How can you compute the tension in the string which holds the falling weight? If you have your notes from University Physics I, or a textbook, look in the section about "Atwood's machine".
If not, consider the figure to the right. The weight is accelerating downwards. Can you fill in the table below? Don't forget to include all the appropriate signs.
force
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gravity
tension
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total
You measured the acceleration a, which should be negative (right?), and you know the mass of the weight m. The acceleration due to gravity is just g = 980 cm/s^2. So you must be able to compute the tension T ....