Session 1: ‘Fakir bed’ or the pressure applied on a solid

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Objectives 1: Understand that the pressure is proportional to the surface on which a same force is applied

Explanation

The following activities will show by experiment the concept of pressure, representing the force applied on a given area. If the same force is applied on a larger area, the pressure decreases.

Another important aspect is that the pressure on a solid depends on the contact area, while for a fluid (i.e. a liquid or a gas), the body is immersed in the fluid so the pressure is applied on all the surface…

The following experiments correspond to the fakir bed experiments, difficult and dangerous to manage in a classroom!

Material

• Bucket full of sand and sand pit
• Shoe
• Snowshoeing
• Stilt
• Flat big container full of sand or sand pit
• Wooden plate with 3 nails
• Wooden plate with plenty nails
• Scale
• Plastic container full of modeling dough
• Plastic containers of the same size

Maximum duration

45 minutes

Main question to be asked

Introduction/Starters

Go to a sand pit and ask the children to flatten the sand.

Then you tell them that you are going to put your feet in the sand with the stilt, the normal shoe and the snowshoeing.

What do your students expect to see?

Ask what is the difference for them?

You can also show them the fakir and ask them why he seems so comfortable on his nail bed?

Main activities

Step 1 :3 nails

1. Take a wood plate with 3 nails and put on the top a plastic container full of play dough. What do they observe?
2. Put a light plastic glass on the top of the container. Add water until the nails touch the bottom of the container.
3. Weigh the amount of water you add to reach the bottom

Step 2 : 20 nails

1. First add the same quantity of water as for step one
2. Then apply the same procedure with a wood plate with 20 nails. Add water until it reaches the bottom of the plastic of water
3. Ask to the students to explain you need much more water before you reach the bottom level

The way this experiment is proposed here is just to understand the principle of pressure with solid. You can always apply a more scientific approach by varying parameters such us number of nails, thickness of the play dough, changing the position of the glass of water, …

Conclusion/Plenary

When applying a given force on a solid surface, the resulting pressure will be function of the surface.
If the surface increases, the pressure decreases, and if the surface decreases the pressure increases.

Again quantification of this law will be made with students above 11 years old.

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