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rolling ball is initially heading straight for a wall, but a steady wind begins
blowing from the side, the ball does not take any longer to get to the wall.
In the case of projectile motion, the force involved is gravity, so we can say
more specifically that the vertical acceleration is 9.8 m/s
2
, regardless of the
horizontal motion.
Relationship to relative motion
These concepts are directly related to the idea that motion is relative.
Galileo’s opponents argued that the earth could not possibly be rotating as
he claimed, because then if you jumped straight up in the air you wouldn’t
be able to come down in the same place. Their argument was based on their
incorrect Aristotelian assumption that once the force of gravity began to act
on you and bring you back down, your horizontal motion would stop. In
the correct Newtonian theory, the earth’s downward gravitational force is
acting before, during, and after your jump, but has no effect on your
motion in the perpendicular (horizontal) direction.
If Aristotle had been correct, then we would have a handy way to
determine absolute motion and absolute rest: jump straight up in the air,
and if you land back where you started, the surface from which you jumped
must have been in a state of rest. In reality, this test gives the same result as
long as the surface under you is an inertial frame. If you try this in a jet
plane, you land back on the same spot on the deck from which you started,
regardless of whether the plane is flying at 500 miles per hour or parked on
the runway. The method would in fact only be good for detecting whether
the plane was accelerating.
Discussion Questions
A. The following is an incorrect explanation of a fact about target shooting:
“Shooting a high-powered rifle with a high muzzle velocity is different from
shooting a less powerful gun. With a less powerful gun, you have to aim
quite a bit above your target, but with a more powerful one you don’t have
to aim so high because the bullet doesn’t drop as fast.”
What is the correct explanation.
Section 6.1Forces Have No Perpendicular Effects
B. You have thrown a rock, and it is flying through the air in an arc. If the
earth's gravitational force on it is always straight down, why doesn't it just go
straight down once it leaves your hand.
C. Consider the example of the bullet that is dropped at the same moment
another bullet is fired from a gun. What would the motion of the two bullets look
like to a jet pilot flying alongside in the same direction as the shot bullet and at
the same horizontal speed.