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: Kinematics of Throwing Balls



David Bowman, referring to a desire to make the problem
simple enough so it can be done by the intended introductory
students who may be having trouble understanding the concept
of acceleration itself, wrote:

It's just that finding an illustrative example that it both simple
enough and yet still realistic can be a very tricky proposition.

That is why idealizations are necessary. Here is the same
message but slightly corrected again.

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

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) Suppose that, after leaving the barrel, the bullet enters
a lake. How large would the acceleration be in water
if the water resistance force were 100 N?

David considered a situation in which the rifle is immersed
in water. Presented with this problem, and assuming that
v2=300 m/s, I would answer that the acceleration is still
v^2/(2*x) = 45,000 m/s^2. But that is not what he had in
mind. I agree that if v2=300 m/s in air it would be much
smaller for the same rifle in water. Nothing very
profound but perhaps useful.
Ludwik Kowalski