# Physics with Calculus/Mechanics/Newton's Laws

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Newton's laws describe what causes motion of objects. There are three laws, which can be briefly stated:

• First law: In an inertial reference frame, unless acted on by a net force, an object at rest stays at rest.
• Second law: In an inertial reference frame, force is equal to mass times acceleration (${\displaystyle {\vec {F}}=m{\vec {a}}}$)
• Third law: For every force, there is an equal and opposite reaction force (${\displaystyle {\vec {F}}_{\mathrm {a} b}=-{\vec {F}}_{\mathrm {b} a}}$)

Each of these laws will be discussed in more detail in each section.