Many of these inertia balls also have a third loop on the side of the ball. I ask students what would happen when I pulled the string from this loop directly to the side. "Are you doing it quickly or slowly?" they ask and I tell them they can think of it either way but when they share out they will have include their choice in their description. Again groups are split, some think that if I pull slowly the top string will break again, others think that if I pull it quickly the side string will break. I pulled the side string slowly at first and students saw the ball shift to the side but the top string held. I briefly said that this looks like a force in the horizontal direction did not affect the vertical direction. I reminded them that we saw a similar directional independence in our projectile unit. I let the ball hang freely again then pull the bottom string quickly and it breaks.
While I gave these instructions on my whiteboard I made a powerpoint that has a visual for students as well as the questions.
For high school teachers, this connects well with the NGSS Science & Engineering Practices. Below are the excerpts I thought were most aligned to this activity:
O that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.
O to clarify and refine a model, an explanation, or an engineering problem.
O Develop, revise, and/or use a model based on evidence to illustrate and/or predict the relationships between systems or between components of a system.