I have some questions relating to proportionality and circular motion.

1) Given a set of data points:
x-values: 1.2, 2.5, 3.4, 4.2, 5.5
y-values: 11.38889, 10.32, 10.17301, 10.113379, 10.06612
I know that for example if y=2(1/x^2), y is definitely proportional to 1/x^2, what if there is a y-intercept that is not zero? I found that the equation that relates the above data points is y=2(1/x^2) +10, the y-intercept is not zero, does that still mean y is proportional to 1/x^2 ?

2)

Has anyone done this lab before?
In this UNIFORM circular motion lab, I have to measure the time taken for 20 cycles, varying the number of washers hanging at the bottom. The radius measured to be 50cm at the beginning. The mass of the rubber stopper is constant at 11.94g. I need to plot a graph of force (force of gravity of the washers) vs speed and try to find the relationship. But I don't get how this lab is a controlled experiment. How fast the rubber stopper is moving depends on how much (how violently) you whirl the string, right? What I mean is that your hand, instead of the weight of the washers, is controlling the speed of the rubber stopper. Then, what's the point of doing all this experiment? How is it possible to find the relationship between force and speed if speed depends on how violently you whirl it?