## Key Idea: Motion energy (kinetic energy) is associated with the speed and the mass of an object.

*Students are expected to know that:*

- The motion energy of an object depends on both the speed and the mass of the object and that motion energy depends
*only*on these two factors. Motion energy does not depend on other factors such as size, shape, material the object is made of, or direction of motion. - Any object that is moving has motion energy (kinetic energy) and the motion energy of an object that is not moving is zero.
- Objects that have the same mass and are traveling at the same speed have the same amount of motion energy.
- Increasing an object’s speed will increase the motion energy of the object (regardless of how much the speed is increased) and decreasing an object’s speed will decrease the motion energy of the object (assuming the mass of the object does not change).
- When the motion energy of an object increases, the speed of the object increases and when the motion energy of an object decreases, the speed of the object decreases (assuming the mass of the object does not change).
- For objects that have the same mass, the object with the greatest speed will have the greatest motion energy and the object with the lowest speed will have the least motion energy.
- For objects that have the same mass, the object with the greatest motion energy has the greatest speed and the object with the least motion energy has the least speed.
- For objects traveling at the same speed (greater than zero), the object with the greatest mass will have the greatest motion energy and the object with the least mass will have the least motion energy.
- For objects traveling at the same speed (greater than zero), the object with the greatest motion energy has the greatest mass and the object with the least motion energy has the least mass.
- For objects traveling with the same amount of motion energy, the object with the greatest mass will have the lowest speed and the object with the least mass will have the greatest speed.
- For objects traveling with the same amount of motion energy, the object with the greatest speed will have the least mass and the object with the lowest speed will have the greatest mass.

*Boundaries:*

- We feel that “motion energy” is a more descriptive label than “kinetic energy” for this form of energy. Assessment items will use the phrase “motion energy (kinetic energy)” to avoid confusing students who are familiar with the phrase “kinetic energy.”
- Students are not expected to know or use the formula ½mv
^{2}. The sub-ideas above describe qualitative relationships. - Students are not expected to compare situations where both the mass and speed vary. In assessment items, either the mass or the speed will be held constant while the other varies so that both variables will not be changed at the same time. However, the case where one object is moving and the other is not (regardless of their masses) is valid.
- Assessment items will use miles per hour as the unit of speed.
- Note: The students are not expected to know the difference between “weight” and “mass.” All of the context used in the assessment items will be ones where “mass” and “weight” are proportional to each other. When two objects are being compared, they will be in the same location.
- This idea refers to motion with respect to the surface of the earth. An object is not moving if its position with respect to a point on the surface of the earth is not changing. Students are not expected to know that because every object is moving relative to some other object, no object has a unique claim to be at rest.
- This idea is limited to translational kinetic energy. Students are not expected to know about other forms of kinetic energy such as vibrational kinetic energy
and rotational kinetic energy.

Misconception |
Student Misconception |
Grades |
Grades |
---|---|---|---|

37% |
29% | ||

35% |
33% | ||

32% |
31% | ||

The motion energy of an object depends on its size (AAAS Project 2061, n.d.). | 20% |
17% | |

16% |
16% | ||

18% |
12% | ||

14% |
12% | ||

13% |
11% | ||

The motion energy of an object depends on its shape (AAAS Project 2061, n.d.). | 11% |
11% |

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.