Forces

In this textbook we will talk a lot about forces between objects. Hopefully you remember something about forces from your study of mechanics. Most important is to remember Newton's Laws of Motion:

  1. An object at rest will continue at rest, and an object in motion will continue to move at a constant speed in a straight line, unless acted upon by an outside force.
  2. An object with mass m which is acted on by a net force \(\vec F\) will change its velocity, with an acceleration given by the formula
    $$\vec a={\vec F/ m}$$
  3. If object A exerts a force on object B, B will exert a force on A as well: this force will be of equal magnitude, identical type, and in the opposite direction of the force it feels.

In mechanics, you learned about several different forces: for example normal forces, tension, friction, buoyancy, and gravity. Gravity is unusual in this list, because it's the only non-contact force: you have to touch a block to apply a normal force, water has to touch a boat to exert a buoyancy force on it, but the Earth's gravity pulls on the Moon even though they are not in contact, and are in fact separated by millions of miles of emptiness. Gravity is not unique in this regard: in this book we will study the electric force and the magnetic force, which can also act on objects without being in contact with them.

You have experienced the electric force in its most basic form if you've pulled a piece of plastic wrap or transparent tape from its roll, and had it lunge forward and stick to your hand. This is sometimes referred to as static electricity, and is the reason dust loves to collect on the surfaces of electronics, why you can stick a balloon to the wall by rubbing it on your head, and why you get a shock after walking on a carpet on a dry day. But the electric force plays a role far beyond pranks and parlor tricks: it is what holds protons and electrons together to make atoms, and holds atoms together to make molecules and materials. The contact forces listed above (normal, tension, friction, buoyancy) all arise due to the action of the electric force, and all of the reactions you studied in chemistry depend upon it as well.