Experiment overview

Measuring the free fall acceleration.

Artem Khabibullin

November 13, 2012

Abstract: suggested the equipment for measuring the free acceleration in General Physics I course based on the intensity of the light. In the LabView framework was developed the software (program code). The percentage error obtained during experiment showed the good comparison with actual value of the free fall acceleration.

Experiment overview

Equipment: LabView Software 2010 (32 bit), DAQ device (6009), four regular wires, screwdriver made by NI, two metal stands (no less 1 meter each), ruler, two photodiodes, two laser pointers, two holders (for laser pointers), balls (tested objects).

The general idea of the present experiment was based on applying the classical law

(1),

assume the object is in the rest then we can rewrite above relation in the form

(2).

It is a key point to realize the given experiment.

Let’s notice important steps in making the equipment for experiment: the distance () between two photodiodes should be greater than 0.5 meter it will allow to avoid effect on the percentage error. The laser pointers are placed along horizontals right opposite the photodiodes on the metal stands using the holders. Wire the photodiodes to the “in-pin” (AIO and AI1) and “ground” (AI4 and AI5) respectively of DAQ device. Make sure that positive and ground pins of the photodiodes don’t contact with a metal stand.

Software: in the code was realized the idea based on the formula (2). For that reason we should place the falling object (the ball in our case) right above the first photodiode to reproduce the rest of the object (). We assumed from experimental experience that 3 seconds it’s enough time for the fall from 1 meter height. For this reason the program was realized in the simplest from without any time loops. The execution time of the whole code takes about 3 seconds. It can be reached by choosing the proper "samples to read" and "rate" in the "properties" of DAQ Assistant. The object should be launched from the rest position when program is run (for 1 second). The first photodiode plays the role of a detector to measure the initial time () of the free fall. The time () of free fall will be calculated when the object reaches the second photodiode (). It’s useful to notice that in code is used the time corresponding to the data in array of the voltage, other words the period between each sample (each sample has unique index in the array data of the voltage; minimum of the voltage corresponds to the required time) is reading by DAQ device. The “Select signals” SUBVI’es are used to make distinguish in the signals coming from DAQ device.

Diagram:

Front Panel:

Trials	measured
	10.106
	9.5929
	11.5984
	9.24501
	10.11
	11.104
	10.781
	11.092
	10.072
	9.456
	10.056
	9.788
	9.956
	11.056
	9.456
	10.045
average	10.0592

Data and analysis: the following values were obtained during experiment. The percentage error for 9.81 – given theoretical value is 2.54%. This value is reasonable because we assumed and no any air resistance, although this experiment was handmade and other human factors could affect on the experiment accuracy (proper launching of the ball, vertical position of the stand and etc)..

Conclusion: The value obtained for percentage error shown that we can use suggested equipment and software for measuring the free fall acceleration in the General Physics labs.

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Дата добавления: 2015-11-14