Day 50: Force Diagrams and Balanced Forces


Physics 11 – I tried this problem with two blocks of physics 11.  The first block I had the students work individually and the second block they were put into random groups and did the problem on whiteboards.  As I’ve seen in other situations, the whiteboard groups had much more success. I don’t think it was a case of the strong student doing all the work – there was definite dialogue and sharing of ideas. However, the class ended soon after this question and I didn’t get the chance to do much formative assessment with individual students.

I love this question though, as it hits many topics covered in the past few days:

  • weight is the force of gravity
  • scales measure normal force
  • springs can support a surface, which exerts a normal force (equality of forces, reasoning)
  • you should draw a FBD when solving problems

Day 48: Balanced Forces


Physics 11 – Here are two props that tend to get used a lot while investigating forces.  The bowling ball and the hover craft / frictionless soccer disk.  They both play an important role of demonstrating an moving object with balanced forces on it.

The discussion starts with observations and descriptions of the ball as it rolls across a level table.  How would  you describe the motion? How can you tell? Can you confirm that the motion is constant?  What forces do you know are acting on it?  Every student thinks there are at least two forces.  Some students say there are only two, some say that there are three or four.  Some students intuitively have a good grasp of Newtonian mechanics and sometimes it’s required to get them to not shout their thoughts – a strong and confident voice and silence other dialogue prematurely, especially when other students accept the other voice without critically thinking about it.

The importance of identifying forces acting on the object is not fully developed with this demo.  The students need to elaborate on their ideas, particularly with drawing Force Diagrams.

I also introduced the idea of inertia with them, as a way to describe an object’s desire to keep doing what it’s doing (its resistance to change of motion).