Day 46: The Most Difficult Concept

November 15, 2013 44128 PM ESTCollege-Prep Physics: A student asks about the two highlighted scenarios —

But if the forces are balanced, wouldn’t the bowling ball be at rest? How could it be moving?

The point that is overlooked is that the ball is already moving. Which I tried to help kids see by sequencing the diagrams as speed up, constant speed, then slow down. Interestingly, more kids are OK with the frictionless scenario (they already have first-hand experience with not hitting the bowling ball) than the friction scenario. My best explanation asked them to imagine hitting a rolling bowling ball on a frictionless surface but a split second after they hit it forward (to speed it up), someone else taps it in the opposite direction with the same strength (to slow it down). You hit, they hit. You hit, they hit. What happens to the speed of the ball? (It never changes). Friction is just like those opposing taps.

Lastly, to my delight, the kids suggested 2 more possible force diagrams for the bottom right scenario — which then lead another kid to ask which of the 3 possibilities would be most effective at stopping the ball.

You can download a copy of the sheet here: BalancedUnbalancedFBDMotionInteraction 2014



About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

8 responses to “Day 46: The Most Difficult Concept”

  1. Andy "SuperFly" Rundquist says :

    This is a difficult concept, and I think that I too often set up situations for students (to quickly talk about forces) where no acceleration means not moving. I need to try harder with that.

    I’m curious about possibly teaching the concept of frame changes in this regard. You can always switch to an inertial frame where those balls are at rest. Is that useful?

    • daveeckstrom says :

      I use a small hovercraft I got from Arbor or Ed. Innovations. I shove it across the floor above a section of tape measure and ask the kids to estimate the speed. It doesn’t matter what the speed is, the point is that it’s constant. Next, I turn on the hovercraft (gotta pre-determine a level spot on the floor) and ask them to estimate the speed, which they all say is zero. Then I attach the tape measure piece to a constant velocity car and have the car pull the tape measure beneath the stationary hovercraft and ask now what’s the speed, which they still say is zero. Next I repeat, asking them what is the speed of the hovercraft if you are a little person standing on the tape measure. This eventually leads to the conclusion that “standing still” and “moving at constant speed” are really the same thing and that the interesting thing is what happens when there is a change in motion. We do this all after kicking the hovercraft around on the floor in a circle. I think this was inspired by Kelly O’Shea.

    • Jared Stang says :

      “But if the forces are balanced, wouldn’t the bowling ball be at rest? How could it be moving?” I heard the same comment (not necessarily about bowling balls) many times during the Newton’s laws portion of the introductory college physics course I TA in. I agree that it’s one of the most prominent misconceptions. (Having zero velocity and non-zero acceleration is a pretty big one for kinematics – as at the top of the arc for a ball thrown up vertically.)

      The instructor in my course did not go into reference frames. However, I remember students approaching the idea in at least two conversations I had with them. One of these was in response to the following problem, presented in lecture: ‘A boy is sitting in a box. His brother is pushing the box at constant velocity. What is the free-body diagram for the boy in the box?’ The student and I talked about riding in a car at a constant velocity, in an attempt to ‘be the boy’ and figure out the forces acting on him. I believe the student then came up with the idea that if she were riding along in the box with the boy, it would be clear that there wouldn’t be horizontal forces acting on him. We had this discussion without mentioning different ‘frames’.

      Andy, I don’t think this answers your question of whether inertial frames would be useful in teaching kinematics, but I thought it would be interesting to note a context in which a student brought it up naturally.

    • Frank Noschese says :

      I see where you’re coming from, but I think reference frames might just make things more complicated, particularly the difference between inertial and non-inertial frames. I’m taking a forces first approach this year and so we haven’t even done kinematics yet. (Thanks for stopping by, Andy. And good luck with your blog commenting project!)

  2. Andy Fitz says :

    I used this an inspiration for an opening lab to the constant force unit last year. It felt a lot better than just pushing the hovercraft down the hallway, as a lot of students still aren’t convinced that your force isn’t still acting on the hovercraft, despite the fact that you aren’t touching it. This approach has students clearly identify the way they think, then they must reconcile this with very clear contradicting data. It’s a little “out of order” in that it establishes that constant force provides constant acceleration, but then I can easily switch to “okay, we know what not moving looks like, we know what accelerating looks like, what does constant velocity look like?” That’s a great moment to see the gears turn.

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