Re: Favorite Test Questions

[herbgottlieb@juno.com (Herbert H. Gottlieb)]

 Ken Fox <kfox@stega.smoky.org> writes:

> I don't know if you would count this but:
> A test of projectile motion is easily a plug and chug, 
> but I love to make one sub question be
" What is the acceleration of the ball 2/3 of the way
> through the flight." 

This question seems to be IMPOSSIBLE  to answer unless
you make many assumptions.   For example:

a) Is the ball travelling through air, water or what....
Let us assume air.

b) Is the ball lighter or heavier than air?
Let us assume that it is heavier,

c) Is the air of uniform density? 
Let us assume that the air density is uniform

d) Are there any gusts of wind?
Let us assume that the air is still.

e) If the ball has stitching like a baseball or a rough
 shell like a golf ball, is the ball spinning?
Let us assume a smooth shell or no spinning.

f) Is the acceleration of gravity constant at different
altitudes as the ball rises or falls?
Let us assume that G is a constant.

Now it is easy to answer the question....OR IS IT?

No it is still not easy to answer the question because..

g) The initial speed of the ball was not given. 
At a fast speed the air resistance is much higher than
it would be at a slow speed. As the ball rises and falls,
its speed is constantly changing and the air resistance is 
also changing.

h) The size and weight of the ball was not given.
The air resistance depends on the volume (Stokes' Law).
If the ball is light enough it reaches a terminal velocity
as it falls and there is no acceleration....NOT EVEN "G".

Herb Gottlieb From New York City
(Where none of our students has ever seen the trajectory of 
a ball moving through a vacuum)


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