## Key Idea: Elastic potential energy is associated with the stretching or compressing of an elastic object and how difficult it is to stretch or compress the object.

Students are expected to know that:

1. An elastic object is an object that changes shape when stretched or compressed and returns to its original shape on when not stretched or compressed.
2. An elastic object that is not stretched or compressed has no elastic energy.
3. Elastic objects that are made of the same material and are stretched (or compressed) the same amount have the same amount of elastic energy.
4. The elastic energy of an elastic object can be increased by stretching or compressing the object out of its original shape. Increasing the amount the object is stretched or the amount of the object is compressed, increases the elastic energy of the object and decreasing the amount the object is stretched or compressed, decreases the elastic energy of an object.
5. For elastic objects that are identical except for how much they are stretched or compressed, the object stretched or compressed the most will have the most elastic energy and the object stretched or compressed the least will have the least elastic energy.
6. For elastic objects that are identical except for how much they are stretched or compressed, the object with the most elastic energy is stretched or compressed the most and the object that has the least elastic energy is stretched or compressed the least.
7. For elastic objects stretched or compressed the same amount (greater than zero), the object that is the most difficult to stretch or compress will have the most elastic energy and the object that is the least difficult to stretch or compress will have the least elastic energy.
8. For elastic objects stretched or compressed the same amount (greater than zero), the object that has the most elastic energy is the most difficult to stretch or compress and the object that has the least elastic energy is the least difficult to stretch or compress.
9. There is a limit to how much an object can be stretched or compressed. If an elastic object is stretched or compressed too much, it may break or not be able to return to its original shape.
10. How much an object is stretched or compressed is measured relative to its unstretched or uncompressed state. They should know that the unstretched or uncompressed length of an elastic object does not determine the amount of elastic energy.

Boundaries:

1. Students are not expected to know the meaning of the term “potential.”
2. Students are not expected to know or use formulas associated with elastic energy, such as ½kx2. The sub-ideas above describe qualitative relationships.
3. As a result, students are not expected to compare situations where both the amount of stretching or compressing and the properties of the object vary. In assessment items, either the amount of stretching or compressing or the properties of the object will be held constant while the other varies so that both variables will not be changed at the same time.
4. Students are not expected to know which objects are elastic. Assessment items will use only familiar elastic objects such as springs, rubber bands, and rubber balls. Students are not expected to know the stiffness/rigidity of specific materials or the spring constants of specific springs. In assessment items, the relative rigidity will be provided when necessary.
5. When comparing objects in assessment items, we will not have a situation where one object is stretched and another object is compressed. Additionally, we would not compare an object in a stretched and compressed state.
6. In assessment items, objects will not be stretched or compressed beyond the point where they would return to their original shape (i.e. no plastic deformation).
7. When dealing with springs, students are not expected to know the effect of combining springs in series or parallel on the elastic energy of the springs. Students are also not expected to know about how the geometry of the spring relates to the overall elasticity of the spring.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number
6–8