Our students arrive in classrooms with multiple, interconnected ideas about how the world works. From the students’ perspective, these ideas make sense, and the predictions they make about the world are based on these ideas. So, what occurs when a student has an idea that is incorrect? Oftentimes these types of misconceptions remain unknown and don’t cause issues for students. Students are rarely required to confront these common yet incorrect ideas with observable, contradicting evidence. Thus, when a student is asked, “Why do astronauts float in the International Space Station (ISS)?” the student can simply maintain the idea that astronauts float because of a weaker force of gravity due perhaps (in their mind) to the great distance of the ISS from Earth. This lesson is based on teaching the standards shown in Table 1 (see Online Connections) and is designed to confront this common, yet incorrect, idea.

Student ideas

The ideas students bring to our classrooms are truly like a constellation: Interconnected and sometimes wrong ideas are perpetuated by a network of right ideas. In this case, the idea that the magnitude of the force of gravity weakens between two objects as the distance between them grows is accurate. However, attributing that accurate concept to the specific phenomenon of astronauts floating in the ISS is incorrect. The misconception in this case would be best described as a preconceived notion, according to the five categories of misconceptions presented by the National Research Council (NRC 1997). When describing preconceived notions, the NRC writes:

Campbell, Schwarz, and Windschitl (2016) propose leveraging students’ preconceived notions in the classroom through sensemaking activities supported by the Next Generation Science Standards (NGSS). They emphasize the importance of viewing student misconceptions as an opportunity for student sensemaking. In practice this means student misconceptions or preconceived notions are a resource rather than a deficit inside your science classroom. We encourage you to take this perspective as you enact this lesson.

So, how can we provide students with a sensemaking learning opportunity and ultimately address their misconceptions about gravity in a meaningful way? Gooding and Metz (2011) suggest several different instructional strategies to enable conceptual change for students: clarifying responses, asking for evidence or inferences, using wait time, encouraging peer-to-peer talk, playing devil’s advocate, and not seeking the “right answer.” These strategies also align well with discourse moves advocated for by Ambitious Science Teaching (2015): probing, pressing, re-voicing, peer-to-peer talking, putting ideas on hold, and managing the silence. These strategies and discourse moves are pivotal in supporting student sensemaking and will be important to use throughout this lesson to support students in making conceptual shifts in their understanding. In this article, we share a discrepant event to help foster conceptual change for students through multiple opportunities for sensemaking in their understanding of gravity and objects in orbit around the Earth.

Lesson description

Day 1

This multiday lesson is anchored with a phenomenon. Students are prompted to write down observations as they watch the video “Chris Hadfield’s Space Kitchen,” a YouTube video from the Canadian Space Agency’s page (www.youtube.com/watch?v=AZx0RIV0wss). This video is short (2:26 runtime) and shows how astronauts eat in space. Students will likely notice things such as “the food is floating” or “the astronaut can let go of their food and it doesn’t fall.”

As soon as the video ends, students have minutes to ask questions about the phenomenon they witnessed in the video. These questions can help the teacher guide instruction throughout this lesson and highlight specific concepts as they arise naturally. Then students transition into a modified think-pair-share activity. Students first individually develop a model explaining the phenomenon they just observed by answering the driving question: Why do astronauts float in the ISS? Students are encouraged to use drawings, free-body diagrams, words or full sentences; essentially, students have the agency to explain their observations and create their models in their own way. This portion of the modified think-pair-share activity provides students with an opportunity of sensemaking through the science and engineering practice (SEP) developing and using models. The NGSS describe models as helpful tools for representing ideas and explanations, and students will engage in this by providing drawings, terms, and diagrams to represent and explain their understanding. Students take about five minutes to complete this task, which serves as the “think” portion of the modified think-pair-share activity.

Next, students work in small groups (three to four students); this serves as the “pair” portion of the modified think-pair-share activity. It is important to be intentional in student groupings for this activity. You may want to consider your students’ talents and abilities to draw and/or model, write concisely, and exhibit leadership in discussing ideas. Choosing groups in a purposeful manner is one way to differentiate and ensure you meet all your students’ needs. During this portion of the activity, students discuss their explanations, present their models, and arrive at one consensus model to explain the phenomenon for the group. Once students have adequately discussed the strengths and weaknesses in each of their explanations and have arrived at one unified explanation incorporating a consensus from their group, they receive a large sticky note (25” × 30”) to re-create their consensus model. If you don’t have access to large sticky notes, other alternatives are small whiteboards, 11” × 14” paper, or 8.5” × 11” paper.

