In the stated
example, a
helium balloon does not `
float away from the
pull of gravity'. It
floats up because it is
less dense than the
air - it's a principle of
buoyancy, the same reason an
air-filled balloon floats up out of the
water but falls
down from the
sky. The balloon in a
car crash will only be thrown
backwards relative to the
car if there is significantly greater (enough to overcome the balloon's
momentum)
air pressure in the
front than the
back. You can perform the
experiment yourself - get a
helium balloon and go out to a
parking lot. Go
forward slowly, then
slam on the brakes. Repeat the experiment at faster and faster speeds, and see at what point the balloon goes
backward instead of
forward. I haven't
done the math on this, so I don't know what sort of
velocities and
accelerations you're going to need for this to
occur, or if you can
attain them in your
car. My
instinct says that you can't, but
I could be wrong.
Oh, and
relative to the
ground, you're not being thrown
forward in a
car crash - the
car is just
stopping quickly and you're
not.
A better
example of the
principle that
acceleration and
gravity are indistinguishable would be that of a
spaceship accelerating in
interstellar space, where external
gravity sources are
negligible, or an
accelerating elevator.
In the case of the
spaceship in interstellar
space, if the
ship accelerates at a near constant
9.81 m/s/s, you won't be able to tell the
difference between standing in the
ship and standing on
earth. You will not be able to perform any
experiment that will give a diffirent
result on the
spaceship than it would on
earth.
In the case of the
elevator, by
accelerating it
up or
down one can alter the
percieved force of gravity. Given a sufficiently long
elevator shaft, one could, for example,
accelerate it downward at a suitable rate to
mimic the gravity of the
moon. You would not be able to tell if the elevator was sitting on the moon or accelerating in its shaft (until it hits the bottom, or stops accelerating and moves at a constant velocity).