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To an astronomer or a physicist they are completely
different things.
The mass of a body is a measure of how much matter
it contains. An object with mass has a quality called
inertia.
If you shake an object like a stone in your hand, you
would notice that it takes a push to get it moving, and
another push to stop it again. If the stone is at rest, it
wants to remain at rest. Once you've got it moving, it
wants to stay moving. This quality or "sluggishness" of
matter is its inertia. Mass is a measure of how much
inertia an object displays.
Mass, Weight – Same
Difference…OR IS IT?
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Every object in the universe with mass attracts every
other object with mass. The amount of attraction
depends on the size of the masses and how far apart
they are.
For everyday-sized objects, this gravitational pull is
vanishingly small, but the pull between a very large
object, like the Earth, and another object, like you,
can be easily measured. How? All you have to do is
stand on a scale! Scales measure the force of
attraction between you and the Earth. This force of
attraction between you and the Earth (or any other
planet) is called your weight.
Picture this…
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You are in a spaceship far between the stars
and you put a scale underneath you. The scale
would read zero. Your weight is zero!!!! You are
weightless. There is an anvil floating next to
you. It's also weightless.
Are you or the anvil mass-less? Absolutely not.
If you grabbed the anvil and tried to shake it,
you would have to push it to get it going and
pull it to get it to stop. It still has inertia, and
hence mass, yet it has no weight. See the
difference?
Your Weight on Other Worlds
Mass VS. Weight
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Mass -> how hard it is to push or pull an object.
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The greater mass an object has, the greater force it
takes to move that object.
Weight -> force of gravity pulling down on an
object located on the surface of earth.
The weight of an object depends on its location
in the universe. However, the mass of the
object does not change.
Mass & Weight
What causes an orbit?
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Orbits are the result of a perfect balance
between the forward motion of a body in
space, such as a planet or moon, and the pull
of gravity on it from another body in space,
such as a large planet or star.
An object with a lot of mass goes forward and
wants to keep going forward; however, the
gravity of another body in space pulls it in.
There is a continuous tug-of-war between the
one object wanting to go forward and away
and the other wanting to pull it in.
What causes an orbit?
What causes an orbit?
What if…they aren’t balanced?
These forces of inertia and gravity have to be perfectly balanced for an orbit to
happen. If the forward movement (inertia) of one object is too strong, the
object will speed past the other one and not enter orbit. If inertia or
momentum is much weaker than the pull of gravity, the object will be pulled
into the other one completely and crash.
Our World
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Gravity in Space
Exercise in Space
Just last night…
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Winter X Games
2012: Colten Moore
Crash
 What
role did
gravity and
inertia play
in this crash?
Review
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Draw this diagram in
your notebook. Draw
arrows where gravity
and inertia are acting.
Review
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Inertia
Gravity/Gravitational Pull
Weight and Mass
Balanced and Unbalanced Forces
How force of gravity can change…
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What makes it increase or decrease?