After drawing their consensus model, students stick their models along the four walls of the classroom. Students then engage in a gallery walk and examine every model posted. If you use 8.5” × 11” paper, instead of doing a gallery walk, we recommend either using a document camera or taking pictures of the models and displaying the pictures to share the models. Students then share the similarities and differences they notice among the models, along with which models they feel most accurately describe the phenomenon. This activity serves as the “share” portion of the modified think-pair-share activity. This portion of the lesson provides students with opportunities of sensemaking through the constructing explanations and communicating information SEPs. Students collaboratively represent multiple ideas and explanations through drawings, terms, and diagrams. Doing so allows students to use the process they prefer in their small groups to represent their thinking and serves as a natural differentiation strategy for the class. Lastly, the NGSS describe the SEP of communicating information as critiquing and communicating ideas individually and in groups, which is accomplished through the discussion at the end of the gallery walk as students develop better explanations by listening to their peers. Students typically need about 30 minutes to complete this portion of the activity.

From our experience, students will initially explain this phenomenon by claiming that gravity is weak in outer space. Some groups may even create a free-body diagram indicating how the ISS is far from Earth and experiences a weaker force of gravity. This is not the appropriate moment to correct these claims. Simply accept all ideas and state that you will arrive at an explanation together by the end of the lesson.

Day 1 concludes with the water bottle demonstration, which serves as the discrepant event for this multiday lesson. Table 2 (see Online Connections) provides the materials you will need, and instructions on how to carry out this demonstration (you can also refer to these videos: Demo 1 [www.youtube.com/watch?v=F4nDpEp9I3k] and Demo 2 [www.youtube.com/watch?v=HCFjgX3uyfY] to see how the materials are used together). In short, the water bottle demonstration has you as the teacher demonstrate the difference between two different scenarios. In the first scenario, you simply fill a water bottle that has a hole near the bottom of the bottle. You plug this hole with your finger and demonstrate what occurs to the water when you unplug the hole and hold the water bottle up with your other hand. Water simply flows out of the bottle and empties into a bucket below it. In the second scenario, you follow the same basic instructions, except you let go entirely of the water bottle while simultaneously unplugging the hole. The results can be seen when the footage is captured in slow motion with a phone camera but can also be perceived by the naked eye if you prime your students to pay close attention. Water does not flow out of the hole in the second scenario, and it typically goes against all predictions made by students!

Day 2

The second day begins by revisiting the anchoring phenomenon. Students share examples of astronauts floating (e.g., in popular movies or commercials). Students are then asked: How much weaker is the force of gravity the astronauts in the ISS experience than someone on the surface of Earth? You should anticipate some very low numbers; in fact, some students will claim that they experience 0% gravity since they are in space. Simply make a note of these predictions, either on the board or a piece of paper, as you will address these predictions later in the lesson. This lesson opener takes approximately 10 minutes.

After this opening activity, students are told that there is a way to calculate the force of gravity acting between any two objects. We suggest transitioning back to the models students created during the previous lesson and returning to their original groups. Students are prompted to identify the various components of their models, which could be represented by a variable or numerical value. The key here is to guide the discussion toward the concepts of mass, distance, and force. Students then label certain components of their models with known variables. This is how we introduced Newton’s law of universal gravitation: FG = (m₁m₂)/r².

After introducing the mathematical relationship between the variables, it is important to connect some of students’ models and explanations from the previous day back to this scientific law. Ideally, the posters will still be displayed from the gallery walk the day before, so you can point to the astronaut or ISS in their models and say this is m₁ and Earth is m₂. Additionally, you can discuss how the space between these objects in their drawings can be mathematically represented by r. As you have this discussion, you should focus on the inverse relationship between distance and the magnitude of the force of gravity; this is the mathematics behind their accurate intuition that gravity is weaker as two objects move away from each other. You can model this for them as an example by doing calculations using “your mass” for m₁, Earth’s mass for m₂ and increasing the r value from 10,000 m to 100,000 m by 10,000 m for each calculation to demonstrate the difference and changes in FG. We suggest spending no more than 15 minutes on this direct instruction time. You may need to provide a notes outline or a similar scaffold for students who struggle with taking notes or absorbing information from a lesson.

