Foundation and Higher · compound measures

Speed, density,
pressure.
One method.

the units tell you
the formula

a unit made from two others —

Compound

km per hour · g per cm³

“per” means divide

step 1 of 4

Read the units

1. The slash means divide.

2. So the units give you the formula.

✓ Forgotten the formula? Read the units

km/h= km ÷ h

speed = distancetime

g/cm³: density = massvolume

N/m²: pressure = forcearea

step 2 of 4

Speed

1. Change the time into hours: 2.5.

2. Distance ÷ time.

✓ 30 minutes is 0.5 hours

150 km in 2 h 30 min

2 h 30 min = 2.5 h not 2.3

150 ÷ 2.5

= 60 km/h

step 3 of 4

Rearranged

1. Put the numbers into the formula.

2. Rearrange like any equation.

✓ Substitute first, then solve

density 8 g/cm³, volume 50 cm³

8 = mass50

mass = 8 × 50 × 50 both sides

= 400 g

step 4 of 4

Changing the units

1. Convert the distance part.

2. Convert the time part.

✓ Then divide again

72 km/h into m/s

72 km = 72 000 m × 1000

1 h = 3600 s 60 × 60

72 000 ÷ 3600 = 20 m/s

compound measures —

Speed

distance ÷ time

Density

mass ÷ volume

Pressure

force ÷ area

Minutes are not decimals:
15 minutes is 0.25 hours.

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