Number of Washers . . . Total Mass . . . Total Force . . . Ratio
1 ................................................0.6 kg ................5.9 N ........... 9.83 N/kg
2 ................................................1.2 kg ...............11.8 N ............ 9.83 N/kg
3 ................................................1.8 kg ..............17.6 N ............. 9.78 N/kg
4 ...............................................2.5 kg .............24.5 N ............. 9.80 N/kg
5 ...............................................3.2 kg ..............31.4 N ............. 9.81 N/kg
The ratios are listed in the table.
The ratio seems to be constant, and not depend on the number of washers or the total mass. It's always about 9.8 Newtons per kilogram of mass. There must be something special about that number.