This past week, students in my teaching physics course spent a fair amount of time examining student responses to MC physics, and reflecting on what all of that meant, and how well they were at predicting student responses and the difficulty of those items.

Today we tried to gather together all the things they noticed during discussion. Here’s what they noticed:

Students have trouble with circular motion… sometimes thinking that objects can keep curving even when forces aren’t present (maybe they are thinking about something like a curveball that can keep curing after you throw it)… or thinking that objects will get thrown out of a circle (they mentioned the term “centrifugal” force).

Students have trouble with force:

Students think that contact forces can continue to have influence on objects even after they are no longer in contact. Some thought that maybe students were confusing the concepts of force, momentum, and inertia.

Students think that a force (or more force in the case of competition) needs to be in the direction of motion.

Students have trouble with Newton’s 3rd law: They might think that heavier things exert more force, or that more active things exert more force.

Some students don’t seem to think that a surface can exert a force. Some of us thought no students would, but others thought students would definitely.

We can look at pairs of questions and see that questions that we think are similar are not similar to students:

For example they might think that a ball keeps curving in one circular motion question but that it gets thrown out of the circle in another. We might have thought that students answer inconsistently.

By looking at student responses to shape of trajectory for impulsive and continuous forces, we can see they don’t necessarily understand the difference between impulsive and continuous forces. We couldn’t have know this by looking at one question.

Students sometimes give wildly unexpected answer… like answering that an object dropped out of plane would fall backwards. Sometimes it’s hard to remember what it’s like to “not know the physics”. But on the other hand, if we think about it we can often come up with reasons why a student would answer that way (e.g., from the perspective of the plane, the cargo would fall backwards).

Some questions have one really common incorrect answer, while others there are more scatter.

Many of the wrong answers are similar across different questions: There are many questions where students indicate a force in the direction of motion.

Next, we focused in a bit on some specific questions and tried to come up with rules for what students might do when faced with  similar problem. Here is what they came up with for a velocity question.

They might think that same position at the same time means same speed

They might just look for some really obvious pattern in the representation and based an answer off that

Here is what they came up with for an acceleration question.

They might think that velocity is the same as acceleration

The might think that velocity and acceleration are closely related, but not the same (i.e., more velocity implies more acceleration, or velocity implies acceleration, or acceleration causes velocity)

I then gave each pair of students a new question to work on. They had to solve the problem correctly, and then try to solve the problem as they imagine a student might. While the problems we inferred the rules from were for “strobe diagrams”… I gave them questions that mostly involved graphing problems. Groups shared out what they did. For the most part, we found that our rules worked pretty well even for other problems. We did however need to add a rule and modify a rule them slightly, including

“Students might calculate velocity as position/time”,

and that they might think that “being ahead means moving faster” in addition to “having same position means same speed”