
Planes Of Symmetry
Hi, I've worked these amounts of planes, but I'm not sure if they are correct, could you help me confirm them?
Cuboid  3 Triangular Prism  4 Squarebased Pyramid  4 Cylinder  2 Tetrahedron  3
Thanks you!
 ryos
 Power Member
Re: Planes Of Symmetry
I'm not sure I quite understand what is meant by "plane of symmetry." If it is just "a plane that passes through the object which cuts it into two symmetrical pieces," then a cube would have an infinite number of such planes. Just pass a vertical plane through the center, and rotate it by infinitesimal amounts. It will always be symmetrical, regardless of the angle!
El que pega primero pega dos veces.
Re: Planes Of Symmetry
No, that isn't right, pretend the plane is a mirror line, if you just turn it then you will not always get a cube.
 MathsIsFun
 Administrator
Re: Planes Of Symmetry
For a cube (not a cuboid) there would be the x, y and z planes, but also the planes that pass through the corner points, am I right?
I am not referring to anything other than my imagination, but here goes:
Cuboid (not cube): 3 seems right Triangular Prism: the crosssection in the middle, and if it is regular then each symmetrical plane of the triangle. 1+3=4 Squarebased Pyramid  x,y,x=y,x=y, so 4 Cylinder  the crosssection in the middle, plus if it is a circular cylinder wouldn't there be infinite planes thru the circle? Tetrahedron  you could run a plane through one edge and bisect the face opposite. A tetrahedron has 6 edges, so 6.
"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  Leon M. Lederman
Re: Planes Of Symmetry
Math Student is right. In the same way that squares only have 4 lines of symmetry, cubes will have a finite amount as well.
However, ryos's reasoning is valid for the cylinder, so it would have infinite planes.
I'm not entirely sure on this, but I think a tetrahedron would have 6.
Edit: Rarrrrggh.
Why did the vector cross the road? It wanted to be normal.
 darthradius
 Full Member
Re: Planes Of Symmetry
I agree, except that I believe that a cuboid may have 9 planes of symmetry... 3 that run perpendicular to the sides (in each direction) and then 6 that cut through the diagonal for each side.
The greatest challenge to any thinker is stating the problem in a way that will allow a solution. Bertrand Russell
 MathsIsFun
 Administrator
Re: Planes Of Symmetry
Oh, yes!
This is an interesting mental exercise ... I guess it depends on the cuboid ...
If the cuboid is a cube (all faces square) then any face can be cut diagonally giving 2 × 3 (x,y,z) = 6, and then there are the 3 cross sections parallel to faces, giving a total of 9 planes
At the other extreme, if no face is square, then the diagonals won't work, giving only 3 planes.
Maybe ?
"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  Leon M. Lederman
Re: Planes Of Symmetry
That's right. And in the middle extreme, with a square prism, you can get 5 planes.
But a cuboid is defined as having rectangular faces. They can also be squares, but they don't have to be. So, Math Student's original answer of 3 is correct.
Why did the vector cross the road? It wanted to be normal.
 MathsIsFun
 Administrator
Re: Planes Of Symmetry
"Middle extreme" ... good concept
"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  Leon M. Lederman
Re: Planes Of Symmetry
well i found out a formula that helps oyu determin the planes of symmerty of any 3d shape here its n+1 which means the number of sides of the 2d surface area +1
Re: Planes Of Symmetry
The question is to find a plane that "cuts" (passes through) the triangular pyramid and which has equal distance from all its apices.
 bob bundy
 Moderator
Re: Planes Of Symmetry
hi annagg
See picture below. Is that what you are after?
Don't understand your word "apices" .
Did you mean vertices ?
Bob
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Re: Planes Of Symmetry
bob bundy wrote:hi annagg
See picture below. Is that what you are after?
Don't understand your word "apices" .
Did you mean vertices ?
Bob
Yes, vertices
 bob bundy
 Moderator
Re: Planes Of Symmetry
hi anna_gg
The picture I suggested goes through two vertices and the midpoint of the side joining the other vertices.
So it is the same distance from only two of the vertices, and it is a plane of symmetry.
There is one point only that is the same distance from all four vertices, (see picture below), so you could draw any plane that all that goes through this point. It would not be a plane of symmetry.
Bob
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Re: Planes Of Symmetry
bob bundy wrote:hi anna_gg
The picture I suggested goes through two vertices and the midpoint of the side joining the other vertices.
So it is the same distance from only two of the vertices, and it is a plane of symmetry.
