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UCM: Two Days

April 4, 2017

Here is the gist of circular motion:

1. Elicit initial thinking about UCM through carnival swing ride. Where would rider go if the chain broke? Students asked to record thinking on a sheet.

2. Suggest that this a question about forces and motion. So Then review our ideas about forces and motion in 1D through clicker questions, using physics aviary.  Also  do 2D constant force. 

3. Ss explore how to get hover puck to move in a circle with meter stick. We discuss our observations, then together observe string used to make go in a circle.

4. Ss asked to draw motion diagram for circular motion in both cases, with force vectors added. What’s similar about what meter stick and string exerted forces? Establish a rule: a force pointing toward the center of the circle is needed to keep an object going ina circle. 

5. Why? Clicker questions asks why an inner force is needed: 

A. Inner force balances the force trying to throw the hover puck out.

B. Inner force is all that is needed to keep the puck turning around the circle.

C. A force in the direction of motion is needed to keep the puck moving, while an inner force prevents it from drifting away. 

Call these these the “inner only”, “balanced in and out” and “around the circle” rules. 

6. Testing experiment. Ball rolling in a metal ring. The ring is suddenly removed. Goal is to predict what each rule would predict. Ss reason through a before during after diagram. In the before, Ss asked to draw FBD before ring pulled; in the during, Ss argue about how forces change when pulled away (and why); and for the after, students use rules from part 2 to argue where all should go. 

7. Observe outcome of experiment and help Ss to see how consistent with inner only rule.

8. Introduce textbook description. Practice with a few clicker questions. First couple about identifying what force is the center pointing force. 

9. Harder question is about FBD for bottom of pendulum swing. Debate. Observe. Refine rule to be about Fnet needs to be center pointing.

10. Return to original question. Ss asked to rethink and discuss how thinking now is different than their thinking before. Even if their answer hasn’t changed, their thinking is likely different. 

Day 2:

10.  Revisit the pendulum swing. Ask question about what changes could be made to make the tension force larger! Likely they will say higher mass, faster swing, may need help with tighter circle. Will press for other contexts in which ideas make sense. Curious what other ideas they will have.

11. Ss are asked to design an experiment to test how Fnet depends on either mass, speed, or radius. Not sure what I will actually ask Ss to do. I think Force mass or force vs Radius. Easier to carry out and either proportional or inverse. 

12. Some magic happens depending on what Lab they do vs what Lab data I show them. Introduce Textbook ideas about quantitative uniform circular motion. 

13. Some practice clicker questions or ranking tasks.

14. Problem solving in context of pendulum swing. Given this photogatedata, what is force sensor reading? Compare prediction to obsevartion. 

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