Inertia and linear momentum

Mass describes a body's resistance to a change in motion. Momentum combines that mass with its velocity, including direction.

Inertia concerns a change in motion

Mass measures inertia. Under the same resultant force, a body of greater constant mass has a smaller acceleration. Inertia is not a resistance to motion itself: a moving body does not need a forward resultant force merely to keep a constant velocity.

Mass is a scalar measured in kg. Weight is a gravitational force measured in N. We treat the laboratory frame as approximately inertial, so the ordinary force laws apply without adding a separate backward "inertia force" to a body's diagram.

Linear momentum is mass times velocity

Momentum is a vector in the direction of velocity. In one dimension, retain the velocity's sign. If velocity is given by several components, multiply every component by the scalar mass.

Its unit is kg m/s, also written N s: since N = kg m s^-2, multiplying by s gives kg m s^-1. Momentum is not a force; N s and N describe different quantities.

Momentum can be zero at an instant while changing at that instant. At the top of a vertical flight, velocity and momentum are zero, but gravity still produces downward acceleration and a changing momentum.