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					                  GravityWhat is it?       
es/cych/apollo%2010                    es/cych/apollo
/story/hoi/ball3.html                  10/story/hoi/ball.html
            Gravity is…
A  natural force
   that pulls all
 objects toward
  the center of
    the earth.
        A pull or a
         push on an
It is measured in Newtons (N)
or kg/ m/ s2.
      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
     Newton’s Laws of Motion and describe
     the motion of all objects on the scale
      we experience in our everyday lives.
     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?
         Earth’s Gravity causes things to stop. The opposing
    force that occurs by making contact with another object is
    called Friction
 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.
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).
  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 don’t even feel it).
2nd law: Force equals mass times
   This equation helps teach us that Gravity
    is a force.
   Acceleration is how quickly an object is
    changing speed
   Earth’s gravity has a constant acceleration
    downward. All objects accelerate at 9.8
   If we all jumped out of a plane we would
    fall at 9.8 m/s2. This goes for all other
    objects whether large or small.
Which do you think would fall first?

   A   bouncy ball

               Try It!
                      Or a marble
They fall at the same rate of speed,
   because of Earth’s constant
 downward acceleration. But the
  marble has more mass (weight)
 and so gravity makes it fall harder
and hits the earth with more force!

  Gravity has a stronger force on
     things with more mass!
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)
       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 (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.
Using Force = mass x acceleration
   Calculate which would land with more
       Object 1 has a mass of 20 grams = .02 kg
       Object 2 has a mass of 100 grams = .1 kg
       Acceleration is 9.8 m/s2.

       Force of Object 1?
       Force of Object 2?
 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
 Right now, gravity is pulling you down in
 your seat, but Newton’s Third Law says 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.
          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).
     Satellites move fast enough that they
    overcome earth’s gravitational pull, but
       still stay within the earth’s orbit.
   As you know Satelittes move around our earth. But did
    you know that is a problem?
   “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.”
      When do Satellites become subject to earth’s
The cannon ball must                      gravity?
move very quickly to
orbit the earth.
If an object does not
meet the required speed
it would be slowly
dragged into the Earth’s
* Objects farther from
earth experience less
gravitational pull.
 The closer to earth’s surface the more
 gravitational pull is felt.
    The Moon orbits the Earth and is
         therefore a Satellite.
   Despite that the moon is much farther it is
    still attracted to the Earth’s gravitational
    pull, but it moves so quickly it revolves the
    planet at a fixed distance.
The Moon is also
1/6.25 the size of
the earth and
therefore has
1/6.25 of the
gravitational pull.

Earth’s gravity is
9.8 m/s2.

Moon’s gravity is
1.568 m/s2.

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