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Acceleration
due to Gravity
Experiment
Profile:
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1.
Background
To
introduce the concept of "Acceleration due to Gravity",
drop two objects. Ask the students which object will reach
the ground first (i.e. which will fall faster?). In reality,
both objects will reach the ground at the same time since
all objects accelerate at the same rate. The value of
this acceleration is known as "g". (Note: We must ignore
air resistance here. For example, if a feather and a brick
are dropped at the same time, then the brick will reach
the ground first since it is less susceptible to air resistance.)
2.
Objective
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To
analyse the motion of a freely falling object. |
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To measure the acceleration due to gravity. |
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To
illustrate the operation of a photogate and its use
in kinematics. |
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To
make use of datalogger hardware and software when conducting
experiments. |
3.
Equipment List
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Datalogger
interface connected to a PC |
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Photogate |
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Picket fence (students can make the picket fence by
pasting several 5cm colour paper on a 30cm
transparent plastic each 5cm apart with the same width
as the plastic) |
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| 1.
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Connect
the datalogger interface to a PC with the software
installed.
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| 2.
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Connect
the photogate to the appropriate channel of the interface.
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| 3. |
Screw
the aluminum rod into the hole at the end of the desk.
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| 4. |
Clamp
the photogate to the aluminum rod, as shown in
Figure 1, approximately 60cm from the surface of
the table.
The photogate that will be used in this experiment
consists of an infrared light-emitting diode and a
photodetector mounted on a bracket as shown in the
photograph to the right. The voltage across the photodetector
depends on the intensity of the light incident upon
it.
By monitoring the photodetector voltage, the interface
can sense whether the light path from the LED to the
detector is blocked or not. The datalogger software
can record the times at which the detector becomes
blocked and unblocked.
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| 5. |
Place a box with shredded paper directly under the photogate
to absorb the impact of the picket fence when it lands.
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Start the software to collect data.
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| 7. |
Hold
the picket fence so it is vertical with the bottom edge
of the first stripe approximately 5cm above the photogate
and drop the picket fence through the photgate.
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| 8. |
After
the picket fence drops, stop the collection of
data. |
Analysis:
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Plot
a distance versus time and a velocity versus time graph using
the data collected.
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From the curve, what is the value of g?
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Plot a linear function to the velocity versus time data
and determine the acceleration due to gravity from it.
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Describe the assumptions being made in using the photogate
to measure instantaneous velocity.
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Assessments:
Have the students turn in their data tables, graphs and
calculations for "g" to show that they understand the formulas.
Make sure they include the reasons for their errors. Some
examples for errors are measuring the length, operating
the timer, doing the calculations, counting and graphing.
Extensive:
To bring the concept of gravity into the students' own lives,
ask them to write a story about what life would be like without
gravity.
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What
would the world look like? |
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How
would our daily activities change? |
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How
would sports change? |
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Would
anything stay the same? |
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Which
would you prefer - life with gravity or life without gravity?
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Video
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