An object floats when the weight force (gravity) on the object is balanced by the support force (upwards push or upthrust) of the water on the object.
A plastic toy duck floats in water because the pull of gravity is less than the support force. If we push the toy down into the water, this additional force upsets the balance – making the downwards force greater than the support force of the water. If we let go (stop pushing), the downwards force is less than the support force and the duck will pop up to the surface of the water. If we leave the toy duck alone, it floats because the weight force is once again balanced with the support force.
The article Building Science Concepts: Floating and sinking has additional information that may be helpful when exploring forces and other concepts.
In this activity, students explore the forces that underpin floating and sinking.
By the end of this activity, students should be able to:
- use a variety of objects to experiment with the forces of floating and sinking
- begin to use content vocabulary
- begin to discuss the forces that act on the objects in the water
- draw a diagram to label what they noticed.
Download the Word file (see link below).
Related content
Building Science Concepts: Floating and sinking supports learning about Physical World concepts of density, displacement and forces.
The level 4 Connected article A sinking feeling introduces the science concepts underpinning floating and sinking in the context of a boat race.
The PLD article Physical World – Floating and sinking curates Hub resources on this topic.
Our Floating and sinking Pinterest board is full of related resources.
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.
Buoyancy in water – students make a Cartesian diver to demonstrate the relationship between volume, mass and density.
Useful links
Related Building Science Concepts books
- Floating and Sinking: How Objects Behave in Water (Book 37) – explores floating and sinking in water. This is aimed at levels 1–2.
- Understanding Buoyancy: Why Objects Float or Sink (Book 38) – explores floating and sinking including in other media. This is aimed at levels 3–4.
