1.4Motion, Forces and Energy
Density — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 25 of 91 1 quiz
Lesson summary
1.4 Density — IGCSE PHYSICS notes: Define density, use ρ = m/V and predict whether an object floats using density.Density = the mass per unit volume of a material.Equation — ρ = m/V, where ρ is density, m is mass and V is volume.Rearranged forms are m = ρV and V = m/ρ.A formula t…
- Common units
- Conversion
- Keep units consistent
- Irregular objects
- Low-density materials
- High-density materials
Section 1 of 9Definition and equation
Density
Density the mass per unit volume of a material.
Equation
- ρ = m/V, where ρ is density, m is mass and V is volume.
- Rearranged forms are m = ρV and V = m/ρ.
A formula triangle helps rearrangement: cover the quantity to be found and the structure of the equation remains.
Section 2 of 9Units and conversion
SI units
- SI units density in kg/m3, mass in kg and volume in m3.
Common units
- Common units density in g/cm3, mass in g and volume in cm3.
Conversion
- 1 g/cm3 = 1000 kg/m3.
Water
- Water its density is about 1.0 g/cm3, which is 1000 kg/m3.
Keep units consistent
- Keep units consistent mass is almost always in kg in physics, but density is often given in g/cm3.
Section 3 of 9Finding the volume
Volume is not always given and is calculated from the shape.
- Cuboid — V = length × width × height.
- Cylinder — V = πr2h.
- Sphere — V = 4/3 πr3.
Irregular objects
- Irregular objects the volume is found by displacement of water.
Section 4 of 9Density and particle arrangement
Density and particle arrangement
Low-density materials
- Low-density materials objects of the same size have a low mass, as a bag of feathers is lighter than the same bag of metal.
High-density materials
- High-density materials particles are packed closely, so the same size has a high mass.
Gases
- Gases particles are far apart, so a gas is much less dense than the same substance as a liquid or solid.
Same mass over a larger volume gives a lower density.
Section 5 of 9Measuring density
Liquid
- Liquid find the mass of an empty measuring cylinder, add liquid, read the volume and find the mass again.
- Mass of liquid = the difference between the two masses.
Regular solid
- Regular solid measure the mass on a balance and the dimensions with a ruler, then calculate the volume.
Irregular solid
- Irregular solid measure the mass and find the volume by displacement.
- Use a measuring cylinder and find the rise in level, or a eureka can and collect the overflow.
Section 6 of 9Predicting floating and sinking
Predicting floating and sinking
Object less dense than the liquid
- Object less dense than the liquid it floats.
Object more dense than the liquid
- Object more dense than the liquid it sinks.
Examples in water:
- Cork (0.24 g/cm3), ice (0.92 g/cm3) and oil (0.9 g/cm3) float.
- Aluminium (2.7 g/cm3), iron (7.9 g/cm3) and gold (19.3 g/cm3) sink.
Ships float because the average density of steel plus air inside is less than that of water.
Two liquids that do not mix (Extended)
- Two liquids that do not mix (Extended) the less dense liquid floats on top of the denser one.
- A solid placed in layered liquids settles at the level where its density lies between the densities of the liquids above and below.
Section 7 of 9More from the specification
More from the specification
Density
- Density equals mass divided by volume, measured in kg/m3 or g/cm3, and describes how closely matter is packed.
Weight and gravitational field strength
- Weight is the force on a mass in a gravitational field, W = mg, where g is about 10 N/kg near the Earth's surface.
Measuring density
- Find the mass with a digital balance and the volume of an irregular solid by the water displaced in a measuring cylinder.
Fluids and upthrust
- An object floats when its average density is less than the fluid's, because upthrust then balances its weight.
Interpreting results
- Compare calculated density with data values to identify a material and judge the accuracy of the measurement.
Density
- Density equals mass divided by volume, measured in kg/m3 or g/cm3, and describes how closely matter is packed.
Weight and gravitational field strength
- Weight is the force on a mass in a gravitational field, W = mg, where g is about 10 N/kg near the Earth's surface.
Measuring density
- Find the mass with a digital balance and the volume of an irregular solid by the water displaced in a measuring cylinder.
Fluids and upthrust
- An object floats when its average density is less than the fluid's, because upthrust then balances its weight.
Interpreting results
- Compare calculated density with data values to identify a material and judge the accuracy of the measurement.
Apparatus
- The equipment needed to carry out an experiment.
Variable
- A factor that can change in an investigation.
Gravitational field
- Region where mass experiences gravitational force.
Field strength
- G = Fg/m.
Vernier callipers
- An instrument measuring small lengths to 0.1 mm.
Independent variable
- The variable deliberately changed, here time
Dependent variable
- The variable measured, here temperature
Weight
- Gravitational force on a mass.
Upthrust
- Upward force exerted by a fluid.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Density | Mass per unit volume of a material. |
| Displacement | Measuring the volume of an object from the water it pushes aside. |
| Eureka can | Overflow can used for displacement. |
| Float | Behaviour of an object less dense than the liquid. |
| Sink | Behaviour of an object denser than the liquid. |
| Water density | 1.0 g/cm³ or 1000 kg/m³. |
Section 9 of 9Exam technique
1
Mixing up g/cm³ and kg/m³ — 1 g/cm³ equals 1000 kg/m³.
2
Forgetting to convert cm and g before using SI units — convert lengths to m and masses to kg.
3
Saying heavy objects always sink — an object sinks only if its density is greater than the liquid.
4
Using the final water level as the object volume — use the rise in level.
Marked practiceQuizzes for this lesson
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