Motion is indeed possible without friction! Even if an object is
stationary in one reference frame, in another reference frame moving
with respect the first, that same object will be seen to be moving. So
motion really doesn't need friction at all!
So perhaps what you're asking about is CHANGES in motion. These
also don't require friction. Newton's first law states that objects
with no external forces on them will continue to move in the same
direction at the same speed. Objects at rest with no forces on them
remain at rest. A usual way to apply a force on an object here on earth
is to take advantage of friction, of course, but you can also push and
pull on things that have no friction (just make sure the force is
applied perpendicular to the surface or your hand will slip).
Objects in space can also fall apart into several pieces, turning
one stationary object into a whole bunch of moving ones. This is how
rocket propulsion works -- a rocket throws exhaust gas in one direction
and gets a push in the other. This process is governed by the
conservation of momentum.
There are kinds of friction in space. In the solar system, a very
thin gas of solar wind (protons, electrons, light nuclei streaming from
the sun) permeates the space and interacts with spacecraft and well,
just about everything else. The friction of water sloshing around in
tides on the earth is an interesting example. The earth rotates faster
than the water bulges due to the tides move around, which pushes the
water bulges forwards. This results in an off-axis gravitational force
between the earth and the moon, slowing the earth's rotation down while
transferring energy and angular momentum to the moon, which slowly
rises to a higher orbit. A similar effect is responsible for the fact
that the moon's rotation rate and orbit are locked together, so that
the moon keeps one face towards the earth at all times.
The long-term effect of friction is to make nearby objects move
together. The energy that was found in their relative motion trickles
away into small-scale thermal motions of their parts.
Tom (and Mike)
(published on 10/22/2007)
