Sources of Error In Physics Lab?

We did a lab in which we attracted a mass to a string that went over a pulley and then attached to a train (a small plastic thing) on an air track (frictionless surface). We then used photo gates to find initial and final velocities to find experimental acceleration. My numbers have between .5 and 1.5% errors from... show more We did a lab in which we attracted a mass to a string that went over a pulley and then attached to a train (a small plastic thing) on an air track (frictionless surface). We then used photo gates to find initial and final velocities to find experimental acceleration. My numbers have between .5 and 1.5% errors from the theoretical, but I still have to put down possible sources of error. Anything but equipment not measuring correctly, air resistance, and human error are excepted. I need two. Any ideas? Please help fast!
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