Can't seem to get the correct acceleration

[azn4lyf89]

From a position vs. time graph I calculated the slope of 2 tangent lines which is the velocity. The first slope was 390 cm/sec and the second slope was 486 cm/sec. Now to get the acceleration I thought I take difference of the velocities divided by the time interval 0.9 (1/60 sec) or 0.015sec. After calculations I got an acceleration of 6,400 cm/sec^2 which would be 64m/s^2. The only thing is that the acceleration should be close to 9.80m/s^2 because it was a free falling object and I need to find percent error. What am I doing wrong because I should be getting around 9.80m/s^2 and not such a large number like 64.

 

[Kurdt]

It might help if you could post the graph you're working from. From working out the difference of the slopes one gets an answer of 96 which would get you a reasonable answer if the time interval was 0.1s. Are you sure you're reading the graph scale correctly?