Kim,

Friction is also a type of force. It exists when two objects move (either by sliding or rolling) across each other. The size of the force basically depends on two things, the coefficient of friction (which is for the most part related to how rough or smooth the surfaces are) and how hard the surfaces are being pressed together. The reason for putting rosin on a bow is to increase the coefficient of friction between the bow and the string, so that the bow can better set the string into vibration. The vibrating string pushes (force _again_) on the molecules of gas in the air which causes them to collide with nearby air molecules which collide with other nearby molecules, and so on carrying the vibration through the air. Take away the molecules and there is nothing left to carry the vibration, which is why sound doesn't travel in a vacuum.

Sound will also travel through other materials, often better than it does through air. Everyone who has ever seen a submarine movie knows that sound travels very well through water and that this is the basis for sonar; and anyone who has ever seen a western knows that sound travels even better through steel - ear on the rail to tell if there's a train coming.

And Amos is right - inertia and momentum are not the same thing. The inertia of an object, how easy or hard it is to change its motion, is determined by its mass which is abbreviated by the symbol "m" and is measured in kilograms (or if you're an American engineer, in slugs). The momentum of an object is the product of the objects mass times its velocity, written "mv". (In physics, there is a difference between speed and velocity - speed tells you only how fast an object is moving while velocity tells you both how fast it is moving AND which way it is going. For example: Speed - 30 km per hour or 20 miles per hour; Velocity - 30 km per hour East or 20 miles per hour South)

Momentum is a quantity which is derived from Newton's 2nd Law and is more useful than the concept of force for understanding and analyzing certain events.