Acceleration due to gravity

    By measuring the time of free fall of a body we can calculate 'g' from the equation S = g * t * t / 2 , where S is the distance and t is the time of flight. Time is done in the following manner.
        An electromagnet is powered to hold a small iron ball. Software then releases the ball and marks the time of start. The ball drops on a flat sufface where a loudspeaker is kept face down. The speaker output is amplified and given to the LSB of digital input. When the ball hits the surface the vibrations make the digital input high momentarily. This time interval between releasing the ball and it hitting the plate below is measured by the program using PCs internal timer clock. The values obtained are given below.

Time	Distance	Accn. due to gravity
0.36457	65	978
0.35058	60	976
0.33622	55	973
0.32036	50	974
0.30296	45	981

The same experiment can be done using different types of sensors. Optical sensors made of LEDs and phototransistors can sense the falling object at a particular position.