Dependant
The variable that we obtain from the results and is changed by the independent; the time taken for the ball to reach the ground
Control
The variables that we have not changed; mass of the ball, gravitational field strength.
Diagram
Apparatus
- Steel ball
- Millisecond timer
- Retort stand
- Release and capture mechanism for the ball
- Metre ruler
Method
The experiment was done as follows:
- The apparatus was set up as shown in the Diagram up to (x) cm
- The steel ball was stuck on the magnetic part of the release and capture mechanism for the ball as the power was switched on
- The power button of the millisecond timer was switched on and as it stopped flashing its light, the release and capture machine was turned off and the amount of time for the ball to reach the capture box was recorded by the millisecond timer.
This was repeated 3 times for each reading.
(x) = 12cm, 24cm, 36cm, 48cm, 60cm, 72cm.
Results
All calculations and reasoning are done on the calculations section.
Three graphs where drawn where distance was plotted against time2.
Before reading on please have a look at the calculations and graphs
Conclusion
The acceleration obtained from the experiment 9.56+- 0.2, this is calculated from a graph of distance versus time2 of which the gradient is 1/2a ( half the value of acceleration ). The values obtained are u, t & s since u=0, we can use the equation s=1/2at2, a graph of s v.s. t2 would give us a linear graph of gradient 1/2a. The uncertainties are calculated from the extreme lines obtained from the error bars on the graph. The accepted value for g= 9.81 which does not lie in-between the uncertainty values. The calculated % discrepancy in 2.5%.
Evaluation
From looking at the raw results obtained, I can say that this experiment is precise and from looking at the obtained value of the acceleration I can say that this experiment was not accurate.
The % discrepancy is 2.5% which is just off the uncertainty value. The reasons for this are the following
- Air resistance
- A different gravitational force since the experiment was done in The Hague and The Hague is under sea level.
- The apparatus used resulting in systematic error
From looking at the diagram we can see that the gravitational field strength decreases when under sea level. To perform a good experiment, we would have to do this experiment at sea level where the gravity is exactly 9.81.
There may have been a systematic error resulting from the machines used to measure the time, we cannot see this from the graph, however this systematic error could affect the position of the line by a very small amount. Even though this value is small it does affect the graphs gradient.
Improvements that we can deal with in the school lab
Obviously in the school we can not perform an experiment that is at sea level, so the position of the experiment can not really be improved however there are other improvements:
- The experiment could have been done in a vacuum to prevent air resistance ( drag ) from effecting the balls motion.
- The experiment could have been repeated with various other machines to prevent systematic error from occurring.
