An engineer is studying the density of polystyrene using a rectangular block of it.
(a) Say how to find the mass of the block and, from measurements, its volume.
Our own questions on this topic, marked straight away, so you see what to revise. Free now.
Show model answer for 17.1Hide model answer for 17.1
Place the block on a balance and read its mass. Measure the length, width and height of the block with a ruler. Multiply length × width × height to find the volume.
How the marks are earned
- One mark: find the mass with a balance (scales are also accepted). Saying only 'weigh it' earns nothing.
- Then any two of these: measure the three sides of the block (any names for the three sides are accepted); use a ruler, tape measure or similar to do it; multiply length × width × height to get the volume.
- A displacement method, such as a Eureka can, earns nothing.
Why it scores
The balance gives the mass. For a regular block the three measurements with a ruler give the volume once multiplied together. Together these give the two quantities needed for density. A displacement can would not work well because polystyrene floats and is easily squashed.
Revise Density(b) Polystyrene of density 50 kg/m3 is cut into a 0.60 kg block. Find the block's volume.
See how you would score on DensityShow model answer for 17.2Hide model answer for 17.2
Density = mass over volume. volume = mass over density
Volume = fraction: 0.60 kg over 50 kg/m3, end of fraction
Volume = 0.012 m3.
How the marks are earned
- One mark: use density = mass ÷ volume, written in any form, for example volume = mass ÷ density.
- One mark: rearrange and substitute: 0.60 ÷ 50.
- One mark: the volume 0.012 (the unit, cubic metres, is optional). The right digits, 1.2, with a wrong power of ten earn 2 marks.
- The correct answer, 0.012 m3, on the answer line earns full marks.
Why it scores
Writing the density equation, in any form, earns the first mark. Rearranging to volume = mass ÷ density is credited together with the substitution on the next line.
Rearranging and substituting the values together earn the second mark: 0.60 ÷ 50.
This is the final mark. The answer alone earns all 3 marks. 1.2 × 10-2 m3 is the same value.
Revise Density