Floating and sinking – exploring forces
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.
Floating toy duck
The toy duck floats because the weight force (gravity) is balanced by the upwards push of the water. The toy is hollow, which increases its volume along with the upwards push.
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.
Exploring forces
Pushing down on a floating object causes the object to sink in the water because the downwards force becomes greater than the support force of the water.
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.
