24
Photogate Timer
012-06379B
®
e
Now adjust the position of the photogate as accurately as you can so that the photogate beam
strikes the center of the pendulum bob.
f
Support a meter stick just under the bob, so you can measure the position of the bob but the
meter stick does not interfere with the photogate beam (see Figure 10.2). Pull the pendulum
bob to one side, then move it slowly through the photogate, along its path of oscillation. There
should be no slack in the string. Using the meter stick, note the position of the bob when the
photogate beam is first interrupted (the LED lights up) and again when the bob is out of the
beam (the LED goes off). Record the difference between these two points as
Δ
d in Table 10.2.
g
Now set the Photogate Timer to GATE mode. Pull the bob to one side along its path of oscilla-
tion. Again, be sure there is no slack in the string. Measure the angle the string makes with the
vertical and record this starting angle as
θ
in Table 10.2.
h
Release the bob so the pendulum oscillates. Record the first times you see on the timer display.
This is the time during which the bob blocked the photogate beam as it passed through the pho-
togate. Repeat this measurement several times, starting the bob from the same height each time.
Take the average of your measured times and record this value as t in Table 10.2.
i
Change the starting height of the bob and repeat steps 4 through 5. Do this for at least five dif-
ferent starting heights.
Data and Calculations
Part 1
c
Plot a graph of T versus L, using your measured values from Table 10.1. Is the graph a straight
line? If not, try manipulating the data mathematically until you do get a straight line. For ex-
ample, try plotting T2, L2, etc. When you get a straight line graph, measure the slope of the
graph.
Slope =
Part 2
d
For each value of
θ
, calculate
Δ
h = L-L cos
θ.
e
For each value of h, calculate
Δ
U, the change in gravitational potential
energy of the pendulum as it went from the highest point in its swing to
the lowest.
Δ
U = mg
Δ
h
=
f
For each value of h, calculate Ek, the total kinetic energy of the pendulum
as it passed through the lowest point of its swing:
E
k
= 1/2 mv
2
= 1/2 m (
Δ
d/t)
2
=
Table 10.1
m
L
T
