Posts Tagged: motion

motion analysis: freefall

On February 9, 2022 - course notes, investigations

Adam went to the trouble of throwing a ball up into the air in his office. The ball not only went up; it came back down. Here’s video of the event:

(It’s also available on YouTube, here.)

You can study this motion in a variety of ways. In another video example, I suggest that you analyze the frames while using some kind of timing device. That could work here, especially if you can advance the video one frame at a time. Another way of doing this is with some video tracking software, such as JS Track, available online as a web based application. To use this, do the following, OR take a look at this video I made showing you the steps I use.

  1. Download the video that I’ve provided above; or, you can record your own!
  2. In JS Track, you’ll be prompted to upload the video file there. It’s best if the video is in mp4 format. That’s pretty natural for a lot of videos you record, but sometimes you’ll want to find a way to convert it by doing a quick search on the internet.
  3. Within JS Track, you can advance the video one frame at a time, starting with the first frame that’s of interest. I usually start with when the ball first leaves my hand. Then, you can leave a mark or point on your object (the red ball, in this case), and then the program will advance the video one frame. If you keep repeating this, you can get a collection of points.
  4. JS Track will then provide you with a bunch of position and time data, just like we created with rolling motion in class. It has a spreadsheet you can use if you download a copy and edit it for your own use, either in Google Sheets or Excel or something similar.

There’s more to talk about, but that’s exactly what the point of this assignment could be!

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elevator physics

On September 17, 2020 - course notes, investigations, phenomena

I like to send people into elevators with scales that they can stand on while traveling up and down. It’s a great exercise because they get to see some physics that they are actively a part of. At the same time, it becomes a nice conversation piece as different, surprised observers come in and out of the elevator we’ve turned into a laboratory.

I spent some time myself on the scale on an elevator, and I made a point of recording a round trip from the bottom floor to the top and back again.

Standing on a scale while the elevator goes up and down.

I think it’s really important for you to know that this scale, like many, is a little sticky and is probably only trustworthy within a pound or so. That is, I think anything that between 158 to 162 pounds is really the same. Keep that in mind as you watch.

You can watch the video as many times as you’d like and look for connections between the motions of the elevator and the readings on the scale. What patterns do you see? What do you think the cause and effect relationships are? In other words, what makes the scale reading change; and what does not cause the scale to change from its normal reading?

This might inspire other experiments you can do on elevators. Does it matter if the person is bigger or smaller? If the elevator is faster or slower? What if you were on a roller coaster or other ride that might move you in more drastic ways? Can you model how the pushes and pulls on the rider would change?

Epilogue:

I made a new video, this one without captions but a smoother responding scale. I think it could be useful for another round of observations, or even as a place to start:

Elevator round trip, version 2.0.

Epilogue again!

Another elevator trip with a different scale that is “zeroed” with the weights on it so it gives positive and negative values. We also have passengers coming on board the elevator, which is fun to hear.
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motion analysis: rolling

On September 10, 2020 - course notes, investigations

I love to do this lab or one similar to it in person, but you can also conduct an investigation about motion on your own. I’ve created some videos that you can use to collect data (and maybe these will inspire you to setup a situation from which to collect your own data) and I’ve also given you a little bit of video instruction to help out.

Here’s the basic idea: You want to figure out how to characterize motion, but all we can really measure directly is a position (“where”) and a time (“when”). We look for changes in these two things to describe motion.

I’ve just found a pool ball and a smooth table that the ball will roll on. (Like I said, you could do this as well, but it turns out I have a really nice setup for this.) You will want to compile some data about when (time) the ball is in different locations (positions). By getting this motion of the ball on video, you have the ability to repeat the same motion over and over and collect whatever data you need. In this case, I’m suggesting that you collect data for the time it takes to go from the start position of 0cm to another given position. I’ve marked increments of 10cm, so you can get the time it takes to get to the 10cm mark, the 20cm mark, the 30cm mark, and so on. The biggest distance I have marked on the video is 120cm. By replaying the video and running your stopwatch 12 different times, you can get 12 different data pairs of position and time.

I explain this here:

My overview of what you’re doing with the next two videos.

Then, you can jump into collecting data. Start with this video of the pool ball on a flat table. There’s two different versions of the motion, one in real time and the other in slow motion. Just pick one of these.

Rolling motion on the flat table.

Like I said, you can pause and go back over and over, each time finding the time it takes the ball to go from 0cm to another mark on the table. Record those times with their corresponding positions in your notebook.

Then, you can do the same with this video of a ball rolling on a sloped table:

Rolling motion on the sloped table.

Once you’ve made all your measurements, your data can go into a spreadsheet or another table, and then from this you can create a graph. By tradition, and so that we can all compare our graphs to one another, your graph should have the positions on the vertical axis (“y-axis”) and the times on the horizontal axis (“x-axis”). So, a blank version might look like this:

Example of how a position vs. time graph would look like before you've put data in it.

But you’ll be filling this in with your own data. You can do this by hand, of course, but it’s also straightforward to have a spreadsheet (Excel, Google Sheets, etc.) make the graph for you as you input your data. To give you an idea of what I mean and to get you started, here’s a template for a spreadsheet that you can copy or download. You can then edit your own version to your heart’s content. I’ve set this up so that as you input data in the appropriate columns you should see the graphs form magically, all by themselves. You’re also welcome to change the settings for the graph, although I’ve tried to make it so you don’t have to.

Enjoy! I’m excited to see your data and the patterns your data create. You’ll be thinking about why it looks this way and we’ll talk about what this all means. Your assignment will tell you what I’m looking for in your report.

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