Day 36: Spring Lab

College-prep students worked on a spring lab. After some discussion, we decided to investigate the relationship between how hard you pull on the spring (force) and how much the spring extends (stretch). One thing I did differently this year was to give each group one spring that was close to ideal (no pre-load required before stretching) and one spring with a pre-load. In years past, I had gone all pre-load or all no pre-load, with mixed results. I think having the contrasting cases will make the follow-up discussion richer. All the groups have identical silver springs (no pre-load) and a different colored spring (pre-load) from the Pasco spring set. I think that too will make for rich discussion since all groups will have similar slopes for the silver spring and different slopes for the colored spring. It will also be easier to compare equations for the silver springs if some groups plot Force vs. Stretch and others plot Stretch vs. Force.

Students know nothing of Hooke’s Law right now. Our post-lab discussion of their experimental graphs (what’s the same? different? what’s the meaning of the slopes? the y-intercept? can we use the graph and equation to make a prediction?) will bring all that out. No need to lecture or watch a video about springs and Hooke’s Law.

Note: Judging by the values in their table, it looks like this group is actually measuring the length of the spring, not the change in the length. It’s a common mistake, even though as a class we agreed that, for this experiment, the change in length was more important than the length because springs can be different lengths.