## 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:

1. 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.
2. 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.
3. 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:

1. 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.
2. Test items will involve situations in which forces are constant, not situations in which the forces are increasing or decreasing.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number
6–8
9–12
Select This Item for My Item Bank

56%

65%

53%

57%

52%

59%

45%

49%

42%

52%

42%

50%

36%

49%

38%

39%

33%

37%

31%

34%

28%

36%

30%

31%

22%

22%

Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

6–8

9–12

FMM107

If an object is slowing down, a force that was moving it forward must be decreasing (Clement, 1982; Watts & Zylbersztajn, 1981).

24%

20%

FMM124

When a force acts on a moving object in the direction opposite the object’s direction of motion, the object will move at a constant speed for a while and then slow down (AAAS Project 2061, n.d.).

23%

19%

FMM125

When a force acts on a moving object to slow the object down, the object will slow down for a while and then move at a lower constant speed (AAAS Project 2061, n.d.).

20%

21%

FMM105

If an object’s speed is decreasing, a force that is retarding the object’s motion must be increasing .

20%

19%

FMM123

When the force on a moving object is constant, the object will slow down.

21%

15%

FMM106

Moving objects stop when they run out of force (Twigger et al., 1994). An object’s force can be used up and must be replenished to maintain activity (Watts, 1983).

18%

17%

FMM090

A moving object has a force within it that keeps it moving (McCloskey, 1983; Osborne, 1985; Viennot, 1979).

20%

11%

FMM128

When a force acts on a moving object in the direction opposite the object’s direction of motion, the object will move at a constant speed the entire time (AAAS Project 2061, n.d.).

18%

14%

FMM126

When a force acts on a moving object in the direction opposite the object’s direction of motion, the object will move at a constant speed for a while, slow down for a while, and then move at a lower constant speed (AAAS Project 2061, n.d.).

9%

7%

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.