Have I proved Newton wrong?

 I set out to investigate Newton's 2nd Law using this apparatus. The light gates were connected to a computer to calculate the acceleration. All I had to do was to sort out the forces. I had the trolley pulled along the ramp using weights hanging on the end of a string. The trouble is that as the trolley rolls along the ramp there is friction which acts as a counter force. The overall force on the trolley, the RESULTANT FORCE is weight - friction. I only wanted the weight: I needed to get rid of friction. I decided to cheat as shown in the picture below. I propped the ramp up with books so that the extra bit of gravitational pull cancelled out the friction. Or so I thought! (This method is called FRICTION COMPENSATION)

Here are my results:

The force is in the left hand column. I used a mass of 2kg so 2kg multiplied by the acceleration should equal the force according to Newton's 2nd Law. That clearly hasn't worked! Have I proved Newton wrong? I won't be sending a paper to PRL anytime soon... I must just have got my friction compensation wrong.