Day 50 – Misconceptions of Gravity

3 particles.PNG
Physics 11 – Today was our Misconceptions in Gravity class. We did a lot of voting and discussion centered on three themes: what things affect gravity?, do small things also have a force of gravity?, and do interacting objects have an equal force of gravity acting on each other?.

There’s too much on these subject to list here, but the above diagram is one of the more convincing arguments. Students see that as they add additional particles to the diagram (X, Y, Z, etc), each new particle has a force of gravity acting between it and A. The extension of this is that large objects, or objects with millions or billions of particles, will have the same number of force arrows as does the small object (A).

Some students were a bit too “white flag-ish” for this class.  It’s so easy to tune out and say to yourself “I don’t know.”  Those students can make this a difficult lesson to teach. But other students really like it. It’s interesting and challenging.  It’s solving a mystery or a puzzle.

Day 40 – First Quantitative CA Quiz


Physics 11 – Today the students had their first constant acceleration “equation” quiz. I tried to incorporate some smart questions into it. The first question is “low floor”. Everyone should have success with it. The second question is a boilerplate physics 11 question that the students are expected to be proficient at. The third question is designed to reveal a common mistake. Students will be tempted to use the d=1/2(vi + vf)t equation, using vo = 0. This turns a constant velocity question into one that (mistakenly) has acceleration. We went over the quiz and several students made this mistake. I wonder if hammering on this mistake will prevent it from happening… No, it won’t be it will hopefully reduce its frequency.

Day 74: Universal Gravitation


Physics 11 – Today the students worked through the formula for Universal Gravitation.  The students felt pretty good about how gravity works, having covered it conceptually already.  However, there were still a few surprises (mostly for me).

It became apparent to me that many students consider the re-ordering of variables in a formula as something that makes a new formula.  For example, after rearranging the formula to solve for r, separation, I was asked several times if the students would be given this formula on the formula sheet.

The second big surprise was being asked if the N*m^2/kg^2 units for G were variables, and what numbers should be plugged into them.

The above two misconceptions will have to be something that I specifically address again the future.

Day 45: What Affects Gravity?


Physics 11 – The classes continued with their examination of things that affect gravity.  In this lesson, we went through how gravity between two objects results in equal forces acting on each object, regardless of the mass of the objects.  I closely followed the outine from Preconceptions in Mechanics, and how Frank Noschese worked with his classes.

I opted to have up to three students pull on spring gauges, with each of their gauges hooked onto a gauge of another (fourth) student.  What we saw was that the total force of the three students (3 x 3N each) equaled the force experienced by the fourth student (9 N).  Students verbally expanded on this idea, but commenting how if there were a billion students pulling with 3 N each, the total of their pull would be 3 billion Newtons, as would be the pull of the singular student.  This was our model to represent small particles, where the Earth is made up of billions of small particles.

We followed this lesson by looking at normal forces. I was surprised that almost all the students immediately saw what a normal force was, and that it balanced against the force of gravity.  In our discussions, there seemed to be very little misconceptions.  Students were also interested in extending their idea of force equality in pairs.  I still zoomed through some demos of a chair spring, foam and vehicle suspension spring (increasing spring constant), and ended with the deflected laser on the desk demo.  They found the laser deflection to be interesting albeit not surprising.