After this introduction to Newton’s law of universal gravitation, students go back into small groups (three to four students) and are prompted with the following question: How can you use Newton’s law of universal gravitation to validate your claims and explanations from yesterday? Students have about five minutes to discuss this question in their small groups. At this point, the discourse moves and scaffolding is key. As teachers we ask: How can we help students arrive at the conclusion that they can mathematically calculate the difference in force of gravity between a person on the surface of Earth and one in orbit some hundred miles above Earth’s surface? More importantly, how can we question students effectively so that we do not provide this path immediately and instead provide our students with a valuable opportunity to explore?

Our advice is to be patient and allow students to explore in this portion of the lesson long enough so that most, if not all students, agree there is one approach that can provide the evidence their claim requires. Students need to get to the idea that they can calculate the gravitational force between Earth and the astronaut when the astronaut is on Earth and when the astronaut is in the ISS. The mass of Earth and the mass of the hypothetical person will remain the same in both calculations. You may even want to give students the option of converting their own weight into kilograms so they can be the astronaut in this scenario. (We don’t recommend requiring this as some students may have weight or eating issues.) If you need to speed things along, you can provide the two varying r values or you can have students find these numbers, which are typically in miles and often differ. If you go with the latter option, there are a variety of numbers they can find online or in texts, so having students put the r values they find (converted into meters) into a shared spreadsheet is one way for the class to create an average value. After calculating the gravitational force in both situations (on Earth and in the ISS), students can divide the smaller, weaker force (for those in orbit) by the bigger, stronger force (for those on the surface). The final calculation will reveal an approximate weakening of the gravitational force of only 12%!

When students complete this final calculation, they are often very surprised. The force of gravity astronauts experience in the ISS is certainly weaker but relatively unchanged. Students then come to the conclusion that this does not explain why the astronauts float. It is important to have students articulate this idea and prompt them with: So, why do they float? To answer this question, tell students that you are going to revisit the water bottle demonstration. Ask them: Why did no water escape the bottle in the second demonstration?

Often, no students are able to provide an answer and that is OK! To help students understand, you can walk them through analyzing the images of the bottle and water falling that they created when making their initial predictions of the outcome. First, have them think through the force of gravity action on the water in both situations and diagram the forces at work. In the first demonstration, the bottle is stationary and the force of gravity on the water causes it to flow out of the hole. In the second demonstration, the bottle is falling and the force of gravity is acting on the water and on the bottle at the same time. The bottle and the water are moving toward Earth at the same rate when they are dropped. In other words, the water simply “floats” within the bottle as they both fall to the ground.

We connect this well-known phenomenon—that all objects fall at the same rate—to our astronauts in the ISS. The reason astronauts float is not due to a weakening in the force of gravity but instead can be explained by the same phenomenon we observed in our classroom. Both the astronauts and the ISS are in a constant state of free fall accelerating toward Earth at the same rate. This produces the effect of floating, or “virtual weightlessness,” we observed in the opening video on the previous day. Students often need time to digest this information and to restate it in their own words, so give them time to write down their own explanations to answer the driving question: Why do astronauts float in the ISS?

Assessment

To assess students in this lesson, students work in their original small groups to edit and revise their original group models for about 20–30 minutes. You will want to prompt them to use their new understanding of Newton’s law of universal gravitation and to use those variables where appropriate as part of their explanation. Groups receive a new large sticky note and can stick their new model over their old one once they are done creating their models. Students participate in one last gallery walk and discuss the differences and similarities across the new set of models. Table 3 (see Online Connections) shows the assessment rubric, which can be adapted to fit your needs.

Conclusion

The lesson described addresses a rather difficult and advanced concept for high school students, but it does so in an engaging, thought-provoking way. Students are engaged by the anchoring phenomenon and driving question, take ownership of their learning as they create models, and are pushed to make sense of evidence that contradicts their preconceived notions about gravitational force. Through this lesson—and with the support and patient questioning of an instructor—students’ initial ideas can be successfully leveraged to help them move toward a more accurate understanding of these concepts.

Online Connections

Table 1. Alignment with the Next Generation Science Standards: https://bit.ly/42pd8Di

Table 2. Materials and instructions for water bottle demonstration: https://bit.ly/3Txm0ms

Table 3. Modeling assessment rubric (Adapted from Boughey and Henriques 2020): https://bit.ly/42sG5yk

Yamil Ernesto Ruiz (ruizye@uscupstate.edu) is director of online learning & program support at USC Upstate, Spartanburg, SC. Brooke A. Whitworth (bwhitwo@clemson.edu) is an associate professor at Clemson University, Clemson, SC.