Title : Free Fall Acceleration

Apparatus :

     Science Workshop Interface 750                                          1

        Motion Sensor ( PASCO Model CI 6742 )                              1

        Falling Object ( Basketball )                                                 1

        stand - and - clamp                                                               1

        metre rule

Objective :

1.    Measure the acceleration of free falling basket ball using a motion

       sensor

2.    Study the s - t, v - t and a - t graphs for a bouncing ball

3.    Show that acceleration is rate of change of velocity

        ( the slope of a graph of velocity against time represents acceleration )

4.    Show that the area under v - t graph represents distance

5.    Show that the slope of a graph of displacement against time

        represents velocity

Theory :

The Motion Sensor sends out pulses of ultrasound and picks up the echoes of ultrasound that bounces back from objects in front of it.

    2 X distance to object = ( round - trip time ) X ( speed of sound )

The speed of sound through air depends on several factors, including the temperature of the air. Because the temperature of air can change, the speed of sound can change. Calibration of the motion sensor is needed before taking measurement.

Velocity is the rate of change of displacement.

  The slope of a graph of displacement against time represents velocity.

Acceleration is the rate of change of velocity.

The slope of a graph of velocity against time represents acceleration. The area under such a graph represents distance.

Click to play movie

In this experiment, a basketball is released from rest. A motion sensor fixed above the basketball is used  to monitor the position of the basketball as it falls. s - t, v - t and a - t graphs are plotted automatically by the computer. By measuring the slope of an appropriate region of the v - t graph, the acceleration due to gravity can be found.

Procedure :

Hardware setup :

1.  Connect the Interface to the Computer, turn on  the interface, and turn on the computer.

2. Connect the Motion Sensor to the interface

3. Use the Motion Sensor to measure the position of the basketball

Software setup :

Sensor Calibration

Calibrate the Motion Sensor so it can use an accurate measurement of the speed of sound in air.

1.       Place the Motion Sensor so it is exactly one metre away from a flat surface that can 

        reflect the pulses from the Motion Sensor.

2.    Open the Motion Sensor's calibration window.

3.    Set the trigger rate to 50 Hz and  click the 'calibrate' button.

4.    Click 'OK' to return to the Experimental Setup window.

Data Recording :

1.    Press the Sampling Option button to open a window. 

2.    Select the Periodic Samples to 2000 Hz and the Stop Condition to 5 seconds. 

3.    Click the Record (REC) button to begin recording data.

Data sampling would stop automatically.

Data Analysis :

Determine the gravitational acceleration and study the motion of free falling of an object

Discussion :

1.  What are the precautions of this experiment ?

2.  How does the value for 'g' compared to the accepted value of

     the acceleration of a free falling object ( ) ?

3. What factors may cause the experimental value to be different