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 (2): An investigation sheet for the students to examine volume and density.
Enquiry Skill - Observing and measuring
Using the senses and taking measurements, using a range of equipment, to make observations about a scientific enquiry.
Enquiry Approach - Research
Using information from a variety of sources to answer scientific questions.
If heavy objects sink, why do large ships float? Ask the children 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, the children 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 the children's understanding, including how to calculate density.
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 children to complete the following steps to calculate the density of a variety of regular-shaped objects.
Note: To provide extra assistance to the children, you may wish to measure the volume of each object in advance and provide this to the children instead of asking them to measure the volume themselves. The formula for volume is Length x Width x Height, and it is measured in cm3. More confident children may be able to do this themselves.
Challenge: Springs come in all sorts of shapes and sizes. Ask the children: 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 children: 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 รท 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.
