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Re: [Phys-l] conservation versus constancy



On 10/17/2006 10:04 AM, Robert Cohen wrote:

Fnet,avg dot Delta x = Delta (1/2 mv^2)

That's the work/KE theorem.

Defining the right-hand-side as the change in the object's kinetic
energy (macro), I find that in a collision the KE "lost" by one object
is "gained" by the other object only if the total displacement of each
object during the collision is the same (since the average net force on
each object is equal and opposite; assuming no other forces contributing
to the net force).

Is this a valid way of looking at things?

Mostly valid.

I say "mostly" rather than "entirely" because is a bit hard to quantify
the idea of "during the collision" in the context of a gravitational
slingshot, since gravitation is an infinite-range interaction.

If so, during the "sling
shot" is the displacement of each object the same during the
"interaction"?

Not necessarily.

There is no law that says KE should be conserved.

If the two displacements are not equal, then some PE is converted
to/from KE.

Indeed in a gravitational/orbital situation, the virial theorem makes
it seem very likely that PE will be converted to/from KE during a
slingshot maneuver, or practically any maneuver.