Day 73: Cart and Spring Scale Lab
"What is the relationship between the pulling force and the resulting acceleration of the cart?"
This is the paradigm lab for the Unbalanced Force Particle Model (aka Newton's Second Law). I like this version better than the modified Atwood's machine. Sure, the data may be a bit messier, but I think kids need a kinesthetic feel that constant unbalanced force produces acceleration not constant velocity.
Again, Kelly O'Shea has an awesome and expansive post about this: http://kellyoshea.wordpress.com/2011/11/16/building-the-unbalanced-force-particle-model/