Nov'18

Experiment: Sink or Float?


by STEM Scouts



By Brooke Myers, Capitol Area Council

Download the Lab Guide: Density_Buoyancy

Supplies Needed:

  • Beaker/Measuring Cup
  • Scale
  • Calculator
  • Honey
  • Water
  • Dish Soap
  • Foosball
  • Parent/Adult Supervision

Background:

Density can often be a hard concept for students to understand. Density for many students is equivalent with weight. If something is heavy, then it will sink. This misconception can hinder students from understanding why a typically dense material, such as metals, are used to build ships! This experiment helps students to understand the mathematical relationship between densities of an object and fluid. Because of the mathematical principals in this lesson, I recommend this experiment for students 4th grade and above. Using critical thinking skills, students will be able to use a mathematical equation to determine whether something will sink or float. Before you begin the experiment, use a real-life example of density to help frame this experiment. For example, compare two sheets aluminum foil that are the same size, then crumple one up, place both in water. Even though the same weight, one should float and one should sink. This context could help the student to confront their misconception during the experiment. This experiment can still be scaled for younger students to help them differentiate that weight is not what determines if something sinks or floats.- check out an additional experiment in the resource section for help with scalability for younger students.

Common Misperceptions:

  • Students might think… Objects float because they are lighter than water
  • Guide them to observe… Objects float because they are less dense than the fluid they are immersed in
  • Students might think… Objects sink because they are heavier than water
  • Guide them to observe… Objects sink because there is not a strong enough upward force to oppose the downward force of gravity. To increase upward buoyant force, increase the volume of the object to displace more mass of fluid
  • Students might think… Mass/volume/weight/density may be perceived as equivalent
  • Guide them to observe… Use examples of small heavy items and large light items to help students understand the difference between mass and volume and the relationship these two measurements have on the density of an object.
  • Students might think… Wood floats and metal sinks
  • Guide them to observe… The density of an object will determine whether it sinks or floats when compared to the density of the fluid

Parent Guide:

  • The upward force is called buoyant force
  • Many patterns can be present: Density of object less than fluid will float, if more than fluid will sink, the ratio of densities will result in the % submerged, if there is equal density between object and fluid than the object will neither float or sink
  • % submerged is equal to the density of the object divided by the density of the fluid
  • Using observation skills, they should see that honey stays on the bottom because it is denser than water
  • Layering Density: https://www.youtube.com/watch?v=-CDkJuo_LYs
  • Encourage exploration, with changing fluid densities and object densities, students will get a better understanding of what will sink and float according to ratios of densities rather than the “weight/mass” of an item to help confront and disprove their preconceptions.

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