1.8.3 Speed, Density & Pressure - Harder

What are speed, density and pressure?

• Speed, density and pressure are compound measures – they are made from other measures
• Speed is related to the measures distance and time
• Density is related to mass and volume
• Pressure is related to force and area
• The relationship between each of these sets of measures follows the same pattern – what we refer to as “blue triangles”

• If you do not remember the blue triangles – do not worry, these can often be deduced from information given in the question – see the examples below
• It is important you understand and can do the basic questions with speed, density and pressure

Speed, distance and time harder problems

• Speed is commonly measured in metres per second (m/s) or
miles per hour (mph)

• There are other possibilities such as kilometres per hour (kmph)
• The units indicate speed is distance per time
ie speed = distance ÷ time
• “Speed” (in this formula) means “average speed
• In harder problems there are often two journeys – or two parts to one longer journey

Density, mass and volume harder problems

• Density is usually measured in grams per cubic centimetre (g/cm3)
or kilograms per cubic metre (kg/m3)

• The units indicate that density is mass per volume
ie density = mass ÷ volume
• In harder problems there are often two metals (alloys), liquids or gases that have been combined rather than working with a single substance

Pressure, force and area harder problems

• Pressure is usually measured in Newtons per square metre (N/m2)
The units of pressure are often called Pascals (Pa) rather than N/m2

• The units indicate that pressure is force per area
ie pressure = force ÷ area
• Remember that weight is a force; it is different to mass

Exam Tip

Do look out for a mixture of units:

• Time can be given as minutes but common phrases like “half an hour
(ie 30 minutes) could also be used in the same question.
• Any mixed units should be those in common use and easy to convert
g to kg (and vice versa)
m to km (and vice versa)

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