What important physics idea is this question targeting?
This task is designed to see if students know that motion stops when the net force on an object is zero. (I)
This task reveals whether students believe motion implies a continuing forward force, or whether they can correctly reason that friction is the only unbalanced force after the push ends, producing a deceleration. (C)
This task examines if students can apply Newton's Laws in real-world settings. (V)
Which answer best describes what student A's response indicates about their physics understanding?
The student understands that friction continues to act after the push ends and correctly identifies it as opposing motion, which is a productive idea. However, they fail to realize that gravity does not play a role in slowing down and stopping the object. (I)
The student consistently identifies gravity, friction, and normal force with appropriate magnitudes, suggesting a decent grasp of force types and directions. Still, they’re struggling to distinguish between a force that causes motion and a force that persists during motion — evident in their statement that the push “sends it” but still influences the box afterward. (C)
The students diagrams and text indicate that they know how to correctly draw force diagrams and use those to interpret motion. They do not fully describe how they came to their conclusions. (V)
Which of the following options would best support Student B in being able to correctly respond to this question?
I’d give the student two diagrams — one of a box being pushed just before the hands stop making contact and one just after the hands stop making contact — and ask them to discuss how the speed changes, then how the acceleration changes, and then how the forces change. I would have them check their answers against Newton's Law that no net force means no acceleration (as opposed to 'no speed').” (C)
I would have them imagine doing this activity in space where there is no gravity and ask how that might change things. We would then add gravity and then add friction. This would build up the idea that unless there is friction the force of the push cannot disappear. (I)
I’d ask the class to brainstorm what causes motion to stop and then map their ideas to force types, including contact forces like friction and normal force, and non-contact forces like gravity. (V)
Which answer best describes what student B's response indicates about their physics understanding for this task?
The student realizes that the box is losing its forward motion because of friction, but in trying to describe that they use the wrong word; they are describing inertia — but probably just don’t know the right word for it yet. (I)
The student believes motion is sustained by a “leftover” or fading push after Kate stops pushing. This suggests they are thinking of motion as indicating the presence of a previously applied force. (C)
They have two of the four forces drawn but forgot about gravity and the normal force. You cannot have friction without those. (V)
Which of the following options would best support Student B in being able to correctly respond to this question?
I would explain that contact forces, like a push or friction, only exist when the objects are in contact. We would then try to explain a range of scenarios like this one using freebody diagrams that obey that rule. Students would put those diagrams on whiteboards to compare and discuss. (C)
I’d introduce the term ‘momentum’ and explain that motion continues briefly because of it, but then goes away. (I)
I’d have them read this section in their book, and then journal about what they think happens to motion and force after the hand lets go, and how that changes until the box stops. (V)