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Self-Check
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 flesh 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 finds, 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 flag in your hand, will the flag wave. If so, which way. [Based on a
question from PSSC Physics.]
pool
ship's direction
of motion
Discussion question A.
Discussion question B.
(1) The effect only occurs during blastoff, when their velocity is changing. Once the rocket engines stop firing, their
velocity stops changing, and they no longer feel any effect. (2) It is only an observable effect of your motion relative
to the air.
Discussion question D.
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.
D. The bottle is sitting on a level table in a train’s dining car, but the surface of
the beer is tilted. What can you infer about the motion of the train.
Chapter 2Velocity and Relative Motion