Applying a predict–observe–explain sequence in teaching of buoyant force

In Serbia, buoyancy, Archimedes' principle and the concepts of floating and sinking are taught during the seventh grade of elementary education, as part of the theme 'Equilibrium of objects'. Floating and sinking are conceptually difficult scientific topics. Complete understanding of the causal structure behind floating and sinking demands nontrivial knowledge, which includes analysis of the relationship between buoyant force and the force of gravity. Textbook authors and teachers often hide behind the concept of 'relative density' when 'explaining' these phenomena. However, even the idea of density is, by itself, problematic for many students [1] and, as a consequence, difficult to teach [2].

Most of the students hold certain ideas and concepts about floating and sinking. These concepts are formed through limited everyday experiences, and therefore most of them are either misconceptions or valid only in special circumstances. They are not in accordance with proper scientific explanations, and are usually referred to as 'alternative conceptions'. However, since the phenomena in question are deceptively simple, students tend to cling to their ideas with strong conviction, and are often surprised by the possibility that they might be wrong. For this reason, overcoming students' alternative conceptions about floating and sinking is often a daunting task for a physics teacher.

Research [3] has identified the ten most frequent alternative conceptions of students.

• (1)  
  Big/heavy things sink, small/light things float.
• (2)  
  Hollow things float; things with air in them float.
• (3)  
  Things with holes sink.
• (4)  
  Flat things float.
• (5)  
  The sharp edge of an object makes it sink.
• (6)  
  Vertical things sink; horizontal things float.
• (7)  
  Hard things sink; soft things float.
• (8)  
  Floating fillers help heavy things to float.
• (9)  
  A large amount of water makes things float.
• (10)  
  Sticky liquid makes things float.

Yin et al have designed a test [3] comprised of ten multiple choice questions aimed at discovering these alternative conceptions regarding why things sink or float. We applied an expanded version of this test to three seventh-grade classes, totalling 74 13-year-old students, during the spring semester of the 2011/2012 school year. The test was administered before teaching the lesson on buoyancy, floating and sinking, and then repeated two weeks (and four lectures) later. We saw that the students had indeed formed their own mini-theories, based on a very limited set of experiences. In addition to this, presenting the topic on buoyancy and Archimedes' principle using classical teaching methods did not lead to complete removal of commonly held alternative conceptions and, in consequence, to a proper understanding of the floating and sinking phenomena.

In this case, the lesson on buoyancy and Archimedes' principle was given using a traditional approach, with a classic verbal method, followed by periods in which the students were given common written problems to solve. Some of the time was reserved for the application of new teaching strategies, based on elements of active learning, to determine students' reactions and whether there would be any improvement in the results.

1.1. The predict–observe–explain sequence in physics learning

This experiment [8, 9] was designed with the POE teaching strategy in mind, and was performed after the lecture on buoyant force and Archimedes' principle. Students were given a worksheet to record their predictions, observations and comments. They followed the teacher's instructions, which included writing down answers to the questions they were asked. The stages of the experiment were as follows.

2.1. Part one: 'measure–explain'

Using a permanent marker, two parallel lines are drawn on the neck of a deflated party balloon, 1 cm apart (figure 1(a)). Then the balloon is filled with water. The balloon is held by its tip, so that it hangs freely in the air (figure 1(b)).

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Notice the change in distance between the lines. A ruler is used to measure the new distance between the lines. Students are asked to note the new distance on the worksheet and explain why the balloon's neck is now elongated. This stage has an additional advantage when compared to the traditional demonstration of buoyancy using a weight and a dynamometer, since it is leads to a more noticeable visual impression as the whole balloon changes shape when water is added.

2.2. Part two: 'predict–measure–explain'

The teacher announces that the water-filled balloon will now be immersed in a transparent water container, so that only the neck of the balloon will remain above the waterline. The students are asked to predict the distance between the two lines marked on the balloon's neck in this case. Several possibilities are offered and the students should describe the reasoning behind their answers. Then the balloon is actually immersed in water (figure 2). The students are asked which prediction has been proven right, as well as to mark the new distance between the lines on their worksheets. Then, they are asked to explain the observed situation in detail.

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2.3. Part three: 'evaluate'

In part one, when asked why the balloon's neck is elongated when the water is poured in, 48 students gave acceptable answers which included: increase in total mass and weight of the balloon due to the added water (40 students); stipulating that the balloon is made of elastic material and with added water the increased force of gravity will extend the balloon (five students); increase in the mass of the balloon and thus increase in the force of gravity acting on it (three students).

Typical students' answers were as follows.

• 'The mass of the balloon has increased due to the added water, and thus the neck has elongated.'
• 'The balloon has gained in weight when it was filled up with water and so the neck has extended.'
• 'After the water was added, the weight increased and since the balloon is made of elastic material, its neck stretched out.'
• 'Because of the increase in mass, the force of gravity acting on the balloon has increased and therefore the neck has extended.'

Twenty-three of the students answered that the neck of the balloon elongated because of the added water, without any additional explanation. These students were either really unable to apply their knowledge of physics to everyday problems, or there was a lack of written communication skill on their part. This test, with its current design, cannot help us to differentiate between the two. Since a significant percentage of the students are involved (figure 3), future modifications to the worksheet should pay more attention to overcoming this issue through the addition of extra questions. Interviews with students could also be used to overcome the possible presence of written communication barriers.

