# Q & A: Motion without friction

Q:
Motion is not possible without friction.What type of friction occurs in space? ( I mean where there is no air)
- Mehak
St.Bonaventure’s High School, Pakistan
A:
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)