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Day 57: Balanced Force Lab Practical


College-Prep Physics: Today we did a balanced force lab practical to tie together all our work on forces. It’s similar to the ones I’ve written about in past years. However, this year we used the green buggies and whiteboards instead of wood blocks and carpet/rubber. (This is because this year, in previous labs, some groups already worked with wood blocks and carpet/rubber.)

Given only a green buggy, a whiteboard, a spring, a 200-gram mass, and a ruler:

  1. Determine the spring constant of your spring.
  2. Determine the weight of your green buggy.
  3. Determine the force of kinetic friction between your buggy’s rubber tires and your whiteboard.
  4. Determine the coefficient of kinetic friction between your buggy’s rubber tires and your whiteboard.
  5. Predict the force of kinetic friction when 500 grams is added to your buggy. Have your teacher test your prediction!

UPDATE 2014 DEC 3: We found that when the 500 gram mass is added to buggy, the buggy rolls (rather than slide) when pulled. A binder clip on a rear wheel works great to lock the wheels so the buggy slides.


NGSS Science and Engineering Practices:
#2. Developing and using models
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking


Day 54: Kinetic Friction

College-Prep Physics: Last Thursday, students investigated the factors that might affect kinetic friction and how kinetic friction compares to static friction.

2015 CP 02 BFPM (1)

Today, students looked at the relationship between normal force and kinetic friction. Is the relationship proportional, like our previous experiments with static friction? If so, how do the slopes for kinetic friction compare to that from our static friction experiment?

2015 CP 02 BFPM (2)

NGSS Science and Engineering Practices:
#2. Developing and using models
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking

Day 52: Balanced Forces in Motion

College-Prep Physics: We’ve been looking solely at forces in static cases. Now it’s time to look at moving cases. So I asked students to complete the following chart on a whiteboard:

2015 CP 02 BFPM (1)

What’s true about the forces in the constant speed cases? In the speeding up and slowing down cases?

Then I tried to address the misconception/difficulties kids have with constant speed = balanced forces (wouldn’t it just not move?) by demo’ing dueling fan carts. First with fan carts off, then with both on — what happens after I push?


NGSS Science and Engineering Practices:
#2. Developing and using models

Day 44: Tug-of-War!


College-Prep Physics: We discussed the results from Wednesday’s friction lab:


What does the slope mean? How does the slope relate to the “grippiness” of the two surfaces?


Then we repeated the experiment with our shoes to determine the “grip factor” between our shoes and the tile. Then used the grip factor to extrapolate to how much friction there would be when we’re wearing our shoes.

Theoretically, the tug-of-war team that wins is the team with the most friction. So we used the friction calculations of the students who wanted to participate in the tug of war to determine the winning team.


Time to test our prediction!


NGSS Science and Engineering Practices:
#2. Developing and Using Models
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking

Day 40: Factors Affecting Friction


College-Prep Physics: First, we brainstormed possible factors that might affect the maximum strength of static friction between two surfaces. Then students designed their own experiments to determine which of those factors actually mattered. Finally, we tried to use our “interlocking bristles” model to explain our results.

— Weight/mass: Definitely affected friction. Why? The bristles interlocked more, making it tougher for them to slide past each other. This is easily demonstrated and felt using toothbrushes.

— Surface Area: Surprisingly, this did NOT matter (with the exception of groups that used highly irregular surfaces like carpet, felt, and cork). Why? Well, a larger surface area means more bristles in contact, which should mean more friction. But a larger surface area also means the surfaces are less compressed, which would reduce the the friction. This is easily demonstrated with weights and foam.


So the net effect is no change in friction.

— Surface material: Changing the material of either surface also affected the maximum amount of static friction between the surfaces. This is similar to changing the material and arrangement of the toothbrush bristles.


NGSS Science and Engineering Practices
#2. Developing and using models

#3. Planning and carrying out investigations


Day 34: Levis Jeans

Levis Tension (3)Levis Tension (2)

College-Prep Physics: Which situation is more likely to rip the Levi’s jeans?

Today we did another round of voting to get at the idea that tension in springs, strings, and ropes are constant all the way through. Today’s slides:

During which, we do one of my favorite demos: What does the middle scale read?


Click the picture to reveal the answer

In the end, we talk about how a seemingly unstretchable rope or spring actually stretches under tension, much like seemingly unbendable surfaces like tables deform under compression. Just like a solid can be modeled as a network of balls and springs, so can rope and string:


Even metal wire stretches!


NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations

Day 32: Interaction Diagrams and Force Diagrams


College-Prep Physics: Now that we have gravitational forces, spring forces, and normal forces under our belts, we can analyse more complex situations. Today was a direct instruction lesson* on drawing interaction diagrams and forces diagrams. You might notice some changes from how I drew them from last year.

I’m using agent-object notation on the force diagrams, rather than last year’s force type + agent, in order to combat the misconception that the force diagram represents what the object is doing, rather than what is being done to the object. This also helps with getting the students to focus on the objects that are exerting the forces, because “every force has a source.” To make the force diagrams easier to read and label, we’re not including the force types on the force diagram vectors. Force types are labeled on the interaction diagram only, to help reinforce that a force is a single interaction between objects.

I’m also starting with complex scenarios early, and also asking students to draw more than one force diagram for a given situation. Last year, some students had the misconception that there must always be one force up, down, left, and right. The didn’t realize you could have 2 forces in one direction or no forces at all.

Drawing multiple force diagrams also allows for identifying 3rd Law pairs (the two vectors with circles in #4 above, though we haven’t formally called them 3rd Law pairs).

We also started with numerical values early. Although the scenarios don’t ask a specific question, we determined the values for as many forces as we could based on what was given.

In hopes of avoiding another common misconception, you’ll see that in both scenarios the normal forces aren’t equal to the weights of the objects.

We are only looking at static cases right now. Up next is tension, then friction. After friction, we’ll consider the dynamic cases.

The two scenarios pictured are taken from Preconception in Mechanics, though PiM doesn’t have the students draw interaction diagrams or force diagrams — a fault I found out too late last year. You can get the entire handout here: ForcesSchemaFBDDevelopmentStatic2015

PS: I haven’t been using the HW sheets from PiM at all. Rather, I’ve been using the occasional PiM HW problem as a bell ringer/do now/warm up.

*If you have a more engaging way of introducing interaction diagrams and force diagrams, please share!


NGSS Science and Engineering Practice #2: Developing and Using Models