There is one point only that is the same distance from all four vertices, (see picture below), so you could draw any plane that all that goes through this point. It would not be a plane of symmetry.
Bob
Hi Bob, Actually we are looking for a plane that has equal (vertical, obviously) distance from ALL FOUR vertices, so I guess this plane must not pass from any of the sides (because in this case, for the 2 of them, distance will be zero).
Anna
 bob bundy
 Moderator
Re: Planes Of Symmetry
hi anna_gg
I have, of course, assumed that the triangular pyramid is regular; ie. all its faces are equilateral triangles.
If it is 'sitting' on one face with one vertex in the air, then, a line drawn straight down from that vertex to the base, will go through the centre of gravity (centroid) of the base. Let's call that line (don't know the proper term for it) an axis. Turn the pyramid so that a different vertex is on top and repeat the construction. This new axis will intersect the first. You can do this four times altogether and get four axes, all intersecting at the same point. This point is the centre of gravity of the pyramid and it is the same distance from all four vertices.
Furthermore it is the only point that is the same distance from all four vertices. But you want a vertical plane.
Ok let's try this. I'll find a line in the base (ABC) that is the same distance from all three base vertices. Then make a vertical plane going upwards from this line. See picture. The base is ABC. The line is shown dotted. It's distance is the same from all three base vertices.
This plane is, therefore, (i) vertical (ii) the same distance from all three base vertices.
Now suppose we are looking straight down from above on the pyramid. Let D be the last vertex. You can see it will be much closer to the plane.
Conclusion. I think it is not possible to create the plane you want.
Bob
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Re: Planes Of Symmetry
bob bundy wrote:hi anna_gg
I have, of course, assumed that the triangular pyramid is regular; ie. all its faces are equilateral triangles.
If it is 'sitting' on one face with one vertex in the air, then, a line drawn straight down from that vertex to the base, will go through the centre of gravity (centroid) of the base. Let's call that line (don't know the proper term for it) an axis. Turn the pyramid so that a different vertex is on top and repeat the construction. This new axis will intersect the first. You can do this four times altogether and get four axes, all intersecting at the same point. This point is the centre of gravity of the pyramid and it is the same distance from all four vertices.
Furthermore it is the only point that is the same distance from all four vertices. But you want a vertical plane.
Ok let's try this. I'll find a line in the base (ABC) that is the same distance from all three base vertices. Then make a vertical plane going upwards from this line. See picture. The base is ABC. The line is shown dotted. It's distance is the same from all three base vertices.
This plane is, therefore, (i) vertical (ii) the same distance from all three base vertices.
Now suppose we are looking straight down from above on the pyramid. Let D be the last vertex. You can see it will be much closer to the plane.
Conclusion. I think it is not possible to create the plane you want.
Bob
Just to clarify, I did not say that the plane must be vertical. I only clarified that the (vertical) distances of each vertex from this plane must be equal. Sorry for the confusion!
Last edited by anna_gg (20121112 04:13:07)
 bob bundy
 Moderator
Re: Planes Of Symmetry
Oh right. Sorry I misunderstood.
In that case a horizontal plane half way up the height will do it. (see picture)
The distance of a point from a plane has to be the perpendicular distance. In the second part of the picture, BC not BA.
So all the points on the base are the same distance from the plane and, as it is half way up, so is the top.
Will that do?
Bob
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Re: Planes Of Symmetry
Bob, I think this is it, although I do not understand your second drawing.
So I guess we have four such planes, correct?
Thanks, Katerina
 bob bundy
 Moderator
Re: Planes Of Symmetry
Four would be my thinking too.
Bob
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Re: Planes Of Symmetry
Nope! There are three more, one for each pair of midpoints of opposite edges of the tetrahedron.
The limit operator is just an excuse for doing something you know you can't. “It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman “Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
Re: Planes Of Symmetry
anonimnystefy wrote:Nope! There are three more, one for each pair of midpoints of opposite edges of the tetrahedron.
So 4 + 3 = 7 such planes? Can you make a drawing for the 3 last planes?
Thanks!!
 bob bundy
 Moderator
Re: Planes Of Symmetry
I think Stefy means this.
Thanks Stefy.
Bob
You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei
Re: Planes Of Symmetry
Yessss!! And there are only 3 such planes, because for every two sets of sides we get the same plane. Thanks Stefy!!
Re: Planes Of Symmetry
Those are the ones. You are welcome.
The limit operator is just an excuse for doing something you know you can't. “It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman “Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
