| Trial: | Length (m) | t_1 (s) | t_t (s) | t_2 (s) | v_1 (m/s) | v_2 (m/s) | %diff |
| 1 | 0.012 | 0.123 | 0.32 | 0.20 | 0.098 | 0.061 | 38 |
| 2 | 1.23 | ||||||
| 3 | 0.53 |
Explain terms where needed and
especially say/demonstrate how calculations were done:
t_1 --- the time it took the cart
to pass through the first photogate
t_t --- the total time recorded
by the photogate timer for the cart to pass through the first and second
gates
t_2 --- computed
from t_2=t_t-t_1 to get just the time for the cart to pass through the
second gate
v_1 --- the average velocity of the cart
during the time it went through the first gate; v_1 = L/t_1
v_2 --- the average velocity of the cart
during the time it went through the second gate; v_2 = L/t_2
%diff --- the percent difference between the first
and second velocity computed by %diff=100%*(v_1-v_2)/v_1
Start with what the lab was about
and what you expected (only briefly):
State what results were, whether
they were good or bad. Pull important numbers from the rest of the lab
Discuss errors without making
lame excuses. Rank them if possible:
Another possible source of error is the timers. Inasmuch
as possible, I tried to test the timers by comparing to my watch.
while this certainly would not indicate small errors,
the timers seemed to work for long periods. I would imagine that
the error presented by the timers is small.
Say how an expected error would
have affected your results and whether you observed this where possible:
Overall, we are confident Newton's laws hold, and would
expect as friction is decreased and 'bow'ness is removed,
the percent difference would go to zero.
In this lab, we measured the velocity of a cart on an
air track at two different points along the track after pushing
it with our hands. By our pushing, we gave the cart an
initial horizontal velocity. Using Newton's laws and the
fact that ideally no force is acting in the horizontal
direction once the cart leaves our hand, we expect that the
velocity will remain the same.
so the reader doesn't have to
go back and hunt it down:
As a measure of how constant the velocity was, we computed
the percent difference between the initial and final
velocities. This percent difference reflects the error
of the experiment as ideally it would be zero. We ran the experiment
three times with different carts and with the gates in
different positions to help randomize the errors (see below).
For these three trials, the average percenter difference
was 25%. This value seems rather high, but I expect that
the errors described below account for it.
Ideally, the air track eliminates friction by supporting
the carts with air. However, the carts are still exposed to drag, as
well as some friction with the track because the air
flow out of the holes is not uniform. This error would manifest itself
as a slowing down of the carts, which is indeed what
we saw. If we had seen the velocity go up, then this error
would help explain nothing.
More importantly though, I believe the fact that
the air track was somewhat bowed (as evidenced when we tried
to level the track and the cart would slide to the low
spot) contributed the most error to this experiment. Indeed,
because of our observation of this possible error, in
trial #3, we purposefully placed the photogate timers symmetrically
about the bow so as to minimize its effect on the results.
While this is in some sense rigging the experiment, it seemed
the best method with such equipment. The results yielded
the lowest percent difference as would be expected.