Re: Conceptual focus on kinematics

[Brian McInnes <bmcinnes@PNC.COM.AU>]

Fabian,
Yoyr first post with the attachment has caused some consternation as it has the
Happy 99 virus attached.


> My answer was (E) because the average speed of the upper block and
> of the
> lower block during interval 3 to 4 is the same and equal to 80
> units of
> lenght / second, the speed of the lower block is constant and of
> the upper
> block is increasing during the interval and if their average speeds
> are
> equal it means that at some time during interval 3 to 4 the blocks
> have the
> same instantaneous speed equal to 80 units of lenght / second. (It
> is easier
> to explain with the Mean Value Theorem of Calculus). I would like
> to know
> your explanations. I am not as much interested in the answer itself
> as in
> the efficient +ACo-use+ACo- of the concepts.

It seems to me that the simplest conceptual treatment of the question is to note
by inspection that (1) the lower block has a steady speed and (2) the upper
block is initially travelling faster than the lower block and finally travelling
slower.  Therefore somewhere along the way it must, however briefly, have the
same speed as the lower block.  No calculations are required.
Brian McInnes