Ingrid’s 4th grade students designed and carried out an investigation to see the relationship between speed and energy. Students let go of a marble placed at the top of a ramp and observed its collision with a second marble at the bottom of the ramp. She asked students 1) to consider what could be changed to make the collision “bigger” or “smaller” and 2) to illustrate their explanation of the phenomenon using white boards.
Focus Practice: Constructing Explanations (SEP6)
1. What strategies did Ingrid use to help students develop their explanation?
2. How would you have reacted to the students’ reference to the marble’s speed (e.g. 2 mph) at the top and bottom of the ramp?
What is an Explanation?
When making observations of a particular phenomenon, students often identify patterns or relationships between variables (e.g. as X increases, Y decreases). However, the goal of science is to understand the mechanisms that explain rather than simply describe observations in the world. Explanations are step-by-step, causal accounts made up of both visible and non-visible parts of the phenomenon, the relationships between those parts, and the underlying processes. An explanation’s validity is determined by its ability to account and explain the observations.
Why do you think the marbles went down and had more collisions?
Ingrid’s use of “why” was geared towards eliciting an explanatory student response. Students initially considered the marble’s magnetic properties. Although magnetism was not a salient part of this explanation, Ingrid asked the students to justify their ideas (“How do you know if was magnetic?”) and considered it an opportunity for students to practice supporting their answers with evidence.
Shift from describing to explaining
The student’s account was focused on describing what was happening (e.g. “this [marble] was going down for 60 milliseconds”), which prompted Ingrid to ask them to explain how and why the marble went faster (“So you're telling me which marble went faster. […] Tell me why do you think that happened.”). To prompt this discussion, she asked the students to explain their use of “2 mph” and “2.5 mph” to reference an increase in the marble’s speed as it traveled down the ramp.
Explaining why a lighter marble moves faster
The students attempt to explain why placing a lighter marble at the bottom of the ramp could result in faster speeds after the collision. First, the students explain that the second marble moves because the moving marble transfers something to the “smaller marble.” Second, the students show some evidence of associating a marble’s size and the amount of energy it can transfer to another marble (“the bigger one could hit the small one harder”). Third, the students show some evidence of an inverse relationship between mass and speed (“the lighter one has more speed because it's lighter”).
What other factors might be important?
Ingrid used the original prompt to help the students to consider another factor, the ramp’s steepness. In response, the students considered the role of gravity and angles. In doing so, the students consider not only how and why the marbles move, but also another factor that could influence this movement.
In this example, we observed students’ attempts to construct an explanatory account composed of relevant components (e.g. marble or ramp), relationships (e.g. mass and speed), and underlying processes (gravity “pushes” the marble down the ramp). To support students’ construction of their explanation, Ingrid helped the students 1) move from describing a system to explaining how and why, 2) justify their responses, and 3) consider additional factors that might be relevant for explaining the situation.
Watch the classroom video from Ingrid’s perspective as she explains her decisions and highlights features for teachers to notice when supporting students in constructing an explanation.