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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?