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Re: [Phys-l] momentum dissipation?



Let me first comment on the statement

Momentum should be covered in it's entirety before acceleration comes
up (except for a fancy word that means 'Slope of a Velocity Graph').

The way I see it, students should be able to work out any dynamics problem using Newton's 2nd, Newton's 3rd and the definitions of velocity and acceleration before they are ever even exposed to momentum and mechanical energy.

Momentum and mechanical energy are convenient ways to integrate changes in position under conditions of varying forces (and sometimes masses) whose profiles may not be entirely known, but they are just that - a tool, rather than the fundamental factors that are involved in the cause-effect relationships that define dynamics. (Forces, masses, delta-positions and delta-times.)

An object does not change its motion because it knows it has to conserve momentum. It changes its motion because it is being pushed on. That is what students have to realize. Ask the simple question:

When two pool balls collide and bounce, why does one go in one direction, and the other go in another direction?

If the answer is, "To conserve momentum" then they don't get it. If the answer is "Equal forces were applied to each, in the opposite directions" then they understand a little better.

Regarding the airplane moving through the air, if it is moving at a constant velocity in a straight line, then its momentum isn't changing (ignoring fuel being burned and exhausted). So whatever it does to one piece of the air, there has to be an opposite motion to another piece of air.

Ask your students to brainstorm this:

In order to stay in the air, the wings and the air must exert equal down/up forces on one another. It usually appears that the air is being pushed down. So if there is zero net momentum change on the airplane, and the air is being blown downward, what is moving upward?

Scott

On 4/26/2010 12:00 PM, phys-l-request@carnot.physics.buffalo.edu wrote:
Date: Sun, 25 Apr 2010 19:42:15 -0400
From: chuck britton<cvbritton@mac.com>
Subject: Re: [Phys-l] momentum dissipation?
To: Bob Sciamanda<treborsci@verizon.net>, Forum for Physics Educators
<phys-l@carnot.physics.buffalo.edu>
Message-ID:<a06240807c7fa80a0c533@[198.18.140.193]>
Content-Type: text/plain; charset="us-ascii" ; format="flowed"

PLEASE don't ! ! ! !
Momentum should be SO much easier to understand than this.
My first real understanding of how momentum could be totally
misconceived was in freshman physics class.
The problem was a classic - a railcar that has a load of coal dumped
into it as it rolls down the track.
How much does it slow down? No big deal.
Then the cat coasts across a raised trestle and DUMPS it's load.
Does it speedup?!?!?!?!
No, the prof tells us - because the coal is still moving forward with
its own momentum.
"oh" a student proclaims "I get it, the car doesn't speed up until
the coal hits the ground!!!".
And somehow the US nuclear navy survived this NROTC kid's enlistment.

Pick your system.
Extend it enough to keep momentum constant or use N2 in it's delta p
form to account for any gain/loss.

Momentum should be covered in it's entirety before acceleration comes
up (except for a fancy word that means 'Slope of a Velocity Graph').

At 6:20 PM -0400 4/25/10, Bob Sciamanda wrote:
>Consider the case of a motorboat accelerating by exchanging momentum with
>the fluid water. It would seem that in such cases it is legitimate to speak
>of the dissipation of momentum by the water. Similar cases occur with
>aircraft/atmosphere interactions.