Have you ever wondered why some things float and some things sink in water?
All things are made up of very tiny particles called molecules. Density is how loosely or tightly packed the molecules in an object are. It refers to the degree of compactness of an object.
Objects will sink if it is more dense — that is, if its molecules are more tightly packed — than the liquid it is put in.
Objects will float if it is less dense — if its molecules are less tightly packed — than the liquid it is placed in.
Some things float at first but then sink as they absorb water or take in water through holes (and thus become denser).
Buoyancy, also known as upthrust, is the force that pushes an object up and makes it seem to lose weight when immersed in a liquid.
When people talk about the buoyancy of an object, they usually mean the tendency of that object to float in liquid or in air.
In more scientific terms, buoyancy is the upward force that a fluid exerts on an object. It is the force that opposes the weight of a partially or fully immersed object.
An object floats when the buoyant force or upthrust becomes large enough to counter its weight.
Changing the object’s shape will also change its ability to float because it will hold less (or more) air and upthrust will have less (or more) space to push up on the object.
But although an object’s shape can affect its ability to float, some materials — such as styrofoam and cork — float no matter what their shape is.
Note on the Worksheets
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Will the objects below sink or float? Make your predictions, then conduct an experiment, and record your findings.
Objects you will need: (1) empty plastic bottle with the cap on, (2) full plastic bottle with the cap on, (3) empty glass bottle with the cap on, (4) full glass bottle with the cap on, (5) small rubber ball, (6) big rubber ball, and (7) marble.
In this worksheet, you get to choose what objects you will include in your experiment. Try to choose things made of different materials and of different shapes.
Sort the items in the worksheet below according to whether they would sink or float on water. If you have them in your house, try to check your answers by conducting an experiment.
This quiz lets you assess what you’ve learned in this lesson about density, buoyancy, and why objects sink or float. Fill in the blanks before the number with the letter of the correct answer.