Future physics teachers’ initial explanations for why a student might say that a dropped stone falls and quickly reaches a constant speed:
“A student could think that since it’s a rock or a heavy object as compared to a sheet of paper or a feather that it would fall faster and might reach terminal velocity faster.”
“A student may put this because they understand the concept of terminal velocity but assume that it happens quicker than it actually does.”
“Students would choose this because of their understanding of objects reaching terminal velocity”
“Someone who has taken some physics and has some knowledge of terminal velocity can easily choose this answer. Someone without experience in physics might still pick this answer because from personal experience, it is difficult to notice objects accelerating as they fall.”
“This might be chosen because if you imagine the situation, the stone falls for such a short time, that you can’t really tell that it’s speeding up. You might think that it quickly gets up to speed.”
“A student might pick answer A because they know about terminal velocity but not realize that 12 feet is not nearly high enough for a stone to reach terminal velocity.”
“If a student had an incomplete understanding of terminal velocity or the distances involved than they would choose this answer.”
Some thing I notice:
Many of the students are situating student responses in terms of the disciplinary concept of terminal velocity… some saying that they might know about terminal velocity, but not understand the conditions in which it is likely to apply and other suggesting that such a student has an complete understanding of terminal velocity. One student suggests an intuitive explanation for why someone might think terminal velocity happens quickly for a heavy object-that because it falls faster (being heavy and all), that it would also reach its top speed rapidly.
I also notice students situating this answer with respect to everyday experience and constrains on our ability to closely observe what’s happening. One suggests that falling happens so quickly that it’s hard to tell that it’s speed up, and another suggesting that it’s hard to notice acceleration.
These two ways of making sense of student ideas are different: One is rooted in student misunderstanding or misapplying disciplinary knowledge. The other is rooted in pointing to a feature of our everyday experience and the consequences of that experience for how we might think about falling objects. Of course, these two might interact.
Both kind of ways of seeing student thinking is important: How does this make sense form students’ experience and set of ideas? What relationship does this have with disciplinary ways of knowing?
One thing I see missing here is how this everyday experience and idea can interface with students learning that the acceleration due to gravity is a constant 9.8 m/s/s. Many students I encounter interpret this as meaning that objects fall at the speed of gravity, which is 9.8 m/s. Some will even interpret this to meant that it takes a second for a falling object to reach the speed. Some will say that air resistance results in the speed being less than 9.8 m/s.
This last piece here is a third way of seeing student thinking: How their everyday experience and ideas can interface with learning formal physics concepts?