In this clip the teacher is continuing the group discussion about the shape of the graph the class created for question 2.
Worksheet 1, Question 2, Clip 2, 3:31
In continuing the discussion, the teacher points out important aspects of the graph that the students will need to understand for subsequent activities.
Notice that the teacher requires that the students describe why their graph is concave down. She also requires that the students describe what they mean by slope and why steeper means faster. It is important that teachers make sure the students are discussing the mathematics in a meaningful way.
The teacher also makes the move to ask them how to calculate the truck's average speed. A student responds correctly about how the car and the truck are at the same places at each endpoint, but the teacher rephrases this to be in terms of distance and time (they went the same distance in the same amount of time). Such reinforcement will be necessary, especially early in the semester, if this type of terminology is to become normative.
She uses this to discuss how to calculate average speed and its relationship to slope. She is helping students connect their new knowledge to their previous knowledge.
