Toppling bottles

In this class activity students tilt three bottles to see which one topples first. You can use it to introduce the idea of stability.

Equipment

Each group of students will need:

• 3 identical small clear plastic bottles with caps
• Beaker with at least twice the capacity of one of the bottles
• Enough coloured water (use food colouring) to fill two bottles
• Thick cardboard
• Pencil
• Sticky tape
• Protractor (optional)

Preparation & safety

Using coloured liquid makes it easier for students to see the mass distribution for the bottles. Add a drop of food colouring to a beaker of water for each group of students.

Try to find bottles with flat bases so there is less ambiguity about the area of the bottle in contact with the board.

Procedure

Ask students to:

1. Fill one bottle with coloured water to the very top.
2. Half-fill another bottle (leaving the last one empty).
3. Using sticky tape, secure the pencil to one end of the cardboard to make a lip.
4. Place the three bottles in a row in increasing mass order along the board next to the lip.
5. 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?
• Why is the middle bottle the most stable?

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 idea of centre of gravity explain that it is the point on an object at which we can consider the gravity force to act. It is a sort of average position for the mass in the object. 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 which 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.

Learning outcome

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

This experiment was safety-checked in March 2020.