## Day 75 – Newton’s Third Law

Physics 11 – Today in physics we formalized Newton’s 3rd Law.  Students had already worked out equal and opposite forces when discussing gravity, but we didn’t put a name to it or generalize it to all forces. We also watched a video of two carts colliding, where each cart had a steel hoop attached to the front.  When they collide, the hoops on both carts compress the same amount. By recalling Hooke’s Law, the students reasoned that since the compression was the same, the forces had to be the same.

## Day 74 – Trying to be Clear

Science 9 – I’ve been trying to be extremely clear with my students on what their learning outcomes are and what they’re responsible for knowing for quizzes. They’re doing a unit on reproduction, and there are a gazillion small facts that could be tested. However, while we work through these details as part of scientific literacy and understanding, I’m emphasizing that there are a few key ideas to know, understand and be assessed on. For example, while there are several steps and activities inside the nucleus during mitosis, what I really want them to know is the function of mitosis, which can be summarized in one or two sentences.

## Day 73 – Unbalanced Forces

Physics 11 – Students were applying their understanding of force diagrams, Fnet and unbalanced forces to problems today.  I usually don’t get too prescriptive on problem solving techniques, but I spent three days emphasizing #1 and #2 in the photo. As all teachers know, kids really resist doing things they think are not necessary. Unfortunately for them, skipping steps 1 and 2 will almost certainly result in mistakes even for the smartest kids with the most intuitive understanding of physics.

# The Big Tip for Physics Teachers

Ok, so we all know that kids try to skip steps. My students would often mix up writing out an equation for the unbalanced forces (Fnet equation, Fnet = ….) with Newton’s 2nd Law (Fnet = ma).  Here is the new thing I found out this year: after kids draw and label their FBD, they become paralyzed on the next step because they realize they don’t know what all the forces are.

I used to think that they don’t write out Fnet = .. + … + … because they were trying to skip a step. What’s really happening is that when they start to write out Fnet = …. what they’re expecting is that they should already know everything on the right hand side of the equation.  They often don’t, and this becomes a discrepant event for them.

There is a strong link between this and a phenomenon in math.  In math, students often have a difficult time understanding the “=” in an equation.  Many people don’t see equations as being a statement of equivalence.  Instead, they see an equation as a question on the LHS (what is?) and the answer on the RHS. This is what happens with Fnet. The intuition is to have an answer for everything on the RHS.  Note: just because students can give a definition of what the equals sign means, doesn’t mean they treat it like that when working with math or physics.

## Day 72 – Particle Model

Science 8 – I love this lab. Students predict what they’ll get when they mix 50 mL of water with another 50 mL of water, and then they find out that it’s almost 100 mL. They do the same with 50 mL of water with 50 ml of ethanol. To their surprise, it only adds up to around 97 mL. Finally they mix 50 mL of sand into 50 mL of marbles. They now have a model for how matter is made up of particles and how there are spaces between the particles.  Students also learn about miniscus, graduated cylinders and brainstorming errors (why did 50 mL of water + 50 mL of water = 99 mL?).

## 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 70 – Probing for Prior Knowledge

Science 8 – We started a new unit on matter and fluids today.  First task was to figure out what everyone already knew about the topics.  Students worked together on whiteboards and compiled lists of prior knowledge.  Common things that my students wrote down included: everything is made up of matter and particles, atoms are the smallest types of particles, atoms are made up of electrons, proton and neutrons,  states of matter.

This activity took about 45  minutes because there were lots of questions being asked as we went around the whiteboards.  When I do KWL (Know, Want to know, Learned) activities, they’re usually done within 5 minutes.  Today’s class was much more rich than that.