Dynamical friction
From Free net encyclopedia
Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space. It is sometimes referred to as gravitational drag, and was first discussed in detail by Subrahmanyan Chandrasekhar in 1943.
The effect must exist if the principle of conservation of energy and momentum is valid since any gravitational interaction between two or more bodies corresponds to elastic collisions between those bodies.
E.g. when a heavy body B moves through a cloud of lighter bodies, the gravitational interaction between B and the light bodies causes the light bodies to accelerate and gain momentum and kinetic energy (see sling effect). Since energy and momentum are conserved, B has to lose a part of its momentum and energy equal to the sums of all momenta and energies gained by the light bodies. Because of the loss of momentum and kinetic energy of the body under consideration the effect is called dynamical friction.
Another equivalent way of thinking about this process is that the light bodies near B are attracted by its gravity toward its position and therefore the density at that location increases and is referred to as a gravitational wake. In the meantime, B has moved forward. Therefore, the gravitational attraction of the wake pulls B backward and slows it down.
Of course the mechanism works the same for all masses of interacting bodies and for any relative velocities between them. However while in the above case the most probable outcome is the loss of momentum and energy by the body under consideration, in the general case it might be either loss or gain (when one body loses momentum and energy in an elastic collision the other one gains them). In a case when the body under consideration is gaining momentum and energy the same physical mechanism is called sling effect.
The fractional rate of energy loss due to dynamical friction is constant for slowly-moving bodies, but drops rapidly at high velocities. Because of this, dynamical friction is unimportant for objects that move relativistically, such as photons.