Picket Fence Free Fall
We say an object is in free fall when the only force acting on it is the earth’s gravitational force.
No other forces can be acting; in particular, air resistance must be either absent or so small as to
be ignored. When the object in free fall is near the surface of the earth, the gravitational force on
it is nearly constant. As a result, an object in free fall accelerates downward at a constant rate.
This acceleration is usually represented with the symbol g.
Physics students measure the acceleration due to gravity using a wide variety of timing methods.
In this experiment, you will have the advantage of using a very precise timer connected to the
calculator and a Photogate. The Photogate has a beam of infrared light that travels from one side
to the other. It can detect whenever this beam is blocked. You will drop a piece of clear plastic
with evenly spaced black bars on it, called a Picket Fence. As the Picket Fence passes through
the Photogate, the LabPro or CBL 2 interface will measure the time from the leading edge of one
bar blocking the beam until the leading edge of the next bar blocks the beam. This timing
continues as all eight bars pass through the Photogate. From these measured times, the program
will calculate the velocities and accelerations for this motion and graphs will be plotted.
• Measure the acceleration of a freely falling body (g) to better than 0.5% precision using a
Picket Fence and a Photogate.
LabPro or CBL 2 interface Vernier Photogate
TI Graphing Calculator Picket Fence
DataGate program clamp or ring stand to secure Photogate
Modified from and reported with permission
of the publisher Copyright (2000),
Vernier Software & Technology
5 – 2 Physics with Calculators
1. Inspect your Picket Fence. You will be dropping it through a Photogate to measure g. The
distance, measured from one edge of a black band to the same edge of the next band, is
5.0 cm. What additional information will you need to determine the average speed of the
Picket Fence as it moves through the Photogate?
2. If an object is moving with constant acceleration