According to our modern world-view, it really isn’t that reason-
able to expect that a special force should be required to make the
air in the train have a certain velocity relative to our planet. After
all, the “moving” air in the “moving” train might just happen to
have zero velocity relative to some other planet we don’t even know
about. Aristotle claimed that things “naturally” wanted to be at
rest, lying on the surface of the earth. But experiment after exper-
iment has shown that there is really nothing so special about being
at rest relative to the earth. For instance, if a mattress falls out of
the back of a truck on the freeway, the reason it rapidly comes to
rest with respect to the planet is simply because of friction forces
exerted by the asphalt, which happens to be attached to the planet.
Galileo’s insights are summarized as follows:
The principle of inertia
No force is required to maintain motion with constant velocity in
a straight line, and absolute motion does not cause any observable
physical e ects.
There are many examples of situations that seem to disprove the
principle of inertia, but these all result from forgetting that friction
is a force. For instance, it seems that a force is needed to keep a
sailboat in motion. If the wind stops, the sailboat stops too. But
the wind’s force is not the only force on the boat; there is also a
frictional force from the water. If the sailboat is cruising and the
wind suddenly disappears, the backward frictional force still exists,
and since it is no longer being counteracted by the wind’s forward
force, the boat stops. To disprove the principle of inertia, we would
have to nd an example where a moving object slowed down even
though no forces whatsoever were acting on it.
self-check E
What is incorrect about the following supposed counterexamples to the
principle of inertia?
(1) When astronauts blast off in a rocket, their huge velocity does cause
a physical effect on their bodies — they get pressed back into their
seats, the esh on their faces gets distorted, and they have a hard time
lifting their arms.
(2) When you’re driving in a convertible with the top down, the wind in
your face is an observable physical effect of your absolute motion.
Discussion Questions
A A passenger on a cruise ship nds, while the ship is docked, that
he can leap off of the upper deck and just barely make it into the pool
on the lower deck. If the ship leaves dock and is cruising rapidly, will this
adrenaline junkie still be able to make it?
B You are a passenger in the open basket hanging under a helium
balloon. The balloon is being carried along by the wind at a constant
velocity. If you are holding a ag in your hand, will the ag wave? If so,
which way? [Based on a question from PSSC Physics.]
C Aristotle stated that all objects naturally wanted to come to rest, with
the unspoken implication that “rest” would be interpreted relative to the
surface of the earth. Suppose we go back in time and transport Aristotle
to the moon. Aristotle knew, as we do, that the moon circles the earth; he
said it didn’t fall down because, like everything else in the heavens, it was
made out of some special substance whose “natural” behavior was to go
in circles around the earth. We land, put him in a space suit, and kick
him out the door. What would he expect his fate to be in this situation? If
intelligent creatures inhabited the moon, and one of them independently
came up with the equivalent of Aristotelian physics, what would they think
about objects coming to rest?