Gravity

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Gravity

• What is it?

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Gravity is

• A natural force that pulls all objects toward the
  center of the earth.

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WHAT IS A FORCE?
A pull or a push on an object
It is measured in Newtons (N) or kg/ m/ s2.
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Newton was a Smart Guy

• Sir Isaac Newton (1643-1727) an English scientist
  and mathematician famous for his discovery of the
  law of gravity also discovered the three laws of
  motion
• Today these laws are known as Newtons Laws of
  Motion and describe the motion of all objects on
  the scale we experience in our everyday lives.

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1st law An object at rest tends to stay at rest
and an object in motion tends to stay in motion
unless acted upon by an unbalanced force.

• -Things will keep moving forever unless it is
  stopped by a force.
• -Things will stay stationary (in one place)
  unless an outside force acts on it.
• If a golf ball was hit in space would it stop?
• If a golf ball was hit on Earth, would it to
  stop? What causes is to stop?
• - Earths Gravity causes things to stop. The
  opposing force that slows things down is Friction

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Some Examples from Real Life
A soccer ball is sitting at rest. It takes an
unbalanced force of a kick to change its motion.

• Two teams are playing tug of war. They are both
  exerting equal force on the rope in opposite
  directions. This balanced force results in no
  change of motion.

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Newtons First Law is also called the Law of
Inertia

• -Inertia the tendency of an object to resist
  changes in its state of motion
• -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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More 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 dont even feel
it).
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2nd law Force equals mass times acceleration.

• Gravity is a force.
• Acceleration is how quickly an object is changing
  speed
• Earths gravity has a constant acceleration
  downward. All objects accelerate at 10 m/s2.
  (9.8 m/s2)
• If we all jumped out of a plane we would fall at
  10 m/s2. This goes for all other objects whether
  large or small.

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Which do you think would fall first?

• A bouncy ball

Or a marble
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-All things fall at the same rate of speed,
because of Earths constant downward
acceleration. -But things with more mass
(weight) will fall harder and hits the earth with
more force! (Gravity has a stronger force on
things with more mass!)
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It takes force to overcome gravity!
Throw a bean bag in the air and then Throw a
basketball in the air.
Which one was harder to throw?
The basketball would be harder to throw. You
had to use more force to throw the basketball
because it has more mass (weight)
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What does F ma say?
F ma basically means that the force of an
object comes from its mass and its acceleration.
-Something very massive thats changing speed
very slowly like a glacier, can still have great
force.
-Something very small thats changing speed very
quickly, 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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Using Force mass x acceleration

• Calculate which would land with more force
• Object 1 has a mass of 20 grams .02 kg
• Object 2 has a mass of 100 grams .1 kg
• Acceleration is 10 m/s2.
• Force of Object 1?
• Force of Object 2?

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3rd Law For every action there is an equal and
opposite reaction.

• For every force acting on an object, there is
  an equal force acting in the opposite direction.
• -Right now, gravity is pulling you down in your
  seat, but your seat is pushing up against you
  with equal force. This is why you are not
  moving.
• -There is a balanced force acting on you gravity
  pulling down, your seat pushing up.

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Think about it . . .
What happens if you are standing on a skateboard
or a slippery floor and push against a wall? You
slide in the opposite direction (away from the
wall), because you pushed on the wall but the
wall pushed back on you with equal and opposite
force.
Why does it hurt so much when you stub your toe?
When your toe exerts a force on a rock, the rock
exerts an equal force back on your toe. The
harder you hit your toe against it, the more
force the rock exerts back on your toe (and the
more your toe hurts).
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Satellites are objects that move fast enough that
they overcome earths gravitational pull, but
still stay within the earths orbit.

• (As you know Satelittes move around our earth.
  But did you know that is a problem?)

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• There are currently over eight thousand
  satellites and other large objects in orbit
  around the Earth, and there are countless smaller
  pieces of debris generated by spacecraft
  explosions and by collisions between satellites.
  Until recently, it has been standard practice to
  put a satellite into orbit and leave it there.
  However, the number of satellites has grown
  quickly, and as a result, the amount of orbital
  debris is growing rapidly. Because this debris is
  travelling at orbital speeds (7-8 km/s!), it
  poses a significant threat to the space shuttle,
  the International Space Station, and the many
  satellites in Earth orbit.
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When do Satellites become subject to earths
gravity?
-Objects must move very quickly to orbit the
earth. - If an object does not meet the required
speed it would be slowly dragged into the Earths
atmosphere. -Objects farther from earth
experience less gravitational pull.
-The closer to earths surface the more
gravitational pull is felt.
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-The Moon orbits the Earth and is therefore a
Satellite.

• Despite that the moon is much farther it is still
  attracted to the Earths gravitational pull,
• -but it moves so quickly it revolves the planet
  at a fixed distance.

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-The Moon is also 1/6.25 the size of the earth
and therefore has 1/6.25 of the gravitational
pull. -Earths gravity is 9.8 m/s2. -Moons
gravity is 1.568 m/s2.