OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Use of such measurements to determine densities of solid and liquid objects.
Full specification wording
Materials: Use of such measurements to determine densities of solid and liquid objects.
Density tells you how much mass is packed into each cubic centimetre or cubic metre of a material, and you find it by dividing mass by volume.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Density tells you how much mass is packed into each cubic centimetre or cubic metre of a material, and you find it by dividing mass by volume. For a regularly shaped solid, measure length, width and height with a ruler and multiply them to get volume in cm³, then read mass on a balance in grams and divide. For an irregular solid, use a displacement method: record the water level in a measuring cylinder, lower the object in fully, and take the rise in level as the object's volume. For a liquid, pour a measured volume into a measuring cylinder and find the mass of that volume, subtracting the mass of the empty container. Keep units consistent, for example g and cm³ give g/cm³, and convert to kg/m³ by multiplying by 1000.
What earns the marks
- Divides mass by volume to calculate density and states the unit as g/cm³ or kg/m³.
- Reads volume of a regular solid by multiplying measured length, width and height.
- Finds volume of an irregular solid from the rise in water level when it is fully submerged.
- Subtracts the mass of the empty container when finding the mass of a liquid sample.
- Converts between g/cm³ and kg/m³ correctly by multiplying or dividing by 1000.
Common mistakes and how to put them right
Adds length, width and height instead of multiplying them.
Put it right: Multiply the three dimensions to obtain volume.
Forgets to subtract the mass of the empty measuring cylinder.
Put it right: Subtract the container mass to isolate the liquid.
Mixes grams with cubic metres without converting.
Put it right: Keep mass and volume in matching units before dividing.
Exam tips
- Write the equation density = mass ÷ volume before substituting numbers so units stay clear.
- Show the volume calculation separately for irregular solids, stating the starting and final water levels.
- Check whether the answer needs g/cm³ or kg/m³ and convert with a factor of 1000 if required.
Try the lesson
The start of the Materials: Use of such measurements to determine densities of solid and liquid objects. lesson, made from checked teaching notes for this specification point.
Have a go first
A block of wood has a mass of 150 g and a volume of 250 cm³. What is the density of the wood?
Listen
Density tells you how much mass is packed into each cubic centimetre or cubic metre of a material, and you find it by dividing mass by volume. For a regularly shaped solid, measure length, width and height with a ruler and multiply them to get volume in cm³, then read mass on a balance in grams and divide. For an irregular solid, use a displacement method: record the water level in a measuring cylinder, lower the object in fully, and take the rise in level as the object's volume. For a liquid, pour a measured volume into a measuring cylinder and find the mass of that volume, subtracting the mass of the empty container. Keep units consistent, for example g and cm³ give g/cm³, and convert to kg/m³ by multiplying by 1000. A strong answer earns marks for these points. Divides mass by volume to calculate density and states the unit as g/cm³ or kg/m³. Reads volume of a regular solid by multiplying measured length, width and height. Finds volume of an irregular solid from the rise in water level when it is fully submerged. Subtracts the mass of the empty container when finding the mass of a liquid sample. Exam tip: Write the equation density = mass ÷ volume before substituting numbers so units stay clear.
Key words
Say what each word means before you turn the card.
Density
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Displacement method
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Unit consistency
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Measuring volume of a regular solid
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See every definition
- Density
- Found by dividing mass by volume, telling you how much mass is packed into each cubic centimetre or cubic metre of a material.
- Displacement method
- Used for an irregular solid: record the water level in a measuring cylinder, lower the object in fully, and take the rise in level as its volume.
- Unit consistency
- Keeping mass and volume in matching units, for example g and cm³ give g/cm³, and converting to kg/m³ by multiplying by 1000.
- Measuring volume of a regular solid
- Measure length, width and height with a ruler and multiply them to get volume in cm³.
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