Edexcel A Level Maths: Mechanics:復習筆記1.1.3 Derived Units
Derived Units
What are derived units?
- Derived units for quantities such as velocity, acceleration and force etc. are combinations of S.I. units
- Speed?or?Velocity?= distance ÷ time?=?m/s or m s-1?(Velocity is just speed with direction)
- Acceleration?= velocity ÷ time?=?ms-1?/s =?m/s2?or m s-2
- Weight?or?Force?=?mass?x acceleration =?kg m s-2?or N?(N = Newtons)
Worked Example
a) Convert the following into S.I. units
i) 72 km h-2? ? ? ? ? ? ? ? ? ? ii)? ?50 cm per minute? ? ? ? ? ? ? ? ? ?iii)? ?14 g cm-3
b) A cyclist takes 15 minutes to travel 2.54 km. Calculate the average speed in m s-1?
Exam Tip
- Watch out for exam questions using non S.I. units such as feet and inches or miles per hour.
- Make sure you are confident?converting?between these derived units,
- Remember?speed?is velocity without direction and?weight?is a force measured in?Newtons, not a mass measured in kg.
- If you ever forget a formula, use the units it is measured in to figure it out,
- e.g. m/s = metres?÷?seconds = distance?÷?time = velocity.
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