Day 58 – Force Diagrams

Physics 11 – Students got to practice drawing force diagrams today. They’re were lots of good questions, usually ones that I want to deal with later. They were invoking Newton’s Third Law, friction, and all sorts of things.

The above graphic is a good example of why we label our forces. Students want to draw a force pushing the skater to the right. However, no one can think of where this force comes from. What is its agent? The closest we get is skater on skater. I found this to be hard to explain. The best I could come up with was a thought experiment where a person imagines they are floating in space. Can they imagine themselves moving around and directing their motion? Most people see the need to push off of something. That something would be the agent. In the above case, since we can’t name the agent, it’s probably not a force.

Day 102 – Reason #102 Why I Like SBG

Physics 11 – Today I had a student that has his ups and downs in physics come by and be assessed on a couple of learning objectives. I know that he knows more than what he shows in testing and I just have to draw it out of him. I was able to give him several situations where he had to draw the force diagram and tell me if the forces were balanced or not. He would work through them, talking while he thought about his answers, correcting himself as he went. He did very well on both of these objectives and I was able to score both objectives as Mastered.

With SBG I can do this type of assessment quite easily. It’s not about how many points the test is out of, I don’t need to give him a “chapter test”, etc. He identified two objectives he was ready to be assessed on, and that’s what happened.

My students have a grade/list of assessments that tell anyone what they know in physics. It doesn’t rate the students, it doesn’t judge how fast they learned it, or how many times they failed. If someone wants to assess them on those qualities, they are more than welcome to do it, but I’m not going to.

DAY 71 – Elevator Lab


Physics 11 – What does a coffee maker have to do with an elevator lab? Absolutely nothing, but it’s still very important. I got a new coffee making device for Christmas, called the Clever. It’s not that much different from ye old Melitta system. You slap a Melitta filter in the cone, add some fresh ground coffee and pour in hot water. The Clever does not let water through the bottom until it is placed on a cup though, so the coffee goes through an immersion before filtering. The Clever is half pour over, half French press. I was interested in it because I use a hand grinder and it’s easier if I do a course grind, which is what you use for a French press and Clever. However, unlike the French press, the filter removes all small sediment and particles from the coffee so that you end up with a clean crisp cup of Joe.

Oh, Physics.

The class watched a video of a person standing on a scale while riding in an elevator. The video was focused on the scale readout.  Students had to work out the person’s weight and normal forces acting on them, and draw force diagrams corresponding to when the person was still, moving at constant velocity and accelerating. It was challenging for them but very thought provoking and useful.

Day 54 – More FBD


Physics 11 – Students did more work on FBD and vectors, with calculating some horizontal and vertical forces using trigonometry.

This question also came up from the modelling package.  I wouldn’t expect many kids to get this question correct, I think it is very unintuitive.  However, it’s a great question to go over in front of the class.

Day 52 – Force Diagrams


Physics 11 – Students drew force diagrams and we extended this activity to analyzing forces acting at an angle.  Students began to look at how an angle can be analyzed in separate vertical and horizontal components.  I think this is a good place to start with vectors and components because the force vector is static.

Day 68: Peer Instruction on 3rd Law


Physics 11 – Having discussed and debated about Newton’s 3rd Law last day, today was a chance to put the students on the hot seat with Peer Instruction.  Class #1: first vote was about 70% correct, moving to everyone correct but one person on the second vote.  Class #2: first vote had over 80% correct, no second vote required.


This question was more difficult because the answer is more subtle.  Class #1: first vote was about 60% correct, moving to 75% correct on the second vote.  Class #2: first vote was about 60% correct moving to 90% incorrect on the second vote.

I could see how students were convincing with their incorrect arguments.  The key was in the drawing of a force diagram, but very few students wanted to do this.  However, as time moves on I think I am slowing convincing students just how important a force diagram is.  Now that we are doing more word problems (5e: extend), I am reinforcing the idea that physics covers the force diagram and writing an equation for the net force, and then the rest of the solution is math.  By skipping over force diagrams, the students are not demonstrating understanding of physics.

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).