Compound Measures
OCR Stage 6
Speed - Distance - Time
Part 1
What is speed?
• How far you travel in a certain time
• Speed = Distance ÷ Time
• We call the result “the average speed”
d
s
t
SDT Triangle
You have seen this in Science?
d
s x t
We can use this to find any one thing
IF we know the other two!
By covering up what you want
d
s x t
Time? = d
Speed?
Distance?
=
t
s x÷ ts
d
A train travels 120 miles in 2 hours
What is its average speed?
s = 120 ÷ 2
s = 60 mph
MUST give
UNITS
Speed =
d
MILES per HOUR
s x t
d
t
A runner’s average speed is 8m/s, how far
does he run in 25 seconds ?
d = 8 x 25
Distance
=s x t
d = 200 metres
MUST give
UNITS
Because speed was in
METRES per second
d
s x t
A lorry leaves Stoke, travels 150km to Leeds
at an average speed of 60km/h. How long
did the journey take?
t = 150 ÷ 60
t = 2.5 hours
MUST give
UNITS
Because speed was in
km per HOUR
Time
=d
s
d
s x t
Car travels 40km in 20 minutes.
What is its average speed in kph?
s = 40 ÷ 1/3
Speed =
s = 120kph
d
s x t
d
t
Density – Mass - Volume
Part 2
What is Density?
• Weight of a given amount of a material
• Density = Mass ÷ Volume
m
d
v
DMV Triangle
You have seen this in Science?
m
d x v
We can use this to find any one thing
IF we know the other two!
By covering up what you want
m
d x v
Density?
Mass?
Volume?=
==dmmx
vd
v
A steel girder weighs 2500 kg and has a
volume of 100m3
What is its density?
d = 2500 ÷ 100
= 25 Kg/m3
MUST give
UNITS
Kilograms per
Cubic Metre
Density =
m
v
m
d x v
Population Density
Part 3
A town in S France has a population of
120,000 living in an area of 15km2
What is the population density?
d = 120 000 ÷ 15 Population density
2
=p
people
/
km
d = 8000
a
MUST give
UNITS
People per Square
Kilometre
p
d x a