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 Post subject: 3D shapes
PostPosted: Sat Feb 19, 2011 5:19 pm 
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Joined: Tue Nov 11, 2008 10:06 am
Posts: 54
There seems to be different answers for the number of faces and edges for cones, cylinders and spheres - I think based on different definitions of what exactly a face is etc. Could someone clarify what the 11+ definition would be please for the following:

sphere - 0 or 1 face (0 edges & vertices)
cylinder - 2 or 3 faces ( 2 edges & 0 vertices)
cone - 1 or 2 faces (1 edge & 0 vertices)

many thanks


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 Post subject: Re: 3D shapes
PostPosted: Sat Feb 19, 2011 10:56 pm 
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Joined: Fri Oct 12, 2007 12:42 pm
Posts: 3810
Location: Chelmsford and pleased
The more I have thought about this the more I am unsure.

Faces are supposed to be flat, but both the cone and the cylinder have curved surfaces that could be opened out to be flat.

An edge is supposed to be straight, so that would mean that none of these shapes has any edges.

A vertex is a point where edges join and the cone would therefore have an apex rather than a vertex.

Looking at this I would conclude that 0 for all three for a sphere, one face for a cone and two faces for a cylinder.

Perhaps at primary level these issues are worried about less and therefore a sphere has one face, no edges and no vertices; a cone two faces, one edge and one vertex and a cylinder three faces, two edges and no vertices.

I await with interest the responses of those with more confidence to make a decision than me.


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 Post subject: Re: 3D shapes
PostPosted: Mon Feb 21, 2011 6:44 am 
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Joined: Mon Nov 08, 2010 9:15 am
Posts: 42
Location: Birmingham
Hello,

I do not think there is a 'simple' answer to your question.
If you check the internet there seems to be a whole lot of disagreement between teachers
and the like.
I accidently started reading about the Euler characteristic to help provide you with an answer
and that led me to being more confused than I thought possible when it came to defining
the characteristics of a sphere.
I suggest that your child would be unlikely to receive a question in an exam involving faces on spheres/cylinders as there is too much indecision on the matter.

However, if your daughter/son is asked a question about faces/vertices/edges involving the sphere - tell them to write - Euler said it's 2!

http://en.wikipedia.org/wiki/Euler_characteristic

Sorry this is not more helpful!

_________________
"A teacher affects eternity; he can never tell where his influence stops."
Henry B. Adams


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 Post subject: Re: 3D shapes
PostPosted: Mon Feb 21, 2011 8:07 am 
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Joined: Mon Jun 18, 2007 2:32 pm
Posts: 6963
Location: East Kent
From a quick search through primary strategy it seems that at KS2 they distinguish between curved surface and flat surface

There was a question in the 2005 sats about faces

but it was to complete a table no of flat surfaces, number of curved surfaces


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 Post subject: Re: 3D shapes
PostPosted: Thu Feb 24, 2011 8:01 am 
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Joined: Tue Nov 11, 2008 10:06 am
Posts: 54
Thanks for the replies so far. My son has got homework from his tutor on faces,edges and vertices which inc the cone,cylinder and sphere - hence my question.I think we will have to go with a definition of straight edge and flat face (not curved) and count up on that basis.

thanks


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 Post subject: Re: 3D shapes
PostPosted: Sun Mar 13, 2011 12:19 pm 
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Joined: Fri Oct 12, 2007 12:42 pm
Posts: 3810
Location: Chelmsford and pleased
I looked on mymaths and was disappointed to discover that they felt that a sphere had one face.

I gave the problem to my year 7 top set and they decided that if a face was a polygon and a polygon is defined as having straight edges then none of these shapes has faces, edges or vertices. :D


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