OpenSciEd Unit 8.1 · Lesson 07 · 7.B
Energy transfer in collisions
From Teacher Edition
Facilitation guidance from the lesson plan — what to notice in student thinking and how to respond while teaching.
Building towards
7.B Construct an explanation based on quantitative relationships (scale) for whether decreasing the mass of a moving object or decreasing its speed would have a bigger effect on the peak forces produced in a collision between it and a stationary object and use these ideas to further explain why this would cause damage in some collisions but not others (effect).
What to look / listen for
On Looking back
Explaining and predicting kinetic energy changes in the system, any answers for questions 1 and 2 are acceptable. Many students will say that all three investigations were useful for question 1 (understanding the relationship between kinetic energy and damage). Most students will say investigation 3 in particular was most useful for question 2 (understanding the relationship between changes in mass and speed of a moving object and the amount of kinetic energy it has). Look for the following answers for questions 3 through 5:
Q3)
Reducing the mass of an object in half should reduce the kinetic energy to be half of what it was.
Q4)
Reducing the speed of an object in half should reduce the kinetic energy to be one quarter of what it was.
Q5)
We can predict how much more or less kinetic energy an object has when its speed or mass increases or decreases. These changes in kinetic energy would affect the peak forces produced in a collision. And if those peak forces don’t exceed the elastic limit of the materials or objects that collide, then no damage will occur, but if they do exceed it then damage will occur.
What to do
If students incorrectly answer Q3 or Q4, have them refer to specific data they gathered from investigation 3 where the mass was halved and the speed was halved and have them calculate the related fraction of kinetic energy that resulted from this change, then have them use this calculation to support, refute, or revise their claim. If students don’t connect to ideas about peak forces or elastic limit in Q5, ask them, What ideas can you use about peak forces and the elastic limit of materials to develop a chain of cause and effect between how factors that affect kinetic energy like mass and speed are related to what is happening during the actual collision that helps explain why damage occurs or not?