Education and Manpower Bureau
Using Datalogger in the Teaching of Physics

Acceleration due to Gravity

Experiment Profile:

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

 To analyse the motion of a freely falling object. To measure the acceleration due to gravity. To illustrate the operation of a photogate and its use in kinematics. To make use of datalogger hardware and software when conducting experiments.

3. Equipment List

 Datalogger interface connected to a PC Photogate 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)

 Experiment Collision and Conservation of Momentum Simple Harmonic Motion Acceleration due to Gravity Pushing and Pulling a Dynamics Cart Acceleration of a Dynamic Cart

 Procedure.

 1 Connect the datalogger interface to a PC with the software installed. 2 Connect the photogate to the appropriate channel of the interface. 3 Screw the aluminum rod into the hole at the end of the desk. 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. 5 Place a box with shredded paper directly under the photogate to absorb the impact of the picket fence when it lands. 6 Start the software to collect data. 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. 8 After the picket fence drops, stop the collection of data.

Analysis:

 1 Plot a distance versus time and a velocity versus time graph using the data collected. 2 From the curve, what is the value of g? 3 Plot a linear function to the velocity versus time data and determine the acceleration due to gravity from it. 4 Describe the assumptions being made in using the photogate to measure instantaneous velocity.

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.

 What would the world look like? How would our daily activities change? How would sports change? Would anything stay the same? Which would you prefer - life with gravity or life without gravity?

 Acknowledgements We are particularly grateful to SKH Kei Hau Secondary School for their help in the production of this video.

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