We used the first ~15 minutes of class to discuss and decide (based on the student calculations) if kinetic energy is conserved in each type of interaction. We easily saw that it WAS NOT conserved in explosions. In one class that had poor data to start with, it was difficult to see if the kinetic energy was conserved in the elastic collisions and NOT conserved in the INelastic ones. In the other two classes, this was much easier to see from the data.
After this discussion we looked at a classic example of conservation of momentum… Newton’s Cradle. I really like using this for a few reasons. First, it helps the student conceptualize the difference between inelastic and elastic collisions. It forces them to explain why only one ball swings away when one ball is started, rather than one ball swinging twice as fast… this does not conserve the kinetic energy. Finally, it sets the stage for this:
Our ‘Giant Newton’s Cradle of Physics’. We also looked (qualitatively) at Conservation of Angular Momentum with another classic demonstration:
We tried something different this year to help the students conceptualize spring constants. We created a short practice sheet that involved a few questions and ranking tasks related to spring constants and systems of springs. We used a “Think, Pair, Share” approach. Here is the initial information:
The questions we asked were:
1. If the same mass is hung on each one, rank the springs in terms of the change in length.
2. If 200g are hung on Spring C, how much mass needs to be hung on the others to create the same change in length?
3. If two identical Spring A’s are connected end-to-end (in series) how does the overall change in length for this system compare to when the same mass is added to one Spring A? Does the spring constant for the system change? If so, how?
4. If two identical Spring A’s are hung side by side(in parallel) and connected with a rod on the free end how does the overall change in length for this system compare to when the same mass is added to one Spring A? Does the spring constant for the system change? If so, how?
We used the colors spring sets from Pasco to verify all four answers. The remaining questions on the practice sheet were ones that required the student to use Hooke’s Law to solve a few simple problems.