Skip to main content Skip to main content

Essential Science for Teachers: Physical Science

Density and Pressure Children’s Ideas About Density and Pressure

Children’s Ideas About Density and Pressure

Below are common ideas children in grades K-6 have about this topic, compiled from research on children’s ideas about science. Consider what evidence might refute this idea, and why a child would be likely to believe this? Once you’ve entered all your answers you can click “printable page” at the bottom of this form to print your answers. You can also click “see possible response” for any question to see one possible response from the series content advisors.

1. An object rises or sinks in water based on its size, weight, and/or shape.

Children aren’t able to directly observe density, the determining “intensive” property of matter that predicts whether something rises or sinks in a fluid. As a result, they rely on their direct experience with the “heaviness” or “bulkiness” of objects, and are often surprised when a large object rises to surface of a liquid and floats. Only objects that are less dense than the fluid they are in will rise and float.

2. Objects with holes in them will always sink, especially once water gets into the holes.

Children may carry this idea from their experiences or memories of having seen boats “spring a leak” and sink. However, objects like the plastic piece with a hole in it in the video may have an average density that is less than the fluid they are in, causing them to rise regardless of whether water gets in the holes.

3. Something floats because it has air in it.

Children often believe that air is the lightest thing there is and, since light things float, anything that contains air will float. Solid objects float because they are less dense than the fluid they are in, not because they contain air.

4. “Pressure” and “force” are the same.

Researchers have found that elementary school students often use these terms interchangeably. As stated in the video, force is the pressure times the area on which the pressure acts. Conversely, pressure is force divided by area. Pressure acts in all directions in a fluid and increases with depth. As the children in the Science Studio swimming pool discovered, the amount of force needed to keep an object underwater is the same at any depth.

Series Directory

Essential Science for Teachers: Physical Science


Produced by Harvard-Smithsonian Center for Astrophysics. 2004.
  • Closed Captioning
  • ISBN: 1-57680-749-5