Chapter one watched things move. This chapter asks WHY they move: something pushed. A push or a pull is a force, and its unit is the newton — $1\ \mathrm{N} = 1\ \mathrm{kg \cdot m/s^2}$, the force that gives one kilogram one metre per second squared. Hold an apple in your palm: that gentle press is about one newton. You have been measuring forces your whole life.

Keep the cast straight, because the units keep it for you. Bare kilograms are mass — the amount of stuff. $\mathrm{m/s^2}$ is acceleration — what a force achieves. Newtons are the force itself. And the physicist's opening move, before any formula, is the free-body idea: draw the object alone and give every push on it an arrow. If the arrows balance, nothing changes. If they don't — that is where this chapter lives.