WAEC 2010 Physics · Question 15

What is the density of a fuel of relative density 0.72? [Density of water = 1000kg m-3]

Options

A.72 kgm-3
B.720 kgm-3
C.7200 kgm-3
D.72000 kgm-3

Answer: B. 720 kgm-3

Explanation written with AI and checked against the answer key

Step 1: Analyze the question We are given the relative density of a fuel (0.72) and the density of water (1000 kg/m³). We need to find the density of the fuel. Step 2: Concept Relative density (also called specific gravity) compares the density of a substance to the density of water: Relative density = Density of substance ÷ Density of water So: Density of substance = Relative density × Density of water Step 3: Solution Density of fuel = 0.72 × 1000 kg/m³ Density of fuel = 720 kg/m³ Step 4: Answer Check: A relative density below 1 means the fuel is less dense than water, so it floats on water. This is true of petrol and kerosene, and 720 kg/m³ is a realistic density for petrol. Tip: Relative density has no units because it is a ratio. To get the actual density, just multiply by 1000 (when water is 1000 kg/m³). Moving the decimal point three places to the right is quick: 0.72 becomes 720. Watch out for options that look similar but have the wrong number of zeros.

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