Toppling bottles

In this demonstration, students see that objects with a lower centre of gravity are more stable.

Learning outcome

Students can relate the stability of an object to the position of its centre of gravity relative to its base.

Equipment

• 3 identical clear plastic bottles with caps - bottles with flat bases are best as their is less ambiguity about the area of the bottle in contact with the board.
• Water in a jug or large beaker - enough to fill two bottles
• Food colouring (so students can see the mass distribution in the bottles more easily)
• Strong cardboard or wooden board
• Pencil
• Sticky tape

Procedure

1. Add a few drops of food colouring to the water
2. Fill one bottle with coloured water to the very top.
3. Half-fill another bottle (leaving the last one empty).
4. Using sticky tape, secure the pencil to one end of the board to make a lip.
5. Place the three bottles in a row in increasing mass order along the board next to the lip.
6. Raise the other side of the board slowly.

Discussion prompts

• What force makes a bottle fall over?
• Can you predict the order in which the bottles will topple?

Teaching notes

Students may be surprised that the bottles with the least and most mass topple at the same time. Encourage them to think about the how the mass is distributed in the bottles.

If students are unfamiliar with the terms 'the centre of gravity' and 'stable object' introduce them.

• The centre of gravity is the point at which we can consider the gravity force to act. A sort of average position for the mass in an object.
• A stable object is one that returns to its original position when disturbed.

Explanation

In the full and empty bottles the mass is distributed evenly and so the centre of gravity is at half way up the bottle. The half full bottle is different because it has an uneven distribution of mass and a lower centre of gravity.

When the bottles are upright, the gravitational force acts downwards through the base of the bottle (the area that is in contact with the board). As the bottle is tilted, it will remain in contact with the board until the line of action of the gravitational force falls outside the base -at which point it will topple over and so will no longer be stable. The full and empty bottles topple first because they have a higher centre of gravity and so reach their tipping points first; the half full bottle is more stable because it has a low centre of gravity.

This experiment was safety-checked in March 2020.