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Re: Kinematics of Throwing Balls



I agree with Bob (see below). Would it be more
appropriate, in the second week of a non-calculus
introductory physics course, to simply ask:

a) Assume the acceleration inside the barrel remains
constant; how large it is?

We know that the net force acting on the bullet is
not constant but it is too early to talk about this.
Ludwik Kowalski

Bob Sciamanda wrote:

This may be viewed as a nit-pick, but I think that in such problems, the
word "average" should be further specified as either "time average" or
"space average". My experience is that the thinking student will be
confused until this distinction is made explicit.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Ludwik Kowalski" <kowalskiL@MAIL.MONTCLAIR.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, September 11, 2002 3:33 PM
Subject: Re: Kinematics of Throwing Balls

| Here is a problem to help them memorize the situation.
| A rifle is fired vertically down from a tall building. The
| bullet, initially at rest, leaves the barrel with the velocity
| of 300 m/s. The distance traveled inside the barrel is one
| meter.
|
| a) How large is the average acceleration inside the barrel?
| b) How large is the acceleration outside the barrel (ignore
| air resistance).
|
| They must know that "free fall" is a situation in which
| weight is the only acting force.
|
| Later (when Fnet=m*a is known) you can add the third
| question. Giving the bullet's mass (say 10 grams) ask:
|
| c) How large would the acceleration be in water if the
| water resistance force were 100 N?
|
| Ludwik Kowalski

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