Forces of Fluids: Density and Buoyancy



High School students

Materials For each group:

  • Large beaker about 800-900 ml
  • Water
  • Force probe
  • Balance
  • Spring scale
  • Overflow can
  • Small beaker
  • Small plastic cup with wire handles to hang
  • Various small object of different sizes (small enough to fit in the cup)

Time frame:

Set up:  5 minutes

Activity:  40 minutes

Clean up:  5 minutes


After completing the activity, students will be able to:

  1.  Explain orally and mathematically the relationship between density, mass and volume.
  2.  Describe the difference between volume and relative density determine experimentally the     Archimedean principle.
  3.  Formulate hypotheses about the density of materials.
  4.  Calculate the force of buoyancy or thrust on a body that is partially immersed in a fluid.


A fluid is a substance that continually deforms when a force is applied to it and continues to deform even after the force is not applied anymore whereas a solid may or may not deform when a force is applied to it. However, in a solid the deformation goes on as long as the force is applied, the moment the forces is removed, no more deformation is experienced by the solid. That is one of the building blocks of the study of movement of matter and it’s used to classify the field in solid and fluids mechanics. Density is a property of matter and is defined as the ratio of an objects mass to its volume. Mathematically it can be expressed as:


Where m is the mass of the object and V is the volume. It is generally expressed in units of g/cm3. Density is an important quantity because it plays a key role on the floatation of objects. When you place and object in a fluid it will do one of two things: it will either float or sink. If the density of the object is less than that of the suspending fluid, it will float, if the opposite is true then the object will sink. This can be best explained using the Archimedes principle.


Have you ever felt like you could fly when you’re on the beach or in a pool? That you suddenly lose some weight when you go into the water? That is because any object partially or completely submerged in fluid experiences a lifting force called buoyant force. According to an Archimedes principle this force is equal to the weight of the displaced fluid. But, what does displaced fluid means?  Imagine you want to plant a tree. In order to do so, you need part of the tree (the roots) to be embedded to the ground therefore you must dig a hole in the ground to place the tree.


In other words, you’re displacing a certain amount of dirt so that the tree will fit in and it can stand still. This is exactly the same idea as displaced fluid, except when you put and object in a fluid it automatically deforms and a certain amount of fluid is displaced.

Puerto Rico Science Education Standard and expectative

Stándar: Estructura y niveles de organización de la materia (Structure and levels of organization of matter)

Expectativa: Q.CF1 – La materia y sus interacciones: Relación entre las fuerzas y la energía (Matter and their interactions)


Activity instructions

Set up (5 minutes) – Distribute materials for each group


Introduction (10 min)

Before the activity, students should have studied in class the following concepts:

  1. Force
  2. Weight
  3. Fluid
  4. Newton’s Laws

Begin the activity with the video on fluids and Archimedes principle:


Explain the differences between solids and fluids. Explain that the buoyant force exerted by fluids arise from the difference in pressure in the submerged object. Explain how the density plays a key role on flotation of objects.


  1. Measure and record the mass of the catch bucket using a digital scale.
  2. Place the catch bucket under the overflow can exit hole. Pour water from the beaker into the overflow can until it comes out of the hole. Let the water run until it stops. NOTE: Do not bump the overflow can. You want the water in the water to be just ready to come out of the hole. Empty the water from the catch bucket back into the glass beaker. Dry the catch bucket using the towel on your table. Place the catch bucket back under the overflow can.
  3. Attach the spring scale to the aluminum block. Record the force measurement from the spring scale before submerging the block into water.  Lower it into the overflow can. Lower the block until it is completely submerged. The water that comes out of the spout represents the displaced fluid. Record the force measurement from the spring scale while the block is submerged.
  4. With the water still in the catch bucket, measure the mass of the water and bucket together using digital scale. Record the mass. Subtract from this value the mass of just the cup that you measured earlier. You now have the mass of the displaced water.
  5. Use the density equation to find the volume of the displaced water. Like before, this is the same as the volume of the block since it is totally submerged.
  6. Repeat the process for objects of different masses and shapes and record your data on Table 1.

Table 1:

Object Mdisp Vdisp Wdisp Wair Wapp FB
Aluminum block


  • Mott Robert, (1996). Mecánica de fluidos aplicada. Ed. Prentice Hall.
  • Ortega, Manuel R. (1989-2006). Lecciones de Física (4 volúmenes). Monytex. ISBN 84-404-4290-4, ISBN 84-398-9218-7, ISBN 84-398-9219-5, ISBN 84-604-4445-7.
  • Resnick, Robert & Halliday, David (2004). Física 4ª. CECSA, México. ISBN 970-24-0257-3.
  • Tipler, Paul A. (2000). Física para la ciencia y la tecnología (2 volúmenes). Barcelona: Ed. Reverté. ISBN 84-291-4382-3.
  • Vennard, John K. And Robert L. Street,(1995). Elementary Fluid Mechanics. New York. John Wiley and sons

Volume Diagram

Volume Diagram