Newton's 3 Laws Lab: Solving Problems on Inclined Plane

[Kandycat]

I am doing a lab report for a lab I did in class. The lab is about sliding a mass (in this case a book) down an inclined plane. I collected the data and calculated the acceleration for the mass if it fall at 90 degrees. I have to do an analysis but I having some trouble answering these questions.

1. When I found the acceleration for the mass when it slides down at a 90 degree angle to be 16.9 m/s². The acceleration of gravity of is 9.8 m/s². So why isn't this the value of 9.8 m/s²? Is it because 16.9 m/s² is the weight?

2. How do I find net force on a mass on an inclined plane? I'm pretty sure I have to use Newton's 2nd Law equation: F = ma. But do I use 9.8 m/s² or 16.9 m/s² for a?

 

[Irid]

Clearly the acceleration can't be 16.9m/s^2. Could you please post your measurement data and the equations used to obtain this value? They must contain an error somewhere.