GALILEO ON THE MOON
Video Summary
Galileo used thought experiments to
test many assumptions, including the
notion that heavy objects fall more
quickly than lighter objects when they
are dropped. Lacking access to either
a vacuum chamber or a planetary
body that has no atmosphere, he
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nevertheless correctly predicted that
all falling objects would accelerate at the same rate in the absence of air resistance. In this
video segment from NASA, astronaut David Scott demonstrates the correctness of
Galileo’s prediction.
Topics Covered:
– Earth and Space Science: Earth in the Universe
Recommended for Grades 3-12
Media Type: QuickTime Video
Video Length: 0m 47s
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Discussion Questions
 Predict what would happen if you dropped a hammer and a falcon feather
on Earth: would they hit the ground at the same time or at different times?
Explain your reasoning.
 Why did the hammer and the falcon feather hit the ground at the same time
on the Moon?
 If you couldn’t go to the Moon, how could you repeat the experiment shown
in the video—in a situation with little or no atmosphere—here on Earth?
Background Essay
Long before recorded history, humans observed the world around them, taking note of
the rise and fall of tides, the phases of the Moon, and countless other physical and
astronomical phenomena. Because great thinkers often recorded their observations of
such phenomena, we have a historical record of how our understanding of the natural
world has changed over time. More than 2,000 years ago, Aristotle concluded that
heavier objects fall faster than lighter objects. He also surmised that the rate at which an
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object falls toward Earth when dropped is directly proportional to its mass, i.e., an object
with twice the mass of another falls twice as fast.
We now know that Aristotle was wrong. Although massive objects sometimes fall faster
than lighter objects, mass and weight have nothing to do with the rate of their descent.
Indeed, if we could control for all variables for any two dropped objects, we would find
that they always accelerate toward Earth at exactly the same rate. So, what causes
objects to behave differently in so many cases? For example, why do hammers and
feathers usually fall at such dramatically different rates?
Air resistance causes feathers, leaves, and other objects we consider lightweight to fall more
slowly than more massive objects such as hammers and bowling balls. This force resists the
acceleration of falling objects, and the larger an object’s surface area, the greater the
resistant force. In the absence of air resistance, such as in a vacuum or on the Moon where
there is almost no atmosphere, all objects accelerate toward the ground at the same rate.
Gravity, the force that causes objects to fall toward Earth, is also responsible for Earth’s
atmosphere. Gravity exists between any two objects that have mass. The more massive an
object and the closer it is to another object, the greater the gravitational force it exerts on
the other object. Earth exerts a gravitational force on gas molecules in the atmosphere, such
as water vapor, oxygen, hydrogen, ozone, and carbon dioxide, just as it does on more
massive objects, such as the Moon.
The Moon has almost no atmosphere, and because it is one-quarter the diameter of Earth
and significantly less dense, the Moon has about one-sixth the gravity of Earth. Thus, the
gravitational force the Moon exerts is not strong enough to attract and retain significant
quantities of atmospheric gases. In the near absence of these gases, there is no measurable
air resistance, and hammers and feathers are free to fall just as Galileo predicted.
To learn more about physical characteristics of the Moon, check out
Extreme Temperatures on the Moon, Explore the Moon, and Your Weight on Other Worlds.
To learn more about the Moon’s place in the solar system, check out The Origin of the
Moon and Why Doesn’t the Moon Fall Down?.
To learn more about gravity and weightlessness, check out Free-Falling and “Weightlessness”.
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Curricular Standards Correlations:
NSES, Project 2061, MCREL, and state standards correlations available at
www.teachersdomain.org. (Free registration required for your specific state standards correlated to this resource.)
Source: NASA
Materials used courtesy of:
NASA
Collection developed and produced for Teachers’ Domain by:
Collection funded by:
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