Chronology Current Month Current Thread Current Date [Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

# Re: [Phys-L] The Impulse Momentum Theorem

• From: John Denker <jsd@av8n.com>
• Date: Thu, 06 Nov 2014 09:13:47 -0700

On 11/06/2014 07:45 AM, Paul Nord wrote:

Rather than defining a new term, you just use delta p. That makes
sense.

That's how I've always done it, and always seen it done.
momentum symbol: p
impulse symbol: Δp ... no new or special symbol required

From that point of view, the relationship between impulse and
momentum is not even a "theorem" -- the only new thing here
is the definition of impulse.

We teach F = ma but we don’t seem to carry that forward to talk about
the change in velocity

I don't see impulse as a second law issue, but rather a
third law issue.

There is a pedagogical principle that says "ideas before
names" i.e. "concepts before terminology".

You can introduce the /idea/ of impulse just fine without
mentioning word; just treat it as a direct application of
the third law. Later the term "impulse" can be introduced
and attached to the idea.

==========================

My children had a 4th grade textbook with the following review question:
It takes a lot of ____ to stop an airplane.
The only reasonable choice was Force.

That's terrible. Good pilot technique calls for *not*
using a lot of force, unless needed for some exceptional
reason ... for the same reason that Driver's Ed teaches
you not to charge up to a stop light and slam on the
brakes. A panic stop causes extra stress on the structure
and (!) on the passengers.

Very few passengers have ever seen a maximum-effort
stop. They would be impressed, and not in a good way.

I assume this was a multiple-guess question. Otherwise
"distance" would be no worse than "force" ... and in
fact we know that long runways are required. Distance
and force go together, because there is energy involved.

Similarly "time" would be no worse than force. Time
and force go together, because there is momentum i.e.
impulse involved.

General rule: Pay attention to the conserved quantities!
The arriving airplane has a lot of energy and a lot of
momentum that need to be unloaded.