012-06379B
Photogate Timer
13
®
EQUIPMENT NEEDED:
-Photogate timer with Accessory Photogate
-Air Track System with one glider.
Introduction
In this experiment, you will use Newton’s Second Law (F = ma) to measure
the force exerted on an object by the Earth’s gravitational field. Ideally, you
would simply measure the acceleration of a freely falling object, measure its
mass, and compute the force. However, the acceleration of a freely falling
object is difficult to measure accurately. Accuracy can be greatly increased
by measuring the much smaller acceleration of an object as it slides down an
inclined plane. Figure 5.1 shows a diagram of the experiment. The gravita-
tional force
F
g
can be resolved into two components, one acting perpendicu-
lar and one acting parallel to the motion of the glider. Only the component
acting along the direction of motion can accelerate the glider. The other
component is balanced by the force from the air cushion of the track acting in
the opposite direction. From the diagram,
F = F
g
sin
θθθθθ
, where
F
g
is the total
gravitational force and
F
is the component that accelerates the glider. By
measuring the acceleration of the glider,
F
can be determined and
F
g
can be
calculated.
Procedure
c
Set up the air track as shown in Figure 5.2.
Remove the block and level the air track very
carefully.
d
Measure
d
, the distance between the air track
support legs. Record this distance in the
space on the following page.
e
Place a block of thickness
h
under the support
leg of the track. Measure and record
h
on the
following page. (For best results, measure
h
with calipers.)
f
Measure and record
D
, the distance the glider moves on the air track from where it triggers the first
photogate, to where it triggers the second photogate. (Move the glider and watch the LED on top of
the photogate. When the LED lights up, the photogate has been triggered.)
g
Measure and record
L
, the effective length of the glider. (Move the glider slowly through a photo-
gate and measure the distance it travels from where the LED first lights up to where it just goes off.)
h
Measure and record
m
, the mass of the glider.
i
Set the Photogate Timer to GATE mode and press the RESET button.
j
Hold the glider steady near the top of the air track, then release it so it glides freely through the
photogates. Record
t
1
, the time during which the glider blocks the first photogate, and
t
2
, the time
during which it blocks the second photogate. Use the memory function to determine each time.
k
Repeat the measurement several times and record your data in Table 5.1. You needn’t release the
glider from the same point on the air track for each trial, but it must be gliding freely and smoothly
(minimum wobble) as it passes through the photogates.
Glider
Figure 5.1: Forces Acting
on the Glider
L
d
h{=
Figure 5.2: Equipment Setup
Experiment 5: The Force of Gravity
D
F
g
ϑ
Component of F
g
perpendicular to air track
Force of air cushion
pushing glider away
from air track
