Letter: Sinking feeling

Share
Sir: I am surprised there is so much confusion over Archimedes' principle. (Technoquest, 23 September; letters, 26, 27 September).

A body immersed in water displaces its own volume. If you put an object into a bath filled to the brim, and collect the water that flows over the edge and pour it into a measuring cylinder, you have the volume of the object. (According to legend, when Archimedes made his discovery he had been asked by the king to determine whether his new crown was solid gold. He knew the mass or weight of the crown - as far as he was concerned, the two terms were equivalent and can be treated as such in this case. He also knew what volume that weight of gold should have. He now needed to measure the volume of the crown without destroying it, and the bath gave him the method.)

The displaced water has to be raised against the force of gravity, and the weight of this water leads to the buoyant force resisting the immersion of the object. For a material denser than water, such as lead or gold, this force is less than the weight of the object and it sinks. For a body made of a substance less dense than water, such as wood, or that is hollow, such as a ship, the weight of water displaced is greater than the weight of the body, and the object floats, reaching an equilibrium at the level where the weight of the volume of water displaced equals the weight of the object.

PAUL DORMER

Guildford,

Surrey

Sir: It is common for mass and weight to be confused, but Joy Bampton (letter, 27 September) has managed to confuse mass and weight and volume, all at the same time. Volume and mass are not the same thing.

Whether or not an object floats depends on its density, which is its mass divided by its volume. If its density is less than that of water, gravity will pull an equal volume of water down harder than it pulls the object, so the object will rise and the water sink. If the object's density is greater, it will sink and the water will rise.

ALAN ROBINSON

York

React Now

Latest stories from i100
Have you tried new the Independent Digital Edition apps?
iJobs Job Widget
iJobs General

Associate Recrutiment Consultant

£18000 - £23000 per annum + Uncapped OTE: SThree: SThree Group have been well ...

Trainee Recruitment Consultant

£18000 - £23000 per annum + OTE: SThree: Real Staffing Group is seeking Traine...

Year 6 Teacher (interventions)

£120 - £140 per day: Randstad Education Leeds: We have an exciting opportunity...

PMLD Teacher

Competitive: Randstad Education Manchester: SEN Teacher urgently required for ...

Day In a Page

Read Next
 

Letter from the Political Editor: Cameron's unexpected tax pledges give the Tories home advantage

Andrew Grice
President Barack Obama walks with U.S. Secret Service agents to Air Force One at Los Angeles International Airport in Los Angeles, Calif., May 8, 2014.  

Obama's Secret Service has become sloppy with its delusions of Hollywood grandeur

David Usborne
Italian couples fake UK divorce scam on an ‘industrial scale’

Welcome to Maidenhead, the divorce capital of... Italy

A look at the the legal tourists who exploited our liberal dissolution rules
Time to stop running: At the start of Yom Kippur and with anti-Semitism flourishing, one Jew can no longer ignore his identity

Time to stop running

At the start of Yom Kippur and with anti-Semitism flourishing, one Jew can no longer ignore his identity
Tom and Jerry cartoons now carry a 'racial prejudice' warning on Amazon

Tom and Jerry cartoons now carry a 'racial prejudice' warning on Amazon

The vintage series has often been criticised for racial stereotyping
An app for the amorous: Could Good2Go end disputes about sexual consent - without being a passion-killer?

An app for the amorous

Could Good2Go end disputes about sexual consent - without being a passion-killer?
Llansanffraid is now Llansantffraid. Welsh town changes its name, but can you spot the difference?

Llansanffraid is now Llansantffraid

Welsh town changes its name, but can you spot the difference?
Charlotte Riley: At the peak of her powers

Charlotte Riley: At the peak of her powers

After a few early missteps with Chekhov, her acting career has taken her to Hollywood. Next up is a role in the BBC’s gangster drama ‘Peaky Blinders’
She's having a laugh: Britain's female comedians have never had it so good

She's having a laugh

Britain's female comedians have never had it so good, says stand-up Natalie Haynes
Sistine Chapel to ‘sing’ with new LED lights designed to bring Michelangelo’s masterpiece out of the shadows

Let there be light

Sistine Chapel to ‘sing’ with new LEDs designed to bring Michelangelo’s masterpiece out of the shadows
Great British Bake Off, semi-final, review: Richard remains the baker to beat

Tensions rise in Bake Off's pastry week

Richard remains the baker to beat as Chetna begins to flake
Paris Fashion Week, spring/summer 2015: Time travel fashion at Louis Vuitton in Paris

A look to the future

It's time travel fashion at Louis Vuitton in Paris
The 10 best bedspreads

The 10 best bedspreads

Before you up the tog count on your duvet, add an extra layer and a room-changing piece to your bed this autumn
Arsenal vs Galatasaray: Five things we learnt from the Emirates

Arsenal vs Galatasaray

Five things we learnt from the Gunners' Champions League victory at the Emirates
Stuart Lancaster’s long-term deal makes sense – a rarity for a decision taken by the RFU

Lancaster’s long-term deal makes sense – a rarity for a decision taken by the RFU

This deal gives England a head-start to prepare for 2019 World Cup, says Chris Hewett
Ebola outbreak: The children orphaned by the virus – then rejected by surviving relatives over fear of infection

The children orphaned by Ebola...

... then rejected by surviving relatives over fear of infection
Pride: Are censors pandering to homophobia?

Are censors pandering to homophobia?

US film censors have ruled 'Pride' unfit for under-16s, though it contains no sex or violence