Day 86 – Forces

Science 8 – Today’s lesson has students pushing a hover disk back and forth along a desk. While doing this, students are asked to make observations and try to answer the question, “what forces are acting on the soccer disk, and what is the result of force?”

There are a few key things at play in this activity. First, I ask students to identify forces acting on the disk. Kids will have a variety of ideas from 1 to 4 different forces. The two most common forces identified are the push and gravity. We then break it down a bit by looking at a stationary disk on the desk. From this, students are convinced that there must be a force pushing the disk up (otherwise gravity would pull it down to the ground). Eventually we get the point that maybe there are 3 forces acting on the disk.

Next, I put some restrictions on when we are looking at the forces – I deliberately set it up that we are looking at the forces while the disk is moving between two markers, and no one is touching the disk at this point. Students will say there is a push on the disk. The next question then is, if there is a push, who/what is doing the pushing? Obviously the kid, they’ll say. But wait, how can this be if the kid isn’t touching the disk? This causes some serious reflection from the kids. Eventually we get to the idea that there are only two forces acting on the disk (gravity and normal force), that these forces are balanced, and that it results in constant motion. Whew.

This can be one of my favorite lessons because it forces students to really think about where forces come from, and what it means to have balanced forces. It is also a good challenge for getting the students to make useful observations. However, there is a big downside to the lesson. With a class of 30 students, it can be very hard for every student to have a voice. In particular, students that aren’t as quick as others to catch on will not truly participate.


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