What is a compound measure?

• A compound measure is something that is calculated by using more than one measurement
• Compound measures can be used to measure rates
• This measures how much one quantity changes the other is increased by 1
• Examples include:
• Speed – how much the distance changes for each unit of time
• Density – how heavy something is for each unit of its area or volume
• Pressure – how much force is applied to an object for each unit of its area
• Flow rate – how much the volume changes for each unit of time
• Population density – how many people there are for each unit of area

How do I find the units for a compound measure?

• You can use the formula for a compound measure to derive its units
• Use the units for the quantities in the formula to derive the units of the compound measure
• You write a division as a/b or ab ^-1 and pronounce it as “a per b”
• Examples include:
  • 
    • If the distance is measured in km and the time is measured in mins then the speed is measured in km/min
  • 
    • If the force is measured in N and the area is measured in cm ² then the pressure is measured in N/cm ²

How do I find the formula for a compound measure?

• You can use the units for a compound measure to help remember its formula
• You just need to remember what each unit measures
• If the unit is a/b then the formula will be the quantity that a measures divided by the quantity that b measures
• Examples include:
  • kg is a measure of mass and cm ³ is a measure of volume
  • Therefore 
• Density can be measured in kg/cm ³

Can I use a formula triangle?

• Yes – if you like them!
• STEP 1. Use the units in the question (or other info) to write down a formula

• STEP 2. Create a formula triangle

  • e.g. for Speed, Distance and Time

• STEP 3. For each part of the question write down 
• • what you know
  • what you want to know
• STEP 4. Use the formula triangle to rearrange the formula (if necessary)
• STEP 5. Substitute numbers in and solve