# StudySquare

# Direct proportion for AQA A-level Maths

This page covers the following topics:

1. Direct proportion

2. Converting units

Two quantities are directly proportional to each other when they are multiples of each other. The notation for direct proportionality is a โ b. Equations can be constructed using a constant of proportionality and evaluating it using given information.

To convert a value from larger units to smaller units, multiply it by the ratio between the units. To convert a value from smaller units to larger units, divide it by the ratio between the units. Sometimes a conversion of units can achieved by multiplying or/and dividing a value in steps.

1 km = 1000 m

1 m = 100 cm

1 cm = 10 mm

1 kg = 1000 g

1 hour = 60 min = 3600 s

# 1

z is directly proportional to sยฒ. Given that when s = 3, z = 54, construct an equation for the relationship between z and sยฒ.

Since z is directly proportional to sยฒ, z = ksยฒ, where k is a constant of proportionality.

Substituting s = 3, z = 54 into this gives 54 = k(3ยฒ), thus k = 6.

Therefore, z = 6sยฒ.

z = 6sยฒ

# 2

x is directly proportional to y. Given that x = 10 when y = 2, construct an equation for the relationship of x and y.

Since x is directly proportional to y, x = ky, where k is a constant of proportionality.

Substituting y = 2, x =10 into this gives 10 = k ร 2, therefore k = 5.

Thus, the equation is given by x = 5y.

x = 5y

# 3

Convert 100 m to cm.

100 m = 100 ร 100 cm = 10000 cm

10000 cm

# 4

How many hours are in 72 seconds?

72 seconds = 72 รท 3600 hours = 0.02 hours

0.02

# 5

The density of an object is given by 1200 g/cmยณ. Calculate the density of the object in kg/cmยณ.

1200 g = 1200 รท 1000 kg = 1.2 kg

Therefore, the density is given by 1.2 kg/cmยณ

1.2 kg/cmยณ

End of page