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Re: pushing vs. pulling



At 03:44 PM 10/14/00 -0700, John Mallinckrodt wrote:

So what distinguishes a push from a pull?
...
Is it where the "agent" is located?

No.

Or is it that "pushes cause compression" and "pulls cause tension"?

That's what I would have said.

Fortunately, it is seldom if ever a useful distinction anyway.

I disagree.

1) It turns out that certain things are very strong in compression but not
tension, while other things are just the opposite. Examples include:

compression-only tension-only
================ ============

a pile of blocks a string

unreinforced concrete reinforcing bars

That last pair of items can be combined to make reinforced concrete. Some
people think this is important. The amount of money US industry spends on
reinforced concrete greatly exceeds the budget of the National Science
Foundation.

2) The following puzzle is based on my recollection from something in
Leighton & Vogt. This is a side view of a latticework bridge. Which
elements of the bridge must be rigid, and which could be replaced by
non-rigid cables? This is a D=2 problem; forces perpendicular to the page
need not be considered. The weight of the load is large compared to the
weight of the structure.

. __________________
. /\ /\ /\ /\
. / \ / \ / \ / \
. /____\/____\/____\/____\
. v | o
. pivot | wheel
. |
. load

For me, this puzzle is emblematic of the Caltech approach to freshman
physics. There are two ways to solve this problem. The clever way is its
own reward. The other way is its own punishment.