Download gravity - Northside Middle School

GRAVITY
 Gravity is a
force.
 It is a force of
attraction
between any
two objects
GRAVITY
 Gravity works close to the
Earth.
 Objects and the Earth are
attracted toward each
other. Since the Earth
cannot move toward the
object because it is so
big, the object moves
toward the Earth.
 Gravity works out in space.
 Our moon is in the
gravitational pull of the Earth.
Because the moon is smaller
than the Earth, it is pulled
toward the Earth.
 The Earth is in the gravitational
pull of the sun. Because the
Earth is smaller than the sun, it
is pulled toward the sun.
 The farther away two objects
are, the less gravitational pull
the objects will have. So, for
instance, Pluto will have less
gravitational pull than Earth,
because it is farther away from
the sun.
GRAVITY
 Gravity makes things fall down because





it is drawn toward the center of the
Earth.
When you drop an object it starts falling
slowly, but then falls faster.
It accelerates.
This is called acceleration due to gravity
(g). All objects on Earth have the same
acceleration due to gravity.
On Earth g= 9.8m/s2
Mass does not affect the acceleration of
falling objects, IF gravity is the ONLY
force acting on that object.
Acceleration Due to Gravity
The rate of
falling
increases by
9.8 m/s every
second.
On a side note: Remember that kinetic energy is
energy of motion. We learned that an object has the
greatest kinetic energy right before it hits the ground.
Does it now make sense why? As an object falls it
accelerates, so it has to be moving as fast as it can
right before it hits the ground. Faster motion equals
higher kinetic energy!
 I said earlier that mass does not affect the
speed of falling objects, assuming there is
only gravity acting on it.
 Is there ever a situation, here on Earth, in
which gravity will be the only force acting
on the object?
 Are there any other forces acting on a
falling object?
GRAVITY
 When something is falling
gravity pulls it down.
Air friction
 But another force, AIR
friction, pushes it up.
 Remember that friction
acts in the direction
OPPOSITE the motion of
an object
Gravity
GRAVITY
 When something starts to
fall, gravity makes the
object accelerate. At first
they the object isn’t going
very fast, so air friction is
small.
Air
Friction
 Think about when you put your hand
out the window on a car drive. There
is more drag when you are flying
down the highway then when you
are driving 25mph down Main street!
Gravity
GRAVITY
 As the object falls
faster, the force of
friction gets bigger, so
the acceleration of the
objects begins
decrease. The object
doesn’t slow down, it is
just not accelerating as
much anymore.
Air
Friction
Gravity
GRAVITY
 After a while , the air
friction force becomes the
same size as the force of
gravity.
 The object stops
accelerating. It is still
moving, but not
accelerating anymore)
 It is moving at a constant
velocity
 It is now moving at its
terminal velocity
Air
Friction
Gravity
Terminal Velocity
 Terminal Velocity is the maximum speed
an object can go.
 If a person jumps out of a plane, he will fall
until he reaches his top speed. Once the
force of gravity is equal to the force of air
resistance, he cannot accelerate anymore.
He could fall forever and not reach a
higher speed than his terminal velocity.
GRAVITY
Air Friction
Gravity
Air Friction
Gravity
velocity
time
Air
Friction
Gravity
What Affects Terminal Velocity?
 Mass can affect terminal velocity. The more
mass an object has, the higher its terminal
velocity will be. More massive objects have a
greater gravitational pull toward the earth.
 A more massive object will also take longer to
reach its terminal velocity. This is due to the
object needing more time to reach a higher
terminal velocity than a smaller object.
 Surface area also affects terminal velocity. If an
object falls with a large surface area, it will
experience a higher air resistance and will have
a smaller terminal velocity.
GRAVITY
 What happens when a
parachute opens?
 The surface area gets
bigger.
 The friction force gets
bigger because of air
resistance.
 The parachutist slows
down because the force
of air friction is larger than
the force of gravity.
GRAVITY
Air Friction
Gravity
GRAVITY
Air Friction
Air Friction
Gravity
Air Friction
Gravity
velocity
time
Air Friction
Gravity
Gravity
GRAVITY
 The falling object has slowed down.
 It now has a lower terminal velocity.
 So, Why does rain fall faster than snow?
So what would it look like if there
was no air resistance?
 On the moon, there is no air resistance to
slow a falling object.
 Because of this, objects fall at the same
speed.
 http://www.youtube.com/watch?v=L55jImE
hNZ0
Without air resistance
With air resistance
Gravity is different on different planets!!!
The more mass a planet has, the more gravity it has.
Planets which have more mass than Earth would
have more gravity than Earth. A person would weigh more
on these planets than they do on Earth.
How much would you weigh?
Location
Moon
Mercury
Venus
Mars
Jupiter
Saturn
Uranus
Neptune
Weight on Earth
Gravity
X
X
X
X
X
X
X
X
0.17
0.38
0.86
0.38
2.87
1.32
0.93
1.23
Calculated Weight
=
=
=
=
=
=
=
=
You just calculated your weight on
different planets. Question….
Are mass and weight the
same thing?
NO!!!!!!!
 Mass is the amount of matter in an object. It is how much
of something you have.
 Weight is the force of gravity acting on an object. Your
weight is a measurement of how much force, the force of
gravity, you are applying to a scale.
 If you hopped from planet to planet, your weight would
change because of change in the gravity of each of the
planets, but your mass would still stay the same. There
would be the same amount of you here and on the
moon….unless you cut off your arm on the space ride to
the moon. Make sure to keep you arms and legs inside
the space ship at all times. Safety first!