Floating and sinking

Introduction

Floating and sinking is a common activity in early years classrooms. Students' ideas about floating and sinking are intriguing. The strategies for developing their understandings discussed in this topic are examples of the probing, investigative and challenging activities that characterise effective science teaching and learning.

Key concepts of floating and sinking

The activities in this topic are designed to explore the following key concepts:

Early years

• Whether something floats depends on the material it is made of, not its weight.
• Objects float if they are light for their size and sink if they are heavy for their size.
• An object can be light for its size if it contains air, such as a hollow ball.
• Materials with a boat shape will float because they effectively contain air.
• Water pushes up on objects with an upthrust force.

Middle years

• Objects float if the upthrust force from the water can balance their weight (gravity force).
• Objects float depending on their density compared to water; for an object to float its density needs to be less than that of water.
• Objects float when air is enclosed in an object; their density is lowered, thereby increasing the likelihood of floating.
• The upthrust depends on the amount of water displaced.
• Objects float better in salt water (density of salt water is greater than that of pure water).
• Water surfaces have a cohesive force (surface tension) that makes them act like a 'skin'.
• Small, dense objects (e.g. a pin; a water spider) can 'float' on the surface of water without breaking it, due to surface tension effects.

Scientific terms associated with floating and sinking

• force: a push or a pull.
• density: amount of mass per unit mass of an object (i.e. the concentration of mass, or how 'heavy for its size' an object is). The density of water is 1 kg per litre.
• pressure: amount of force applied per unit area. At a given pressure, twice the area will experience twice the force.
• Archimedes' principle: A floating object will experience an upthrust force from water, equal to the weight of water displaced (pushed aside). It will sink into the water until it reaches the point where the weight of the water pushed aside equals its own weight. For an object that is floating, the mass of the material equals the mass of water that is displaced by the object (1 kg = 1 L of water). Dense objects cannot displace enough water to provide an upthrust force to counterbalance their weight, so they plummet below the surface. Objects made of material denser than water (e.g. a boat made of iron) can still float if they contain air so that the mean density is less than that of water.

Students' alternative conceptions of floating and sinking

Students will have views about at least three aspects of floating and sinking that differ from science views. These alternative views centre around the questions:

• What do we mean by 'floating'?
• What determines whether something will float or sink?
• What causes things to float (i.e. what are the forces involved in floating)?

Interviews reveal that students can attach different meanings to the term 'floating' and that these meanings vary depending on the context (such as observing real objects in water as opposed to viewing line drawings). The students still seem to be at the formative level with respect to this idea and there are likely to be students in most classrooms whose understanding of 'floating' differs from scientists. Some students could become confused if teachers do not recognise this.

Students have a range of views about why some things float while others sink. Younger students (7-10 years) often do not realise that there could be a single explanation. Their response is to give explanations for individual materials. The explanations offered could be described as partial explanations. They focus on specific aspects such as lightness or heaviness and fail to take into account other aspects (such as size) needed to formulate a general rule that would explain all cases. Very few students seem to have an understanding of flotation that approximates that of scientists. Others realise that they do not really know why things float or sink, but they appear interested to know.

A number of students think that the length of floating material, or the depth of water underneath or on top of an object, affects flotation level. Some further believe that floating material will sink if the part above the water is cut off, or if it has vertical holes put through it. After initial experiences with reshaped nonfloating material, almost all students realise that non-floating material can be shaped to float.

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