Near Earth’s surface, the gravitational force on an object is $\vec{W} = m\vec{g}$. Its magnitude is $mg$ and it points downward toward Earth’s center.

The normal force is a contact force exerted by a surface perpendicular to the surface. Its magnitude is not always equal to $mg$; it depends on the other forces and the motion of the object.

Tension is the pulling force transmitted by a rope or cable. An ideal spring obeys Hooke’s law: $\vec{F}_s = -k\vec{x}$, where $k$ is the spring constant and $\vec{x}$ is the displacement from equilibrium.

Static friction prevents slipping up to a maximum value $f_s \le \mu_s N$. Kinetic friction opposes sliding with magnitude $f_k = \mu_k N$. Drag is a resistive force that often increases with speed, sometimes proportionally to $v$ or $v^2$.