## Key Idea: If a force acts on an object in the direction opposite to the direction of the object’s motion, the object’s speed will continue to decrease while the force is acting.

*Students are expected to know that:*

- If a force, either constant or changing, acts on an object in the direction opposite to the direction of the object’s motion, the object’s speed will decrease and will continue to decrease for as long as that force is greater than any force moving the object forward.
- If an object’s speed is decreasing, a force must be acting on the object in the direction opposite to the direction of the object’s motion, and that opposing force must be greater than any force moving the object forward.
- If a force acts on an object in the direction opposite to the direction of the object’s motion for a long enough time, the object’s speed will decrease to zero. If the same force continues to act, the object will move in the direction opposite to its previous motion.

*Boundaries*:

- Students are expected to analyze situations involving no more than two forces acting on an object at the same time, and each force must act along the object’s line of motion or, if the object is not moving, the forces are acting along the same line.
- Test items will involve situations in which forces are constant, not situations in which the forces are increasing or decreasing.

Item ID Number |
Knowledge Being Assessed | Grades 6–8 |
Grades 9–12 |
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Misconception |
Student Misconception |
Grades |
Grades |
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When the force on a moving object is constant, the object will slow down. | See the Original Project | ||

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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.