Re: Am I pushing your leg? (Was: CAUSATION IN PHYSICS)

[Derek Chirnside <DCandPC@NETACCESS.CO.NZ>]

On Sat, 14 Oct 2000, John Denker wrote:

> At 02:55 PM 10/14/00 -0500, John M. Clement wrote:
>
> > To accelerate a block you must push it.
>
> Not strictly true.
>   -- you might pull it instead of pushing it

John Wrote:
I've always enjoyed doing the following demonstration which, I
think points to the generally artificial distinction between
"pushing" and "pulling":
----------

Surely in some cases it is very clear.

Pulling a string.
Pushing a car.
Push against a door to close it.
Pulling a door handle to open a door.

While there are subtleties that can cause ambiguities, I like to start with
what seems clear and in common useage.

The problems come when we consider what "pulls" the car if it's being towed
by a chain.  technically the reactiion force between the chain and the car.

What supports the parachutist falling in a harness?  Not the tension in the
strings, a reaction force.

I believe the use of free body force diagrams helps a lot in sorting out
ambiguities and aids student understanding in picking out what is really
significant in solving a problem.
I also have a list of forces on the wall of my class which students should
choose from.  In many cases several answers apply.  [eg centripetal force
may be supplied by friction or tension or normal reaction etc]  The list has
been slightly different each year!!

At the risk of some scrutiny, the list currently includes:

push, thrust, driving force
pull
lift
friction
tension, spring force, elastic force
air resistance, drag
buoyancy
normal reaction force, support force
centipetal force

Three forces that act at a distance:
gravitational force [or weight]
electrical
magnetic

Odd One Out: centrifugal force.

[strong and weak nuclear forces]
[surface tension]

I have comments and descriptions built up with the students over a number of
weeks.
Great fun!!

As for causality issues, I would find it hard to not teach using the force
causes acceleration ideas.  I may compose a fuller post on this.  :)

Derek