Newtonian mechanics studies how objects move under the action of forces. It works extremely well for everyday speeds and sizes, where effects from relativity and quantum theory are negligible. The central ideas are position, velocity, acceleration, mass, force, and time.

In the SI system, position is measured in meters, time in seconds, mass in kilograms, velocity in meters per second, acceleration in meters per second squared, and force in newtons, where $1\,\text{N} = 1\,\text{kg}\cdot\text{m}/\text{s}^2$.

Scalars have magnitude only, such as mass or temperature. Vectors have magnitude and direction, such as displacement, velocity, acceleration, and force. Newtonian mechanics relies heavily on vector reasoning, because direction matters in almost every real problem.

Physics models real systems by simplifying them. Common idealizations include treating objects as particles, ignoring air resistance, and assuming rigid bodies do not deform. These simplifications make problems solvable while preserving the dominant physics.