Data Sheet

Galileo Galilei, Isaac
Newton, and Inertia
People once believed that a continuously applied force
(a push or pull) was required to keep an object in motion,
even with no other forces resisting its motion. We now
know that an object in motion will stay in motion
unless forces act on it to stop its motion. For example,
an object will stop moving because of friction with its
environment, like the ground it’s moving on or the air or
water it’s moving through.
In an experiment to understand inertia, Galileo rolled
marbles down two inclined planes (ramps) that were
positioned in a “V” shape. He found that when he rolled
a marble down one incline, the height that the marble
would reach on the second incline was about the same
as the height from which the marble was released on the
first incline, only just a little lower.
Even when Galileo made the inclined planes as smooth
as possible, he found that the marble never rose as
high on the second plane. He reasoned that there must
be something acting on the marble preventing it from
reaching the same height. He had discovered friction.
Galileo reasoned that if the second inclined plane was
horizontal and there was no friction, then the marble
would roll forever.
Sir Isaac Newton added to the work of Galileo by stating
that the idea of inertia applies to all objects. He also
found that the amount of inertia an object has depends
on its mass: A more massive object will be harder to
move while a less massive object will be easier to move.
The marble rolls to
almost the same height
on the second incline,
but friction keeps it from
getting all the way up to
the original height.
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