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  • Floating and sinking provides opportunities for students to observe how everyday objects behave in water and to investigate the factors that determine whether an object will float or sink.

    The New Zealand Ministry of Education’s Building Science Concepts (BSC) series presents sets of interlinking concepts that build stage by stage towards big ideas in science. A big idea shows how a fully developed understanding of the concept might look, but recognises that such an understanding might not be achieved until New Zealand Curriculum level 7 or 8.

    This resource is a partial replication of Building Science Concepts Book 37 Floating and Sinking: How Objects Behave in Water. It covers the science notes provided in the original BSC book. The overarching science concepts (big ideas) and how they may be scaffolded in sequence are illustrated in the image below.


    This resource is designed to build on students’ experiences with water and their observations of everyday objects floating or sinking.

    This resource also links to Building Science Concept Book 38 Understanding Buoyancy: Why Objects Float or Sink.

    The themes covered in BSC Books 37 and 38 include:

    • how everyday objects behave when they are put into water
    • why objects float or sink
    • density, volume and displacement.

    Floating and sinking

    We tend to think of objects as floating or sinking but objects can show varying degrees of buoyancy. An apple may float half in the water and half above, while an empty, sealed plastic bottle might bob on the surface of the water.

    We need to think about three elements.

    • The whole system – for example, a boat and a life jacket float and a person may sink, but a person-in-a-boat system or a person-wearing-a-lifejacket system both float.
    • Whether the system is supported by water – for example, the bobbing apple is supported by water so it is floating.
    • The situation at that moment – for example, a stone skimmed across the water’s surface floats while it is moving but sinks as it loses speed.

    Forces acting on an object – why does an object float or not?

    Three factors affect whether an object floats or sinks.


    Effect on floating

    The forces/tōpana on the object

    The force of gravity (weight/taumaha) pulls the object down.

    The support force (upthrust/pana whakarunga) of the water being displaced pushes upwards on the object.

    An empty sealed plastic bottle floats in water because the pull of gravity is less than the support force/upthrust.

    If you push the bottle into a bucket of water, two forces act on it. The push you are exerting downwards (along with a little bit of help from gravity) and the upwards support force produced by the water. This upwards force is what pops the bottle out of the water when you remove the downwards force (let go).

    The volume/rōrahi of the object

    The greater the volume of the object, the more water it displaces as it settles into the water. If the volume of the object is greater than the volume of water displaced, it will float.

    The water that the object is displacing is the water that moves aside as the object settles into the water. For example, think of the amount of water that would spill over the edge of a completely full bath when you immerse your body into it – this is the water that is displaced.

    The density/kiato/apiapi of the object – its mass/papatipu related to its volume

    If the object is less dense than the water it is displacing, it will float.

    If the object is more dense than the water it is displacing, it will sink.

    Shape/āhua/hanga may sometimes also affect the density of an object if the object ‘holds’ air.

    The volume of an object

    The volume of an object is important in determining whether it will float, as the upthrust force of the water is equal to the weight of this displaced water, and the amount displaced is determined by the volume.

    If changing the shape of an object that sinks increases its volume, the object will displace a greater volume of water. This displaced water tends to return to its original position and, in doing so, exerts a force on the object known as the upthrust. More water displaced equals more upthrust. For floating to occur, this upthrust needs to be equal or greater than the force of gravity operating on the object.

    For example, a ball of modelling clay will sink, but if you shape the ball into a boat shape, the clay will float. The boat shape has a larger volume, which displaces more water than the ball shape, until the upthrust equals the boat’s weight. Another way of viewing this concept is that the boat shape floats because it holds air and so the boat+air system is less dense than the water.

    Density of an object

    The density of an object is the relationship between its mass and its volume. Density refers to how tightly packed the particles of an object or system are. A dinghy containing three people might float easily – for its volume, the boat contains relatively little mass. But if an additional 10 people step onto the dinghy, the mass has increased to the point where the overall density of the dinghy+people system becomes more than the density of the water it is displacing, and it begins to sink. The force of gravity, due to the extra weight, pulls the dinghy and its occupants downwards. The force of gravity has become greater than the upthrust of the water on the boat’s surface.


    float because ...

    Apples, icebergs, dry pumice, empty (upright) plastic bottles, dead fish (which contain air) and dry paper

    • they are less dense than water

    • the force of gravity pulling down on the mass of the object equals the upthrust of the water on the object, and part of the object remains above the water’s surface.

    Soup bowls, waka, ships and dinghies

    • their shapes add air to the overall system, which makes their overall density less than that of water.


    sink because ...

    Pumpkins, anchors/punga, stones, plastic bottles filled with water and waterlogged cardboard

    • they are more dense than water

    • the force of gravity pulling down on the mass of the object is more than the upthrust of water on the object.

    Baking trays (inserted vertically into water) and dressmaking pins

    • their density is greater than that of water, and when inserted vertically, there is insufficient surface area to experience upthrust to make it float.

    Alternative conceptions

    These are some alternative conceptions that students may hold:

    • The size of an object determines whether it floats or sinks – small objects float and large objects sink.
    • The weight of an object determines whether it floats or sinks – light objects float and heavy objects sink.
    • Soft objects are more likely to float than hard objects.
    • Objects that float have air inside them.
    • Floating objects must sit wholly above the surface of the liquid.
    • If two objects weigh the same, they will both float or both sink.

    Younger students may equate the smallness and lightness of an object with its ability to float. They are less likely to consider the object’s volume in terms of how much water it displaces.

    If volume is the only factor considered, students may think that objects of the same density but different size/rahi – for example, small and large pieces of candle – will behave differently. They may expect the small pieces to float and the large pieces to sink, whereas pieces of all sizes will float because they all have the same density, which is less than that of water.

    Related content

    Use these articles to further explore aspects of floating and sinking.

    Use these images to gather students’ ideas about floating and sinking.

    Our Floating and sinking Pinterest board is full of related resources.

    Literacy resources

    A sinking feeling is a title from the Connected series and introduces the concepts that underpin floating and sinking in the context of a boat race. The teacher support materials provide curriculum links and include suggestions on how the article can be used to grow science capability in critiquing evidence.

    Activity ideas

    Will this float or sink? uses an interactive or paper-based graphic organiser to consider whether an object floats or sinks. Use it prior to a unit on floating and sinking to gauge students’ thinking and again during and after the unit as formative assessment.

    Investigating floating and sinking is a set of activities that use play and exploration to directly observe how everyday objects behave in water.

    Floating and sinking – exploring forces use play and exploration to explore the support force (upthrust) that keeps objects afloat.

    Temperature, salinity and water density – use this activity to help your students visualise differences in water density.

    Floating eggs – students investigate water density by floating an egg in freshwater and saltwater.

    Buoyancy in water – students make a Cartesian diver to demonstrate the relationship between volume, mass and density.

    Investigating seawater – students investigate some of the properties of seawater.

    The Tip of the iceberg image shows how ice floats in salty seawater. Place an ice cube in a glass of water and observe if it floats in the same manner.

    Useful links

    Related Building Science Concepts books

    Additional information and activities on this topic can be found in the chapter ‘Gravity and Flotation’ in Making Better Sense of the Physical World (Ministry of Education, 1999).

    Assessment Resource Banks

    The Assessment Resource Banks (ARBs) also offer a range of levelled activities that are ready for use in the class. You need to be registered to use ARB resources.


    This resource is a partial replication of the New Zealand Ministry of Education’s Building Science Concepts Book 37 Floating and Sinking: How Objects Behave in Water.

      Published 11 October 2022 Referencing Hub articles
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