MnSTEP Teaching Activity Collection > MnSTEP Activities > Incompressibility of liquids

Incompressibility of liquids

Mick White
Central High School
Mankato, MN
I saw this done and was given a handout some years ago. I can't remember who the demonstrator was and there is no information on the handout.
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Students are observers. As students walk into the classroom I will fill a few bottles with water (80-90% full). I will hit the top of each bottle. The only effect on the bottle will be that the bottom "falls" out of it. I do not explain anything to the students. After the demonstration I have the students discuss what they think happened.

Learning Goals

The concepts that the students are to learn are:
  • Simple hydraulics: that water (and other liquids) do not compress when a force is applied at the open end of a system that is closed at the other end.
  • The transfer of energy: The energy created from the mallet striking the bottle compresses the air. The compressed air pushes on the water. Because the water is incompressible the energy is converted into a shock wave that travels through the water and strikes the bottom of the bottle, causing it to break.
  • Critical thinking skills are developed. The students must reason amongst each other what happened.
  • The vocabulary that will be introduced after this lesson will vary, depending on how it is presented, as either an introduction to hydraulics or the conservation of or transfer of energy.
  • Hydraulics Vocabulary: Hydraulics and Pressure
  • The concept derived will be Pascal's Principle, that any change in the pressure applied to an enclosed fluid is transferred through all parts of the fluid and the enclosing walls without any loss of energy.

Context for Use

This discrepant event is best used to introduce the section on hydraulic force. This could be used at lower levels, but fits nicely in 9th grade physical science or in physics class. It's a quick demo. All you need is a couple of beer or wine cooler bottles (remove the labels), a rubber mallet, safety goggles, a glove, and a wastebasket.

Subject: Physics:Fluid Mechanics
Resource Type: Activities:Lab Activity, Classroom Activity:Short Activity:Demonstration, Activities:Classroom Activity
Grade Level: High School (9-12)

Description and Teaching Materials

  • A couple of beer or wine cooler bottles (remove the labels)
  • Rubber mallet
  • Safety goggles
  • A glove
  • Wastebasket
Have the bottles filled before the students enter the room. After the students are seated, I put on my goggles and glove. I pick up a bottle and the rubber mallet. I begin to hit the top of the bottle with the mallet. Sometimes it takes a few hits, be patient. It is important to hit the bottle with the head of the mallet as squarely as possible. After a couple of hits, the bottom breaks out (over the basket). I lay the mallet and bottle down and ask the students to explain what happened. After a little discussion amongst students I begin to steer them towards the idea of the incompressibility of liquids. I ask them where this might be useful (car brakes, heavy machinery, etc.). This leads into Pascal's principle.

Teaching Notes and Tips


The assessment is done while the students are discussing what happened. A better assessment leads into the lesson on hydraulics when they can apply the event to what we are studying.


Grade 9-12 Strand II Physical Science Substrand C: The student will understand energy forms, transformations, and transfers.

Grade 9-12 Strand II Physical Science Substrand D: The student will use Newton's three laws of motion to qualitatively and quantitatively describe the interactions of objects.

References and Resources