What important physics idea is this question targeting?
The question is meant to see if students know that accelerating an object requires a force in the direction of motion. (I)
This question helps reveal whether students think that constant motion requires a constant net force. (C)
This question explores how students reason about forces acting in different directions on an object in motion.(V)
Student A's response:
"Because of Newton’s Laws, every action has an equal and opposite reaction. So the weight of the bucket pulls down on the rope and the rope will pull up with the same force. So the force of tension will match the weight of the bucket. "
Which answer best describes what student A's response indicates about their physics understanding?
This student clearly understands Newton’s Third Law and used it to correctly solve the problem. However, they have trouble describing their ideas in a claim-evidence-reasoning structure and it is hard to follow. (I)
The student knows about “equal and opposite forces” and is applying it to explain why the rope pulls with the same force as the weight. That phrasing suggests confusion between interaction pairs (Newton’s 3rd) and balanced forces acting on a single object (Newton’s 1st). (C)
The response suggests they’re developing their understanding of the role of force in maintaining motion within a system. (V)
Which of the following options would best support Student A in being able to correctly respond to this question?
I’d ask the student to draw a force diagram for the bucket and label all forces acting on it, then do the same for the rope. We would then label which forces are "equal and opposite" because they are Newton's Third Law force pairs. Then we’d connect their diagram back to Newton’s First Law and the idea of zero net force. (C)
I would give student groups a spring scale to use to lift an object at various speeds so that they can see how the weight of the object gets heavier when you lift it up faster. We would then use this to diagram what must be happening when the rope is lifting the bucket before finally drafting a clear statement about our claims. (I)
I’d ask the student to reflect on the role of forces in sustaining or changing motion, and how those relate to different laws of motion. We would then use those rules to revisit this question. (V)
Student B's response:
"Pulling the bucket up with it’s equal force wouldn’t cause the bucket to move up or down. Pulling with less force would cause the bucket to move downward. The only logical answer is that you would have to use a strength greater than the weight of the bucket."
Which answer best describes what student B's response indicates about their physics understanding?
The student is probably thinking of friction or air resistance — maybe they assume there’s extra resistance that needs to be overcome and so you need to pull more strongly than just the weight. (I)
The student thinks that equal forces result in no motion at all, rather than understanding that equal and opposite forces result in no acceleration. Their logic shows they’re reasoning from an intuitive but incorrect idea that force is needed to “keep things going." (C)
The response shows they are applying force concepts to real-world systems, and just need support aligning that thinking with physical principles in a scientific way. (V)
Which of the following options would best support Student B in being able to correctly respond to this question?
I’d have them use spring scale and a mass — maybe even videotaping with a phone as they go. First pulling to start motion, then maintaining constant speed — and compare what the scale reads in each case, and then create diagrams of the forces in two scenarios. This lets them see that starting motion requires more force, but maintaining constant motion just requires that the upward force equals the weight. (C)
I’d have the student pick up a heavy object like a book or a bucket and slowly lift it upward while reflecting on how it feels. Then I’d ask them: ‘Does it feel like you’re working just to keep it moving steadily?’ and ‘Would you say your muscles are applying more or less than its weight?’ I’d encourage them to notice how it takes effort the whole time, and then draw a force diagram that captures that effort. (I)
I’d guide them in making more precise distinctions between types of motion and the forces required to sustain them; we would capture the ideas on posters around the room and lead a gallery walk to summarize ideas. (V)