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# Re: [Phys-L] Figuring Physics solution Jan 2018

Here is a significant difference between a controllable force
versus a permanent one:
control requires some increment of energy:
a permanent force needs no energy expenditure.

Who cares about unstoppable forces? Not me.
In the usual formulation of "an irresistible force
on an immovable object" the proposition is certainly
a contest of impossible concepts.
Calling the irresistible force an unstoppable force allows
the weasel-wording which I employed to square this
particular circle.

Brian W

On 1/23/2018 9:50 PM, Todd Pedlar wrote:
I don't see any qualitative difference between a solenoid and a
screw-jack. They each can be "turned on" and "turned off" - it just
requires a different action on the part of the operator.

But still... what, again, is the point of a force being "stoppable" or
"controllable" or "permanent" or "not permanent". Who cares? A force is a
force, and its effect on a system is the same regardless of which adjective
you wish to use to describe its "stoppability". It's a meaningless
distinction.

On Tue, Jan 23, 2018 at 4:45 PM, brian whatcott <betwys1@sbcglobal.net>
wrote:

If I apply a force to an object with an electric solenoid actuator,
switching it off removes the force that it applies, does it not? I stop
that force with a switch.
If I apply a force with a screw-jack, then that force is not controllable
or 'stoppable' but may instead be permanent in this context.

Brian W