Science Assessment ~ Ideas ~ A force is a push or a pull between two objects.

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.

Sub-Ideas
Items & Student Performance
Misconceptions
NGSS

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
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%
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%
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.).	24%	20%
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%	22%
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.

NGSS Statements
Code	Statement
PS2.A MS	The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion.
PS2.A HS	Newton's second law accurately predicts changes in the motion of macroscopic objects.