Quantities and units in mechanics

Explanation

• The fundamental S.I. quantities are length in metres, time in seconds and mass in kilograms; velocity uses $\text{m s}^{-1}$, acceleration $\text{m s}^{-2}$, force and weight newtons, and moment $\text{N m}$.
• Convert all measurements to consistent S.I. units before substituting, using for example $1\,\text{km h}^{-1}=\dfrac{5}{18}\,\text{m s}^{-1}$ and $1\,\text{tonne}=1000\,\text{kg}$.
• Since $F=ma$, $1\,\text{N}=1\,\text{kg m s}^{-2}$; multiplying a force by a perpendicular distance gives a moment in $\text{N m}$.
• A common error is to confuse mass with weight: mass is measured in kilograms, whereas weight is a force measured in newtons; a moment is not measured in newtons alone.