Newton's Second Law

When the forces do not cancel, the net force changes the motion: the object speeds up or slows down. How fast its speed changes, in m/s every second, is its acceleration a. Its unit is m/s²: an acceleration of 2 m/s² means 2 m/s faster every second. Slowing down is an acceleration too, pointing backward. Newton's second law says how big it is:

ΣF = m a

The acceleration points the same way as the net force, and a larger mass gets less acceleration from the same force. Force is measured in newtons: 1 N = 1 kg·m/s².

N and mg cancel; the net force F minus f points right, and so does the acceleration.

Solve a problem in three steps: draw every force on the object, choose a positive direction, then write the net force as ma.

A 5 kg crate is pushed right with 40 N while friction pulls left with 10 N: 40 - 10 = 5a, so a = 6 m/s² to the right.