book at rest. Most of the class included an upward force by the spring in their diagrams. A few others argued that because the spring was not alive, it could not “exert” a force.
Mr. Hull asked, “So, how come many of you who said the table does not exert a force are now saying that the spring does exert an upward force? The spring isn’t alive.” The students responded, “The spring moves.” “The spring compresses or extends.”
The teacher asked the students to think about what was similar about the situations in which they were willing to say there was an upward force. They suggested that when the book was on the hand, one could see or feel the muscular activity in order to support the book, and when the book was on the spring one could see the change in the length of the spring. Mr. Hull pointed out that they were responding to evidence for a force by looking at some change in the “thing” that is doing the supporting. He wanted his students to be seeking observational evidence in support of their ideas and inferences.
Mr. Hull: “How about those of you who suggest the table does exert an upward force. In what way does that make sense to you?” While gesturing sideways, one student said, “Whenever anything stays still, if there is a force on one side, there has to be a force on the other side to keep it stopped.” Mr. Hull: “ I see you are talking about horizontal forces, does that also work with vertical forces?” Again, he guided his students to see the consistency across contexts, in this case, explanations of the at-rest condition should be the same whether considering horizontal forces or vertical forces. This gave some rational argument for an upward force.
Mr. Hull asked his students to think about evidence. “What observable evidence do you have that the table exerts an upward force?” A few students suggested the table bent like the spring. Others countered, arguing that the table was a heavy, solid demonstration table, that it was rigid and therefore could not bend. The students suggested the need for a critical experiment. “How could we see whether the table bends at all?” asked the teacher. Not hearing any suggestions, Mr. Hull proposed that they use a “light lever.” Bringing out a light source (in this case a laser pointer), he placed it so that the light hit the shiny table top at a low glancing angle. With the room lights off, one could see where the reflected light hit the far wall. The teacher checked to be sure that the students knew that if the table bends, the light on the wall should move. Although the movement was not readily noticeable with one book placed on the table, as the stack got larger and was taken off and back on, the light could be seen to move.
After exploring ideas about force