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In part two, the students were asked to predict what would happen to the distance between the lines on the neck of the water-filled balloon when it was immersed in a water container so that only its neck remained above the surface. A total of 49 students gave unacceptable answers.

• A wrong prediction was given by 27 students.
• A correct prediction, but without any argument, was given by 14 students.
• A correct prediction, but only mentioning that the distance between the lines would revert to the original value because the balloon was now inside a water-filled container, was made by eight students.

The students made the following kinds of wrong prediction.

• Nine students predicted that the distance would be the same as when the water-filled balloon was suspended in air; five of these students gave no explanation and four explained their prediction by the absence of water in the balloon's neck ('there is no water in the neck of the balloon, so the distance will remain the same as it was in the air').
• Four students expected an even greater elongation, without offering any explanation.
• Another nine students predicted that the distance between the lines would be reduced to a value between 1 cm (when the balloon was empty) and 3 cm (when the water-filled balloon was held in the air). Most of these students understood that a buoyant force would be acting on the balloon, but were unable to estimate its value. They typically explained their predictions by 'the distance will be reduced, but the amount of reduction depends on the buoyant force'.
• Similar explanations, referring to the inability to estimate the value of the buoyant force, were also offered by three of the five students who picked the last option, 'it is impossible to predict'.

A third of the students (25) gave the correct prediction with an acceptable justification, including buoyant force; seeming reduction of the balloon's weight when it is submerged; approximately equal average density of water and the water-filled balloon. Some of these justifications were as follows.

• 'The distance between the lines will be the same as when the balloon is empty because the buoyant force seemingly reduces the balloon's weight and the distance returns to the original value.'
• 'When the balloon is filled with water, it will have the same density as its surroundings. Therefore, the balloon would have floated in water, and the distance between the lines will be the same as before the water was added.'

Only one of the students mentioned equal and opposite forces of buoyancy and weight keeping the balloon in balance.

After the balloon was actually immersed in the water, the students were asked to note which of the predictions had been correct (all of the students answered correctly) and to give an explanation for the observed situation. Compared to the explanations given during the prediction stage, the number of acceptable answers increased from 25 to 54, with the number of unacceptable answers falling from 49 to 20 (figures 4 and 5). Most of the acceptable explanations included the buoyant force acting on the balloon, a seeming reduction in the submerged balloon's weight and comparisons between the densities of water and the water-filled balloon.

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Students that had made the wrong prediction during the first part of the activity mostly explained the observed situation as follows.

• 'Buoyant force is acting on the submerged balloon, seemingly reducing its weight and returning the distance between the lines to the original value of 1 cm.'

Students that had already made the correct prediction during the first part usually offered more detail during the second part. For example, the following explanations.

• 'When the balloon is submerged, the buoyant force starts to act on it, in addition to the force of gravity. The buoyant force is acting vertically upwards, and that is why the distance between the lines is now the same as before the water was added to the balloon.'
• 'The distance has reverted to the original value because the density of the water-filled balloon is approximately equal to the density of water, so the weight of the balloon is balanced out by the buoyant force.'

While the percentage of correct explanations had increased significantly, 20 students still failed to provide an acceptable answer even after the demonstration. This result implies that a more complex activity design might be warranted in the future, with peer-to-peer learning discussions taking place between students after the first stage of individual predictions is complete. Research [10] has shown that exchanging ideas and discussing physical problems with their peers helps students with their understanding of scientific concepts. However, while making individual predictions requires only a small amount of extra time, compared to traditional demonstrations, discussions between students are much more costly in this respect, limiting their applicability. Additional investigation should be made into combining predict and discuss modes to help design a more complex, but still feasible, activity.

3.1. The students' evaluation of the activity

Application of the worksheet for the experiment with the water-filled balloon, designed in accordance with the POE teaching strategy, has clearly demonstrated the efficacy of active methods in physics teaching.

The first stage of the test showed that one third of the students failed to give a concrete answer to a simple question of why the balloon's neck extended once water was poured in. Since this is a large percentage of the students, future designs of the worksheet should include additional activities, such as direct conversation with students, in order to overcome possible difficulties with written communication. This would help to distinguish the inability of a student to apply his or her knowledge of physics to everyday phenomena from communication issues.

The second part of the activity leads to the conclusion that giving students the opportunity to test their ideas directly leads to positive shifts in the way they think and understand natural phenomena. This result, combined with overwhelmingly positive reactions from the students to the described form of activity, clearly underlines the need for a series of activities designed to improve the understanding of buoyancy. Additionally, the percentage of correct predictions could be increased by asking students to discuss their individual answers in small groups, and then make new predictions.

It is clear that the students really enjoyed this activity; one of the elements for its redesign might include asking the students to self-evaluate the contribution it has made to their knowledge and understanding of buoyancy.

Expanding on this, and in the context of results obtained by the diagnostic test for detecting common alternative conceptions regarding sinking and floating, the authors plan to design activities that would enable a different approach to the teaching of buoyancy and Archimedes' principle. These activities will diverge from the traditional teaching methods by giving students multiple opportunities to directly test their own ideas regarding sinking and floating. In this way, the students could progress from common-sense explanations towards the scientific approach in interpreting natural phenomena. These activities will be carried out by the authors in future studies. One of these will be based on improving the described activity and worksheet by adding new elements. The redesigned activity would be more time-demanding, but authors hope it will lead to better results.