Newton's Laws

N1L: no force $\Rightarrow \vec{V}={\rm const}$ (or at rest, $\vec{V}=0$) implies existence of inertial reference frames

N2L: \[ \vec{a}={1\over m}\vec{F}_{\rm total}, \quad \mbox{or} \quad \sum \vec{F}= m \vec{a} \] implies that objects have mass, a measure of inertia

N3L: all forces are due to interactions \[ \vec{F}_{\rm A\> on\> B} = - \vec{F}_{\rm B \> on \> A} \]

Problem-solving strategy, refined
  1. read and re-read; itentify what is known/unknown
  2. make a drawing, or
    draw a vector diagram, identify all [pairs of] forces
  3. use the laws of Physics to establish formal relationships ($F=ma$), or
    use N2L to determine acceleration, $\vec{a}={1\over m}\vec{F}_{\mbox{total}}$
  4. perform algebraic manipulations to get the final result, or
    use kinematics, i.e. $\vec{a} \rightarrow \vec{V} \rightarrow \vec{r}$
  5. substitute numerical values; watch significant figures
  6. check units; does the result make sense?