Work -Energy Theorem

I believe Ryne is working on this lab. Here are some comments from the other specialists.



Getting reliable, repeatable results using the motion sensor will be close to impossible. For example, substituting the motion sensor for the photogate in the Conservation of Energy Lab will give errors that are much larger and erratic.

Tommy Morgan
ASIM Physics Specialist

Good morning everyone,



I present myself to you once more as the resident GLX ignoramus. But at least this time, it’s just questions, not problems. I recently checked the equipment (including GLX) for 8 lab stations for the Work-Energy lab. I wanted to make sure that all Force Sensors, Motion Sensors, and GLX would do their job correctly before dropping it off at a school.

Although the write-up cheerfully proclaims (Equip. Setup, #2) that the GLX will open to a Position-Time graph, all 8 opened to a Force-Position graph. Also, when I moved to different graphs (by changing the axis parameters) I found that “position” was the default measurement of the x-axis for all three graphs. I don’t know how to create or change a default setting. Of course, we can change it every time we move to a graph, but is that how it must be done? If you’re wondering, yes, I had Position-Position graphs instead of Position-Time graphs.

The lab talks about using the portion of the Force-Position graph that is horizontal (Analysis, #6), however every time I ran a test, the Force readings alternated about an average position which was just about right, judging from the mass hanging on the string. What little horizontal part there was, usually did not extend beyond about 4 data points (at the most). This was with 8 different Force Sensors and 8 different GLX. Also, the reading was only to a tenth of a Newton while I could make reasonable guesses to the hundredth of a Newton simply by looking at the position of the data points on the graph. In other words, the graph would bounce back and forth and the cursor readings would be 0.2 N and 0.3 N (alternating) although I could see that the real points were probably 0.24 N and 0.26 N. Is there a reason for this or is that just the way it is?

In regard to using only the horizontal part of the graph: If we only use a portion of the Force-Position graph, say from 0.3 m to 0.45 m, then the Work value we obtain will be the work between those points. Why would we not need to find the change in velocity between those points in order to calculate the change in kinetic energy that correlates to the amount of work done from 0.3 m to 0.45 m? Yet the change in kinetic energy found by the instructions in the lab is from start to finish (Analysis, #7) using the max. vel. found before being concerned about a horizontal part of the Force-Position graph. Should we use the Linear Fit tool on the Force-Position graph and then use the distance from start to max. vel. to get a work value that would correspond to the max. vel. we’re using (from Analysis, #3)? Apparently this has not caused problems in the past, so there is something here I am not understanding.

By changing the measurement parameters of the x-axis on the velocity graphs, I think an interesting question to challenge the students with presents itself. To wit, “Why is the graph of Velocity-Time a straight line, but the graph of Velocity-Position has a decreasing slope, like a root curve?” I was thinking it might be good, although a little tricky, not to mention the Velocity-Time graph and just ask why, if the acceleration is constant, does the Velocity-Position graph curve downward instead of being linear?” They might assume it should be linear, not realizing that applies to the Velocity-Time graph, not Velocity-Position. Then they have to start thinking to make sense of it. I know this is not the purpose of the lab, but understanding what the graphs are saying is part of the COS.

I welcome any help with any of the above, and thank you.



Jeff Woods