Hula-hooping robots

Directions for teachers:
To engage students before reading the article, have them answer the “Before Reading” questions as a warmup in class. Then, instruct students to read the online Science News Explores article “Wiggling robots reveal the physics of how Hula-Hoops stay up. Afterward, have them answer the “During Reading” questions.

As an optional extension, instruct students to answer the “After Reading” questions as a class discussion or as homework.

This article also appears in the June/July issue of Science News Explores. Science News offers another version of the same article written at a high-school reading level. Post this set of questions without answers for your students using this link.

Directions for students:
Read the online Science News Explores article “Wiggling robots reveal the physics of how Hula-Hoops stay up.” Then answer the following questions as directed by your teacher. 

Before Reading
1. Have you ever used a Hula-Hoop? If so, how long were you able to keep it moving about your waist before it fell? Do you remember any techniques or strategies you used?

Answers will vary.

2. To the best of your understanding, describe the forces that keep a Hula-Hoop from falling. Support your explanation by drawing a picture. Use arrows to illustrate the direction of different forces that might come into play when using a Hula-Hoop. Optional extension: Name the forces that you used in your illustration.

Answers will vary.

During Reading
1. Explain how an hourglass shape helped robots keep Hula-Hoops up.

As a Hula-Hoop swings around an hourglass shape, the slope at the bottom of the constricted middle generates an upward force that helps the hoop stay up.

2. What inspired Leif Ristroph to investigate the physics of Hula-Hoops?

Watching Hula-Hoop performers inspired his investigation.

3. What does it mean for an object to gyrate?

Gyrating describes a kind of movement where an object makes quick, small circles.

4. Describe two robot shapes Ristroph and his team tested besides the hourglass.

Ristroph and his team tested robots shaped like cylinders and like upside-down ice cream cones.

5. Pick one of the non-hourglass-shaped robots and describe the problem encountered when it tried to keep a Hula-Hoop in place.

Answers will include one of the following. (1) The cylinder-shaped robot’s Hula-Hoop kept sliding down. (2) The ice cream cone-shaped robot could not keep the hoop in one place. When started high on the cone, the hoop migrated upwards until it slung off. When started near the bottom, the hoop traveled down.

6. Explain how the robot’s shape contributed to the problem described above by comparing it to the performance of the hourglass-shaped robot.

Answers will include one of the following answers. (1) Since the cylinder-shaped robot was the same size from top to bottom, it lacked any constricted section that could help keep the hoop from falling. (2) On the robot shaped like an ice-cream cone, the hoop traveled up or down because the robot’s shape didn’t have sloped sections around a constricted middle to help keep the hoop in place.

7. Which of the tested shapes managed to keep a Hula-Hoop up after adjustments? Describe what adjustments were made.

The cone-shaped robot was able to successfully hold a hoop up after scientists adjusted its gyrations based on the position of the hoop.

8. Imagine you’re teaching a friend how to hula-hoop. Write step-by-step guidelines, including tips from the article on how to keep the Hula-Hoop aloft.

Step 1: Line the hoop up with your body so that when you launch, the body and hoop will move in the same direction (for example, start with your hips out to the right and the hoop out to the right).

Step 2: Start with a fast launch! When you first start to move the hoop, spin it fast.

Step 3: Move your hips quickly in circles.

Tip: The bigger the hoop, the greater likelihood of success.

After Reading
1. Imagine swinging a donut around your finger like a Hula-Hoop. As the donut swings, imagine the donut hole gets wider. You adjust your finger’s gyrations, thus managing to maintain a constant donut-spin speed. Predict how you changed your finger’s gyrations to successfully preserve the donut’s spin speed. Explain how you came to your answer by pointing to information in this story.

Scientists of this study explained that larger hula-hoops require a lower gyration speed than smaller hoops do. So, if you are spinning a donut around your finger like a Hula-Hoop, and the donut’s hole is slowly widening, you would need to slow your finger’s gyration speed to maintain the same spin speed.

2. Imagine a ball-shaped robot and a bottle-shaped robot trying to hula-hoop. Do you think a ball-shaped robot will manage to keep the Hula-Hoop up? What about the bottle-shaped one? For each robot, explain why you predict success — or not. Point to findings from this story to support your prediction.

The ball-shaped robot will probably not hold up a Hula-Hoop because it will not have a constricted middle. Answers will vary regarding the bottle-shaped robot, but some will predict that the hoop will migrate upwards until it slings off, similar to what is seen with the cone-shaped robot.