Resources: Displacement can (can be homemade), ruler, mass balance, beaker, measuring cylinder and a selection of objects to measure (uniform material and some cube or cuboid in shape).
Handout: An investigation sheet for the students to examine volume and density.
The concepts of water resistance, upthrust and buoyancy are introduced in upper KS2. Instigate a class discussion to recap on these terms and ensure the students’ understanding.
If heavy objects sink, why do large ships float? Ask the students to offer suggestions.
Use the presentation to explore why objects float or sink based on the forces applied using force arrow digrams. On slide 6, students will be asked to name the downward force acting on the boat. They may describe this as gravity, but it is in fact weight. Then, introduce the idea of density in order to develop student understanding, including how to calculate density. The practical should be completed to allow students to use the calculation and explore density in both regular an irregular shapes.
Career Film: This is Alex Potts. Alex works as a Nuclear Safety Consultant for Orano. Click on the tour to learn about Alex's job role.
Expert Film: Alex Potts works as a Nuclear Safety Consultant for Orano. Alex explains how density affects buoyancy.
Expert Film: Dr Sam Rowe as he discusses the difference between weight and gravity.
Calculating Density
Ask the students to complete the following steps to calculate the density of different objects.
Support: With the students, model how to convert grams to Newtons.
Challenge: Springs come in all sorts of shapes and sizes. Ask the students: why is the spring in the suspension of a car much bigger and thicker than the springs we use in the classroom? They can draw free body diagrams to help communicate their ideas.
Complete the assessment quiz and discuss at which points in the practical an error could have occurred. Ask the students: how can these errors be overcome in the future?
Our knowledge of forces can help us explain why things float or sink. There are 2 forces acting on an object when it is in a liquid. Weight is the downward force exerted due to the gravitational pull of the Earth - measured in newtons (N). Buoyancy (or upthrust) is the upwards force acting on the object. Therefore, if an object sinks, the weight is greater than the upthrust. If an object floats, the forces are equal.
There is a relationship between the density of an object and whether it will float or sink. The density of an object is worked out by: mass ÷ volume. If the density of an object is greater than one, it will sink but if it is less than one, it will float.
Buoyancy is the upward force exerted on an object immersed in a fluid, which opposes the weight of the object. The buoyant force arises from the pressure difference between the top and bottom of the object due to the weight of the fluid it displaces.
The amount of buoyant force acting on an object depends on the density of the fluid and the volume of the displaced fluid. If the object has a greater density than the fluid it is placed in, it will sink, as the weight of the object is greater than the buoyant force acting on it. If the object has a lower density than the fluid, it will float, as the buoyant force acting on it is greater than its weight.
The relationship between density and buoyancy can be explained using Archimedes' principle, which states that the buoyant force on an object is equal to the weight of the fluid displaced by the object. If the object is more dense than the fluid, it displaces a smaller volume of fluid, resulting in a smaller buoyant force. If the object is less dense than the fluid, it displaces a larger volume of fluid, resulting in a larger buoyant force.
In summary, density affects buoyancy because the buoyant force is determined by the weight of the fluid displaced by the object, which in turn depends on the density of the fluid and the volume of the displaced fluid. If the object is less dense than the fluid, it will float, and if it is more dense than the fluid, it will sink.