GravityWhat is it?
A natural force
that pulls all
the center of
WHAT IS A FORCE?
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
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
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
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
***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
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
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
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
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
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
1/6.25 of the
Earth’s gravity is
Moon’s gravity is