We found out how the acceleration of our hook-string-cart-mass system changed as the external force was changed. Since we talked about systems earlier, it wasn't as hard as usual to use a modified Atwood machine for this lab. The idea of moving mass from the cart to the hook to change the net external force without changing the mass of the system seemed easier to explain this year.
Since we already had a reason to believe that m/s² and N/kg were equivalent units, when the slope reduced to 1/kg, we had to mass one of the systems. Pro tip: I kept a list of the reciprocals of the masses of the systems in my front pocket during the lab. I knew which group I wanted to bring their system up to the front to be massed.