Sink or Swim??

Curriculum links:

• Physics – Forces – Investigating the law of floatation – How things like ships and submarines float or sink

Learning objectives:

1. Make predictions about what different objects will sink or float
2. About different weights, sizes, densities and materials of objects and how these factors may influence floating ability.

Materials:

• Paper and pencil
• Basin of water
• Different objects to test – fruit, bottle cap, paperclip, leaves, small toys, modelling clay, marshmallows, stones etc.

Safety:

• Make sure children are not left alone with water

Instructions:

• Fill the basin with water. Hold up the different objects and ask the children to predict which objects will float or sink. Have them fill it into a piece of paper with SINK & FLOAT columns.
• Test the different objects and see the results. Discuss why some objects float and some sink. Use terms like “heavier than water” or “lighter than water”.
• Try different tests with the same object, a bottle cap will float when facing upwards, will it support a stone? This is how ships work! You can explore this more with some modelling clay. Steel is denser than water but because of the SHAPE they can float.
• An orange will float, but if they are peeled, the segments will sink and the skin will float, this is because the skin is lighter than the water, while the body is heavier. Can you think of an object that can float and sink? – submarines, bottles (full or empty?), scuba divers.

How it works:

If an object is buoyant, it is less dense than the liquid it is floating in or else it will have something attached to it that will help it float, like a life jacket! An object will sink if it is heavy for its size (paperclip), and float if it is light for its size (log of wood). Objects are heavier or lighter depending on how dense they are (how much air they have inside them.) Marshmallows are lower density than stones as they have more air trapped within the material. This is why marshmallows will float and stones will sink in water! Buoyancy also depends on shape and how things can stop water from filling up inside them…. like a ship!