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Language of Physics: Banning more words

October 9, 2010

The more we have discussions in physics, and kids really get a chance to talk out what they think, the more I see language as absolutely critical to understanding. So here are a few more points o n some of my (least) favorite words in physics:

  • deacceleration: this is the grandaddy of all confusion. I used to ban it outright the moment it came up in class. Now we try to talk it out, and figure out why it’s like a mental quagmire. Student: “Deacceleration means slowing down. That’s negative acceleration, right?” Me: “Ok, so if we let the downward direction be negative, then this falling ball has negative acceleration, so it must be slowing down, no?” Oops. IMO, understanding positive and negative acceleration is more than enough difficulty for students without having to worry about what deacceleration means. We’ve got one word—acceleration—and it means to change velocity, and any change in that velocity vector (getting bigger, smaller or changing direction) counts.
  • it’s: I hate all pronouns when referring to graphs. Try this. Here’s a velocity vs time graph.
    Now ask a student to describe the graph. Here are some sample responses:

    • it’s changing
    • it’s accelerating
    • it’s constant
    • it’s negative
    • it’s moving forward
    • it’s positive
    • it will be zero
    • Each of the above sentences can be right or wrong, depending on what the pronoun refers to. My hypothesis is that kids don’t really know whether they are talking about the object, the velocity of the object, or the acceleration of the object, and they cover it up with a harmless pronoun.

      So I often tell student’s that for the next 20 minutes, we’re banning pronouns, and work as they struggle through saying things like the “car is moving with decreasing positive velocity, and the acceleration of the car is negative and constant.” Much better.

  • At rest/not moving/stopped: This sentence, reveals some deep misconceptions: “when you throw a ball into the air, the ball stops when it reaches the peak.” To get to this, I ask them what they mean, in terms of velocity and acceleration, for something to stop. Pretty soon, they see that stopping means “make your velocity and keep it there,” or more precisely, “acceleration and velocity are zero.” This helps students clear up the idea that the acceleration a ball thrown upward at the peak of the ball’s motion is not zero. Instead, the velocity of the ball is zero for an instant, but the acceleration is constant (and non-zero) throughout.
  • Slowing down/speeding up: These terms are actually great for describing motion in physics, but few students know what they mean. It takes some focused study of graphs to see that an object with negative velocity and negative acceleration is speeding up, and then finally, the realization that if acceleration and velocity point in the same direction, the object is speeding up, and when they point in opposite directions, they’re slowing down. This also sets the stage for understanding tangential and centripetal components to acceleration in the future.

I also find that I can get much futher with students if we talk about changing velocity for a long time before we begin to use the term “acceleration” with regularity. Explaining more ideas with a few concepts is one of the central ideas of physics.

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