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Kinematics



I have questions based on the first lab this year.
I took students outdoor. With chalk we marked
positions along a path (at x=0, 2, 4, 6, etc. meters
from the origine. Two students at each mark
recorded time instances at which a runner was
passing by; all stopwatches were pressed at the
same moment, that is when x was zero. Here are
the data I am inventing to ask my questions:

1 x (m) 0 2 4 6 8
2 t (s) 0 1.5 2.5 3.3 3.8
3 dx 2 2 2 2
4 dt 1.5 1.0 0.8 0.5
5 Vav 1.33 2.0 2.5 4.0
6 ACav A B C

SPACES IN THIS TABLE SHOULD BE
PRESERVED IF YOUR FONT IS COURIER.

The goal is to calculate average accelerations,
A, B and C. Also to plot Vav and ACav versus time.

Lines 1 and 2 show row data (positions and times).
Lines 3 and 4 show steps in space and in time
Line 5 shows average velocities (dx/dt).

Question #1
In plotting Vav versus time what value of t should be
associated with each Vav? For example, should it
be (1.5+2.5)/2=2 s for Vav=2.0 and 2.6 for Vav=2.5?

Question #2
What value of dt should be used to calculate average
accelerations? For example, what should dt be in
B=(2.5-2.0)/dt ? Should it be 0.9?

Note: the concept of instantenous v and a has not
been introduced. dx and dt are deltas.
Ludwik Kowalski