CONSERVATION OF MOMENTUM:

This will be a quickie. But please note: momentum is of monumental importance in physics. It has a momentous impact... ok, ok, I'll try to behave.

Momentum is mv, a vector. If the net force on a system is zero, the total momentum (vector sum) is constant. That is it. Know it well. Play with it. Apply it to collision problems especially. During a collision the forces are equal and opposite, so their sum is zero, so the vector sum of mv before = vector sum of mv after (if all other forces add up to zero). Momentum is denoted by p, for some reason. So p = mv. Big deal, save writing one letter.

Newton's 2nd law is equivalent to impulse = change of momentum, where impulse is product of force and time. (If the force varies, use average force multiplied by the time that the force acts. If more than one force acts on a body, use the vector sum of the impulses.)

A special case is the elastic collision, the ideal bounce. (Some call it perfectly elastic, so that there is no question about the meaning.) For the head-on elastic collision, the relative speed of approach before collision is equal to the relative speed of recession after collision. If the collision is not head-on, you need to use separate x and y component momentum equations as well as the energy equation. The discussion of this and examples are in the same file as the head-on.

A rocket shoots out exhaust, and the force on the exhaust in the rear direction is equal in magnitude to the thrust, or force in the forward direction. So we analyze this by conservation of momentum. More about rockets.

One more thing: again if the net force on a system is zero, the center of mass of the system will have constant velocity. You may not need this in your particular course. Do not learn this if you don't need to. Unlearn it immediately before it is too late. The world has enough smart people. If you do need to know how to find the center of mass and how to apply it, you could click here.

Oops, there is another "one more thing": We might as well talk about angular momentum a little. The thing that corresponds to mass is called moment of inertia or rotational inertia or just inertia, I. Don't worry about how to calculate it yet. The thing that corresponds to velocity is angular velocity, symbolized by the Greek letter (small) omega, which looks like a curly w (w if your browser recognizes it). Omega is the rotation rate of an object in radians per second. So angular momentum is simply I times omega. The thing that corresponds to force is torque. Now in the underlined statement at the top of the page, replace the words force and momentum with torque and angular momentum, and you now have a statement of the conservation of angular momentum principle. Next time you watch a figure skater do her spins, note that when she pulls her arms and legs in closer to the center of rotation, her spin rate (w ) increases. Her inertia decreased by pulling arms and legs in, and the product of I and w remained constant (neglecting friction).

Do not click on back, or you will have a virtual collision with virtual reality.

My main pages:

mechanics
fluids, heat, electricity and magnetism
vibrations and waves
quantum

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