IGCSEPhysicsCambridge (CIE)Revision Notes1. Motion, Forces & EnergyMass, Weight & DensityFloating

Floating (Cambridge (CIE) IGCSE Physics)

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Leander Oates

Written by: Leander Oates

Reviewed by: Caroline Carroll

Floating objects

Density and floating

  • Whether an object floats or sinks depends on the relative densities of the object and the fluid it is submerged in

    • If the object is denser than the fluid, it will sink

    • If the object is less dense than the fluid, it will float

  • Upthrust is a force that pushes upwards on an object submerged in a fluid 

  • Upthrust always acts in the opposite direction to the object's weight force

    • This is how objects float

  • The size of the upthrust force depends on:

    • the density of the fluid

    • the volume of fluid that is displaced (which is equal to the volume of the object)

    • The denser the liquid, the greater the force of upthrust exerted on the object 

Upthrust and weight forces on a boat

Upthrust Free Body Diagram, downloadable AS & A Level Physics revision notes

An object will float if the force of upthrust is equal to the force of weight

Factors affecting floating & sinking

  • The factors affecting floating and sinking are:

    • upthrust

    • density

  • If the force of upthrust on an object is:

    • equal to the object’s weight, then the object will float

    • less than the object’s weight, then the object will sink

  • If the density of the object is:

    • greater than the density of the fluid, then the object will sink

    • less than the density of the fluid, then the object will float

  • If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will equal its weight (and therefore it sinks)

Floating, IGCSE & GCSE Physics revision notes

Objects which are less dense than water will float in water, and those that are more dense will sink

  • A polystyrene block will float in water

    • This is because polystyrene has a density of 0.05 g/cm3 which is much less than the density of water (1.0 g/cm3)

  • A wooden block will be partially submerged but will still float

    • This is because the density of a wooden block (0.9 g/cm3) is slightly less than the density of water

  • An iron block will sink

    • This is because iron has a density (7.9 g/cm3) that is much higher than water

Worked Example

Potassium is an alkali metal that reacts with oxygen and water. Potassium is therefore stored in containers filled with mineral oil. The potassium must sink in the mineral oil to ensure it has no contact with any air in the container.

A science teacher has four different mineral oils in which they could store their potassium. Potassium has a density of 0.862 g/cm3. 

Which mineral oil should the teacher choose?

A: White mineral oil, rho space equals 0.864 space straight g divided by cm cubed

B: Paraffin oil, rho space equals 0.890 space straight g divided by cm cubed

C: Liquid paraffin, rho space equals 0.825 space straight g divided by cm cubed

D: Food grade mineral oil, rho space equals 0.880 space straight g divided by cm cubed

Answer: C

  • An object will sink in a fluid if the density of the object is greater than the density of the fluid

  • Therefore, for the potassium to sink, the density of the oil must be less than the density of the potassium

  • Liquid paraffin is less dense than potassium 0.825 space straight g divided by cm cubed space less than space space 0.862 space straight g divided by cm cubed

  • The only oil which is less dense than potassium is C

Examiner Tips and Tricks

The main thing to remember is that density is mass per unit volume

In Physics, mass is almost always measured in kg

Density is the only topic in which physicists sometimes use grams instead

Floating liquids

Extended tier only

  • A liquid with a lower density will float on a liquid with a higher density if the liquids do not mix

1-3-4-density-of-liquids-cie-igcse-23-rn

Lower density liquids float on higher density liquids as long as the liquids do not mix

Worked Example

Liquid A has a density of 0.76 g/cm3 and liquid B has a density of 0.93 g/cm3.

If the two liquids do not mix, which liquid will float on top of the other?

Answer:

Step 1: List the known quantities

  • Liquid A = 0.76 g/cm3 

  • Liquid B = 0.93 g/cm3

Step 2: Determine which liquid has the lowest density

  • The liquid with the lowest density will float on top of the liquid with the higher density

  • 0.76 is less than 0.93

  • Therefore, liquid A has the lowest density

Step 3: State your answer

  •    Liquid A will float on top of liquid B

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    Author
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    Leander Oates

    Author: Leander Oates

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.