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Science Activity Physics • Buoyancy

Archimedes

Why does a huge steel boat float while a small stone sinks? In this activity, you will discover that an object submerged in water experiences an upward force: Archimedes' buoyant force. You will also see that the shape of an object can change its ability to float, even when its mass remains the same.

Duration
25 to 40 min
Difficulty
Easy
Location
Home or classroom

What you will understand

When an object is submerged in water, it displaces a certain amount of water. In response, the water exerts an upward force on it: this is Archimedes' buoyant force. The larger the volume of water displaced, the greater this force.

Key idea: an object floats when the force exerted by water balances its weight. The material matters, but the shape of the object also plays a major role.

Scientific question

Question studied

Can the shape of an object change its buoyancy, even if its mass stays the same?

What we will compare

  • A compact ball of modeling clay
  • A boat made with exactly the same amount of clay
  • Each shape's ability to float and support a load
With the same mass, a hollow and wide shape can displace more water before sinking. It can therefore float more easily than a compact ball.

Materials

Essential

  • Modeling clay
  • A large bowl or basin filled with water
  • Some coins
  • A spoon or a small strainer
  • Something to note observations

To go further

  • A measuring cup to measure displaced water
  • A scale to know the mass used
  • A towel to dry the clay between trials
Precaution: wipe spilled water to avoid slipping and do not drink the water used during the experiment.

Protocol

1. Take one fixed amount of modeling clay. This mass must remain the same throughout the activity.
2. Shape it into a compact ball. Gently place it on the water surface and observe if it floats or sinks.
3. Retrieve the clay, dry it quickly, then reshape it into a boat: a hollow hull with raised edges.
4. Place this boat on the water and observe its behavior.
5. Then add coins one by one to the center of the boat until it sinks.
6. If you wish, improve the boat's shape and try again to see if its stability changes.
A wider boat with regular edges is often more stable and can support more load.

Results table

Trial Shape Floats or sinks Stability Number of coins supported Observations
1 Ball 0
2 Boat
3 Improved boat
You can compare not only buoyancy but also stability and maximum load supported.

Analysis questions

1. What happens with the modeling clay ball?

2. Why does the boat float better than the ball?

3. What does “displacing water” mean in this activity?

4. Which shape supports the most coins? Why?

Help to check your reasoning
  • The compact ball often sinks quickly because it displaces too little water before being fully submerged.
  • The boat floats better because its shape allows it to displace more water while remaining partially at the surface.
  • The greater the volume of displaced water, the stronger Archimedes' buoyant force.

Conclusion

Scientific conclusion

With the same amount of modeling clay, the ball and the boat behave differently in water. The difference comes from their shape: the boat displaces more water before sinking, so the upward force exerted is greater.

Link to the real world

A steel boat can float not because steel is "light," but because its shape allows it to displace a large volume of water. Its average density, considering the air it contains, becomes compatible with floating.

Remember: it is not only the material of an object that determines if it floats or sinks. Its shape and the volume of water displaced are essential.
SVsansT — Activity "Archimedes" • Understand buoyancy, Archimedes' buoyant force, and the importance of object shape.
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