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Re: Impulse



At 12:51 PM -0700 10/30/01, Arnulfo Castellanos Moreno wrote:
For an elementary course, you can define:

Final impulse - initial impulse = Constant Force*(final time - initial
time).

I don't understand the final and initial impulse stuff. For me the right
side of your equation is _the_ impulse (neither final nor initial).



At 11:13 PM -0500 10/30/01, Larry Cartwright wrote:
Collision-safety engineering is all about reducing impulse. You can't
do much about the forces involved, so you try to increase the time of
impact as much as you can.

This does not jibe with my understanding of the example. I always thought
collision safety was about reducing the forces. The way to do so is to
increase the collision time because there isn't much we can do to change
the impulse (which is equal to the \Delta p).



At 9:55 PM -0500 10/30/01, Chuck Britton wrote:
At 8:25 PM -0600 on 10/30/01, cliff parker wrote
Is this view widely held? I have interpreted the definitions of impulse
that I have seen to be the application of a force over a period of time.
Impulse causes a change in momentum but it is not exactly the same thing as
a change in momentum. An equal sign does not mean that both sides of an
equation are the same thing.

Newton's second law can be written (as Newton actually INTENDED it)

F = dp/dt

He NEVER wrote F = ma (even in Latin).
He anticipated Einstein's work and chose the dp/dt form which stays
correct in Relativity. ;-)

cross multiplying gives you the impulse-momentum relation that is
being discussed.

Yes, cross multiplication gives you Fdt = dp, but only the left side of
this equation (with deltas or integrated) is defined to be impulse, the
right side is just _equal_to_ the impulse without _being_ the impulse.



At 10:13 AM -0500 10/31/01, John S. Denker wrote:
Actually, I'm a bit mystified as to why the subject of "impulse" needs to
be discussed in an introductory course. Who needs it? Is there a concept
here, or is it just terminology for the sake of terminology? Why not just
talk about momentum transfer and leave it at that?

I find John's question interesting. I guess I've always taught impulse
because it is in the book. Is John right that we don't need it? Can we
forego the new terminology and just talk about momentum transfer? Even in
the dashboard example? Others: please opine.



Larry