Download Gravity and Air Resistance

Gravity and Air
Resistance
Chapter 3
Section 7-9
The Law of Gravitation
• You exert an attractive force on everything
around you and everything is exerting an
attractive force on you.
• This attractive force is called gravity.
• Anything that has mass is attracted by the
force of gravity.
• The Law of Gravitation states that any two
masses exert an attractive force on each
other.
The Law of Gravitation
• Gravitation force depends on two things:
– The mass of the two objects
– The distance between the two objects
• Why do you suppose the Earth exerts a force
on you that you can feel, but you can’t feel
the force the desk is exerting on you?
Gravitational Acceleration
• When objects fall,
the gravitational
pull is 10 m/s2
• When a falling
object is only
affected by gravity
it is said to be in
free fall.
Gravitational Acceleration
• If you drop a a feather and a coin, which
would hit the ground first?
• How about if there were no air?
Gravitational Acceleration
• Force of gravity is greater on the bowling ball
because of its larger mass.
• The larger mass means it has a larger inertia so
more force is needed to change its velocity.
• Gravitational force on the marble is smaller
because it has a smaller mass
• The inertia on the marble is less and less force is
needed to change the velocity.
• Therefore, all objects fall with the same
acceleration!
Weight
• Even if you are standing still and you
have zero acceleration, the force of
gravity is still present.
• Weight is the gravitational force
exerted on an object.
• Capital W stands for weight.
• Gravitational force = mass x
(acceleration due to gravity)
Weight
• Because the gravitational force is the same as
the weight and the acceleration due to
gravity, the equation can also be written as:
• W = (m)(10 m/s2)
Weight & Mass
• Weight and mass are not the same thing.
• Weight is a force
• Mass is a measure of the amount of matter an
object contains.
• The greater the weight, the greater the
attraction between the object and Earth.
Weightlessness
• To be nearly
weightless, you
would have to be far
from the Earth.
• Astronauts
experience this
feeling when they
are in space.
Air Resistance
• What two forces are
acting on an object
when it falls?
• Gravity
• Air resistance
Air Resistance
• Imagine dropping two
pieces of paper. One is
crumpled and the other is
flat.
• Which one will reach the
ground faster and why?
• The crumpled one because
it has less surface area.
Air Resistance
• When something falls,
air resistance acts in the
opposite direction as
the force of gravity.
• Air resistance acts in
the opposite direction
of the object’s motion.
Air Resistance
• The amount of air
resistance an object
experiences depends
on two things:
• Speed
• Surface Area
Air Resistance
• Why do leaves, papers, and feathers fall
at different speeds than acorns, pens,
and glasses?
• Because of Air Resistance…
• Air resistance not mass is responsible for
the difference in objects falling speed.
Terminal Velocity
• As an object falls, it
accelerates and its
speed increases.
• The force of air
resistance increases
with speed.
Terminal Velocity
• The force of air resistance
increases until it becomes
large enough to cancel the
force of gravity.
• When the forces cancel
each other out the object
no longer accelerates.
• The object then falls at a
constant speed called
terminal velocity.
Terminal Velocity
• Terminal Velocity is the
highest velocity that a
falling object will reach.
• A low terminal velocity
allows the skydiver to
land safely.
Terminal Velocity
• Why would a
skydiver want to lay
out flat versus falling
standing up?
• Think about our
paper example…
crumpled vs flat