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.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID Number 
Knowledge Being Assessed 
Grades 6–8 
Grades 9–12 
Select This Item for My Item Bank 
FM103001

Use of tables of speed versus time, and information about forces acting on an object, to determine the object's speed at various times.
 56% 
65%  
FM095001

A moving object will slow down the entire time that a force that opposes its motion is greater than the force pushing or pulling it forward.
 53% 
57%  
FM069005

Use of tables of speed versus time, and information about forces acting on an object, to determine the object's speed at various times.
 52% 
59%  
FM056003

An object’s speed will decrease the entire time that a force is acting in the direction opposite its direction of motion.
 45% 
49%  
FM102001

A ball thrown into the air goes slower and slower on the way up, stops, and then moves faster and faster as it comes down.
 42% 
52%  
FM097002

A snowmobile sliding across a lake with its engine turned off will move slower and slower the entire tme it is sliding across the lake.
 42% 
50%  
FM094002

If an object is slowing down, a force that is causing it to slow down must be greater than the sum of the forces that are pulling or pushing the object forward.
 36% 
49%  
FM016004

If a school bus is slowing down, any forces moving it forward would have to be weaker than any forces slowing it down.
 38% 
39%  
FM055004

The speed of a sailboat will continue to decrease the entire time the force of the water slowing it down is greater than the force of the wind pushing it forward.
 33% 
37%  
FM101001

If an object is moving forward, and a force pushing it backward is greater than a force pushing forward, the object will slow down, stop, and then begin to move faster and faster in the opposite direction.
 31% 
34%  
FM096001

A hockey puck that is sliding across the ice will move slower and slower the entire time it slides across the ice.
 28% 
36%  
FM098002

A moving snowmobile will slow down the entire time that the force of the wind moving toward the snowmobile is greater than the force moving the snowmobile forward.
 30% 
31%  
FM004008

 22% 
22%  
Frequency of selecting a misconception
Misconception ID Number 
Student Misconception 
Grades 6–8 
Grades 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.