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Back in ancient Greece, the
philosopher Aristotle believed that
solid objects fell because they were
made up of the element “earth” and
were trying to get back under the
“air” that was beneath them.
He figured that heavier objects would
fall faster than lighter objects.
Galileo proved him wrong hundreds
of years later by dropping two rocks
of different sizes from a tall building
Galileo found that the two rocks hit at
the same time, disproving Aristotle.
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Later, Isaac Newton discovered the Law of
Universal Gravitation that explained Galileo’s
results.
Newton found that Gravity acts on all objects
on earth in the same way.
This means that ALL objects in free-fall will fall
the same way.
Newton Also discovered that objects actually
accelerate as they fall
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All objects fall with the same
acceleration on earth
It has its very own symbol
g = -9.8 m/s2
It is VERY important that you
remember….THIS IS NOT
GRAVITY!!!!
g is an acceleration
Gravity is a force
g (“little gee”) is the symbol for
the ACCELERATION CAUSED
BY GRAVITY…NOT GRAVITY
itself
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g is a standard in a vacuum (the
absence of matter), but most things
fall in air.
So why does a rock hit the ground
before a feather?
Air resistance!
A feather interacts with the air more
than the rock thus air slows down its
acceleration considerably.
Because of air resistance objects will
not accelerate forever, they reach a
maximum speed
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Terminal velocity (or speed) is the maximum
speed an object can reach when falling in air.
As an object increases in speed, the effect of air
resistance becomes greater.
The force of air resistance & the force of gravity
will eventually cancel each other out & the
object will stop accelerating.
Terminal velocity depends on air resistance.
Air resistance depends on surface area, more
surface area = lower terminal velocity
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The longer it accelerates, the faster it is going
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How high was an object that was dropped
from rest and fell for 12.6 s before hitting the
ground? (Assume it fell in a vacuum)