Microscopic vs. Macroscopic

Physical phenomena can be observed at different scales.

For example, Newton’s 2nd law:

• At the macroscopic scale, we have $\vec{F}=m\vec{a}$.
• At the microscopic scale, we have $\vec{F}=\frac{d(m\vec{v})}{dt}$, based on the mass and change in velocity of particles.