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  • 1. DETERMINING THE ACCELERATION OF FREE FALL (g) USING THE LIGHT SENSOR By Dana Lin AS3 October 2009
  • 2. DEFINITION OF KEY TERMS
    • Acceleration : change in velocity over time
    • Average Acceleration :
    • Acceleration of free fall (g) : The acceleration experienced by an object falling freely in a gravitational field, a.k.a. the acceleration due to gravity .
    Dana Lin, AS3 October 2009
  • 3. MEASURING g 1. The equipments required:
    • 1 Steel ball
    • 2 Light sensors
    • 1 Timer
    • 1 Measuring tape
    Dana Lin, AS3 October 2009
  • 4. M EASURING g (cont’d) 2. The set-up of apparatus Dana Lin, AS3 October 2009
  • 5. M EASURING g (cont’d) 3. Procedures
    • The ball is dropped by cutting the power to the electromagnet.
    • Electronic timer is automatically switched on when the ball passes through the upper light beam.
    • Electronic timer is switched off again when the ball passes through the lower beam.
    • If the height of fall is h and the time taken is t , g can be calculated using this equation:
    • g = (2 h ) / t 2
    Dana Lin, AS3 October 2009
  • 6. E XPLANATIONS
    • Principle of the experiment : to measure the time taken for a stell bar to drop through a known height, and to calculate the acceleration from this.
    • Calculations : Since s = ut + (1/2) at 2 , and initial velocity u is 0, the formula can be rewritten as s = (1/2) at 2 . Also, displacement ( s ) in this case is the height ( h ) and acceleration ( a ) in this case is g , the formula can be rearranged into g = (2 h ) / t 2 .
    Dana Lin, AS3 October 2009
  • 7. ACCURACY & PRECISION
    • Accuracy : To determine the accuracy in the result, we can compare our answer with the theoretical value of g , which is 9.8 m / s 2 . The closer we are to this theoretical value, the more accurate our result is.
    • Precision : Because the starting and finishing of the timing is done through the sensor, it makes sense to record the time in milliseconds. Also, the precision of the measurement can be determined when comparing different data; the more similar the data are with one another, the more precise the measurements.
    Dana Lin, AS3 October 2009
  • 8. RELIABILITY & SOURCES OF ERROR
    • SYSTEMATIC ERRORS:
    Dana Lin, AS3 October 2009 Systematic errors arise from the equipments used. As long as we ensure that the timer starts at 0 and there is no reaction time between the light sensor and the digital stopwatch, the systematic errors are eliminated. One thing to keep in mind is the calibration of the measuring tape. Just like any ruler, it can be subject to calibration errors.
  • 9. RELIABILITY & SOURCES OF ERROR (cont’d )
    • RANDOM ERRORS:
    • 1. The existance of air resistance
    For most falling objects in air, the air resistance will affect is acceleration. Air resistance is considered a random error because wind-speed varies so it is not constant. But because the ball used in this experiment is heavy and will only fall for a short distance, air resistance has little effect on it. Therefore, the ball’s acceleration is effectively g . Dana Lin, AS3 October 2009 2. Uncertainties in the rule r Assuming that the smallest division on the measuring tape is 0.1cm, then the uncertainty would be + 0.1 cm ( + 0.05cm x 2). This should be taken into account when we calculate g using the equation given earlier on.
  • 10. BIBLIOGRAPHY & REFERENCES
    • Pople, Stephen. Complete Physics . London: Oxford University Press, 1999. Print.
    • “ Measurement of the free-fall acceleration” n.d.
    • < http://e-prolab.com/en/human-env/3motion/!free-fall.html > (retrieved on 01/10/09)
    • “ Free fall” n.d.
    • < http :// courses.wcupa.edu/mwaite/phy170/labs/02)%20 Free %20 Fall .pdf > (retrieved on 01/10/09)
    • “ Light sensor” n.d.
    • < http :// www.vernier.com/probes/ls-bta.html > (retrieved on 01/10/09)
    Dana Lin, AS3 October 2009