Forces, Friction, and Translational Equilibrium

Having met Newton's laws, we now look closely at the specific forces that appear in mechanics problems and at the important special case where all forces balance: translational equilibrium.

Common forces

Weight is the gravitational force on a mass, $W = mg$, directed toward the center of the Earth. Weight is a force measured in newtons and must not be confused with mass, measured in kilograms, which is the amount of matter and stays the same everywhere. On the Moon your mass is unchanged but your weight is about one sixth of its Earth value because $g$ is smaller.

The normal force $N$ is the contact force a surface exerts perpendicular to itself, preventing objects from passing through it. On a horizontal surface with no vertical acceleration, the normal force equals the weight, but this i