Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    OK, so I was working out what the formula for the surface area + volume of a torus would be and I got both right. Then I thought 'right, applying a similar principle to a sphere should work too'.

    So this is what happened:
    VOLUME
    A sphere is basically a semicircle rotated around an axis. (works for a full circle too).
    It is rotated for the length of the full circle's circumference.

    And this is what I got..
    volume = 1/2.pi.r^2.2.pi.r
    = pi^2.r^3
    SURFACE AREA
    Using the semicircle-around-an-axis idea again;
    surface area = 2.pi.r.pi.r = 2.pi^2.r^2

    ... which is certainly not what it was supposed to be.

    So why doesn't this method work? What have I done wrong?
    This is probably a stupid question
    • Thread Starter
    Offline

    0
    ReputationRep:
    Aha, never mind! I have to use the centroid of the semicircle rather than the radius... laziness does not pay of...
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: November 3, 2013
Poll
Is the Big Bang theory correct?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.