Last year (i.e. pre-NGSS) this would have been easy. I would have taught about the connection between mass and kinetic energy. Then I would have given the students a step-by-step procedure to follow to confirm what I had taught. Just in case student data didn't confirm what I'd taught, I would finally explain to students what they should have observed in the lab.
That instructional sequence doesn't fit the vision of NGSS. So I struggled to develop a more meaningful way for students to investigate. What I finally landed on was a time for students to explore with the materials and then a time to develop an experimental procedure as a class. (In an ideal world, students would have developed the procedure on their own, but in the first year of NGSS implementation, students need more scaffolding.)
Since I teach classes on a 46 minute schedule, designing the procedure was the extent of our work today. Tomorrow, the students will be ready to carry out the procedure and analyze the results. If the data doesn't confirm what science already says, we'll be ready to look for possible issues in our procedure.
This method is messier; it takes more time; and it requires my releasing control a little more. In return, I get students who are owning the science. They aren't just mindlessly following a procedure; they are developing the procedure and then analyzing the results.