Satellites Spinning Round: Circles Part 3 (Tangents and Incircle)

Inspiration

Have you ever wondered how far a satellite is from the planet?  Sometimes people think about these objects flying through the sky and rotating around a planet.  Such wonders have even inspired greats like Isaac Newton and Kepler.

Some good folk over at illustrative mathematics put this whole satellite into some simpler geometry with circles, triangles, and tangents.

Inspired by these I sought out to simplify the idea of tangents and how they fit around a circle.  We played with this:

Questions

Warm up with some questions about the applet,
• What shapes do you see?
• Which measures, do you think are congruent?
go into some more typical math questions,
• What is the perimeter of the triangle?
get a little geeky
• What is the total area of the triangle (circle included)?
• How do you calculate the value for the area of the blue shaded area (circle excluded)?

Student discourse:

Along the way here were some awesome teachable moments:

T: What kind of segments do you see?
S1: Tangents
T: How do you know?
S1: ’cause it says ‘tangent’.
T: I’m still not convinced. Maybe I mislabeled it. Prove it.
S2: They’re perpendicular. Tangents are perpendicular.
T: Anything else? Any other types of segments or is that it?
[crickets]
T: What about inside? Any segments inside the circle?
T: Where?
S3: Here?
T: Where? Pretend I’m blind.
S3: You already are blind Butler, that’s why you wear glasses.
T: [fake smile]
S3: Here at AJ. Ohh, and also AF. And these ones to but you didn’t label them.
T: Do you notice anything else? What if you drag around that red thing?
S3: They’re all the same?
T: What are the same?
S3: The radiussus. Does that mean they’re equal?
T: What do you think?
S3: Yeah, equal. They have to be, cause of the red thing.

S: Hey Butler, is that a kite?
T: It sure looks like it huh? What do we know about kites?
S: There’s stuff that’s the same. Like congruent.
T: For example…?
S: The pieces that match.
T: So how can you use that?
S: They’re equal. Oh wait, they’re equal. Sooo….
And then the student goes on to apply that understanding to fill in the blanks for the missing segment lengths.

The idea of using a few measures, congruent relationships, and some triangle calculations helped the students apply the geometry and calculations further than just one calculation.  As a follow up assessment they completed a static version of the question.  This was easy to create by just taking a screenshot of the applet at some particular balance.

Some day I want to advance this further with some clock math.

2 thoughts on “Satellites Spinning Round: Circles Part 3 (Tangents and Incircle)”

1. I love the applet – thanks! I spent a lot of time last year developing how to use geostationary satellites in geometry. I spent a summer working at Lockheed and wanted to bring some of that amazement into the classroom. You can see the lesson I came up with at the link below. The applet is a way to make the lesson much stronger. With the app you can move the satellites where ever you want – but in the real world you can’t, or at least you wouldn’t – then let the students develop an hypothesis as to why that is the case.

https://mrmillermath.wordpress.com/2014/07/29/geostationary-satellites-in-3-acts/

By the way – how you been? I believe we met briefly at TMC14?

1. Thanks for the link to your activity. I like it a lot. I’ve been good since TMC14. I’m a District TOSA right now so things have also been very different. Many new challenges but a lot of new opportunities as well. Overall goal for the year has been to build collaboration and sharing across departments and sites. If you ever have ideas for visualizations and applets I always like a challenge there as well. Shoot one over if you think of it.