| Topic: |
Science > Physics |
| User: |
"" |
| Date: |
04 Jul 2007 04:27:32 PM |
| Object: |
Angular inertia and 5 year olds: a sleepless mix |
It always seems like kids ask the hardest questions :)
Yesterday I was baby sitting a relative's son. He's at that stage
where every answer garners another question. Maybe I should just stop
baby sitting :/
Anyway: We were driving down the street past some multi-story condos
when he asked me "Unca, which is stronger - a big building or a little
building".
Seeing that he likes to play with legos / blocks, I assumed he really
meant "which is harder to push over" (then again, who the hell knows
what goes through the mind of a 5 year old :)
Regardless: I reflexively said "a little building", because common
sense would indicate that a structure closer to the ground is harder
to push over than a tall one.
However, thinking back on it, I'm not so sure my answer was correct.
Perhaps someone here can help me clarify my (rusty) physics?
Assume two buildings of the same mass (1 unit). Building A, a single
story home, has a height of 3m. Building B, a 3 story home, has a
height of 15m.
I supply sufficient force at the topmost right hand side of the
building to cause it to tip over(like a child would with a block).
Doesn't the taller building have a higher angular inertia, meaning
it's harder to move?
IOW:
A= 1kg x 3m = 9kgm^2s
B= 1kg x 15 = 225kgm^2s
where Angular inertia is mass x radius of rotation squared (I=mk^2)
So in fact, a taller building is "stronger", as my little friend meant
it? Or am I mis-remembering my equations? Is there something else at
play here?
.
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| User: "Androcles" |
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| Title: Re: Angular inertia and 5 year olds: a sleepless mix |
04 Jul 2007 05:26:14 PM |
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<Antipodean.Lower.Mammal@gmail.com> wrote in message
news:1183421755.030923.310540@d30g2000prg.googlegroups.com...
: It always seems like kids ask the hardest questions :)
:
: Yesterday I was baby sitting a relative's son. He's at that stage
: where every answer garners another question. Maybe I should just stop
: baby sitting :/
:
: Anyway: We were driving down the street past some multi-story condos
: when he asked me "Unca, which is stronger - a big building or a little
: building".
:
: Seeing that he likes to play with legos / blocks, I assumed he really
: meant "which is harder to push over" (then again, who the hell knows
: what goes through the mind of a 5 year old :)
:
: Regardless: I reflexively said "a little building", because common
: sense would indicate that a structure closer to the ground is harder
: to push over than a tall one.
:
: However, thinking back on it, I'm not so sure my answer was correct.
: Perhaps someone here can help me clarify my (rusty) physics?
:
: Assume two buildings of the same mass (1 unit). Building A, a single
: story home, has a height of 3m. Building B, a 3 story home, has a
: height of 15m.
:
: I supply sufficient force at the topmost right hand side of the
: building to cause it to tip over(like a child would with a block).
:
: Doesn't the taller building have a higher angular inertia, meaning
: it's harder to move?
:
: IOW:
: A= 1kg x 3m = 9kgm^2s
: B= 1kg x 15 = 225kgm^2s
:
: where Angular inertia is mass x radius of rotation squared (I=mk^2)
:
: So in fact, a taller building is "stronger", as my little friend meant
: it? Or am I mis-remembering my equations? Is there something else at
: play here?
Begin with a cube. To topple the cube you must RAISE it's centre
until it is vertically above the lower edge, at which point it is unstable
and will fall either back onto the previous base or onto the previous side.
Thus you must turn it through 45 degrees.
Placing a straw on its end, a puff of air will knock it over.
The pyramid or cone requires an even greater tilt than the cube for
the same height.
Since your "buildings" are not of the same shape the comparison is biased.
Maybe you should first ascertain what your pal means by "stronger"
and "bigger" as he is quite likely to bloody your nose in his attempt
to prove he is stronger than you even though you are bigger than he;
and you can't hit back, so he is.
BTW, the words you are looking for are "moment of inertia".
Dictionary:
a measure of the resistance of a body to angular acceleration about a given
axis that is equal to the sum of the products of each element of mass in the
body and the square of the element's distance from the axis
.
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| User: "Sam Wormley" |
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| Title: Re: Angular inertia and 5 year olds: a sleepless mix |
04 Jul 2007 04:50:30 PM |
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wrote:
It always seems like kids ask the hardest questions :)
Do you remember posting this two days ago?
See: http://groups.google.com/group/sci.physics/browse_thread/thread/6b04e7d86436f408/3ab0026a789102a2#3ab0026a789102a2
.
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