Tag Archive | normal force

Day 41: Teasing Out Friction’s Dependence on Normal Force

2015 CP 02 BFPM

College-Prep Physics: Last class, students determined the factors that affected friction. They had said one of the factors was mass and/or weight. Today, we dug a bit deeper and analyzed a few different scenarios (above) to tease out the real factor. They did fine drawing the force diagrams (my annotations in red):


We skipped the interaction diagram this time, since I figured the scenarios were fairly easy. However, a few groups ended up drawing a single combined downward arrow for C and a single upward arrow for D, rather than 2 arrows to represent each object interaction.

Then I asked the class, “For your ranking, which value from the force diagram aided in your ranking?”

It’s not really about weight or mass. It’s not really about the downward force. It’s about the upward force from the surface!

Then we tried using our interlocking bristles model to explain our predictions. The more the surfaces are compressed together, the more the surface bristles interlock, and therefore the more friction there will be.


NGSS Science and Engineering Practices
#2. Developing and Using Models

Day 31: Equality of Normal Forces

College-Prep Physics: On Friday, we established that the table must be pushing up on the book. Today, we explored a different scenario to determine if normal forces between objects we equal in size. (Based on a similar sequence in Preconception in Mechanics.)

VOTE #1: Compare the forces between the wood stick and the car. (target)


I set up a slow buggy driving into a wood dowel that is hanging down from a ringstand clamp. If you remove the tire treads, the buggy wheels will continue to spin, showing that the buggy is continuously pushing against the dowel.


Some students says the forces are equal, some say the buggy is pushing harder because it’s trying to roll into the stick, and some way the stick is pushing harder to keep the buggy in place.

I don’t give the answer, but give them the next scenario instead.

VOTE #2: Compare the forces between the hand and the spring. (anchor)

Most kids say they are the same. It helps to think of a small, motionless board in place between the hand and the spring. Since the board is at rest, the hand and the spring must be pushing equally on the board. Now gently slide the board out from between the hand and the spring. Have any of the forces changed? So how do the forces compare? If I push harder on the spring, what happens? Are the forces the same now? How does the spring know how hard to push? (A lot of kids talk about the spring adjusting or compensating until the forces are equal. Some even refer to the spring lab we did previously. While the forces are ALWAYS equal, even while the spring is moving, I let that detail slide because we’ll return to the dynamic case in another lesson.)

VOTE #3: Compare the forces between the stiff and loose rubber band. (bridge)


Again, most kids got that the rubber bands pull equally because the ring is at rest. How is this possible when one rubber band is stretched more than the other? What happens when you try to make one of the rubber bands pull harder? What happens if the ring is removed and the rubber bands are tied together? Are the forces still equal?

VOTE #4: Compare the forces between the rubber hose and the car. (bridge)


Now I have the slow buggy drive into a piece of flexible rubber hose. The slow buggy works well because the hose will visibly flex and while keeping the buggy in place.


Again, students say the forces are the same. How does the hose “know” how hard to push? What would happen if we replaced the slow buggy with the fast buggy?

VOTE #5: Compare the forces between the wood stick and the car. (target)

We return to the first scenario and re-vote. Students make the connection that the wooden stick still bends and the force between the car and the stick must be equal. Then I quick run through the book scenarios from the previous lesson and ask them to compare the forces (the same, the same, the same, …)


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

Day 30: Does the Table Push Up on the Book?

College-Prep Physics: Today we did another round of voting (a la Preconceptions in Mechanics) to answer the question “Does the table push up on the book?”

One snafu that happened this year that didn’t happen last year: Because we studied gravitational forces first, kids were confused by the question and thought about the gravitational attraction between the book and the table. This was something I did not anticipate. So I had to clarify the scenario (explaining that table’s gravitational force on the book pulls the book down rather than push the book up as per the question).

Last year, that confusion wasn’t an issue because we did normal forces first, which is the suggested sequence in preconceptions in mechanics. But I was dissatisfied with that sequence because there were questions about normal forces between individual objects that are stacked on top of each other. We were talking about the object at the bottom of the stack having to support the weight of the objects on top. Those complex scenarios are easily analyzed using system schema and free-body diagrams, but we hadn’t talked about gravitational forces yet.

So, despite the confusion this year, I still think gravity should be done before normal force. So for next year, I’m revising the questions. I’m going to start with the hand on the spring question, since the answer is obvious and we just wrapped up the spring lab. Hoping that question puts kids in the proper mindset, then I’ll move to the table on the book question. And instead of the foam question, I’ll replace the foam with springs. (My foam never really deformed much anyway.) wpid-photogrid_1413554685985.png

Here’s the revised slides I’ll try next year:


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

Day 11: Equality of Normal Forces


College-Prep Physics: I’ve been using the normal force vote/discussion lessons from Preconceptions in Mechanics. This is the target scenario for the second lesson. Although you can’t tell, the buggy is turned on and the wheels are spinning with the buggy in place.

Which force, if either, is larger: the buggy pushing on the bolt or the bolt pushing back on the buggy?

The rest of the lesson leads students through several different scenarios (including the Dueling Rubber bands from Day 10) carefully selected and sequenced so that they can return back to the target scenario and have a model/mechanism for the answer.

FYI: That board with the bolt came in very handy today.

  • I put a Pasco matter model over the bolt so I could show how the compression/normal force changes direction as the board changes angle from horizontal to vertical. The bolt kept the model from sliding down the board.
  • Later, I took the matter model off the board and put a book on the board, tilted the board, and the bolt prevented the book from sliding — and we talked about the directions of the normal forces between the book & board and the book & bolt.
  • And finally I used it to introduce the Hooke’s Law lab by hooking a spring on it and pulling with a spring scale (“What do you notice?”).


The previous normal force lesson dealt with the nature of the normal force and introduced the ball and spring model. Here are some responses to the HW for the first lesson: