Floating Bowling Ball

What is needed:

• Three undrilled bowling balls – preferably 8, 12, and 16 pounds
• 10 gallon aquarium
• 2 pounds of salt
• towels for drying hands and the bowling balls

This is a good demo to introduce the concept of density.  Most people view bowling balls as “heavy” but so is water.  The diameter of bowling balls is specified at 8.5 to 8.595 inches.  The weight is specified only at a maximum of 16 pounds.  The list below contains the calculated densities assuming a diameter of 8.59 inches.

The density of pure water at room temperature is approximately 0.036 lb / cu in, slightly less dense than the density of the 12 pound bowling ball.  So any lighter ball will float in varying degrees.

Adding salt to the water will increase the density of the water to allow the 12 pound ball to float.

Demo 1:

A good way to introduce this is to ask if people swim and if they float or sink.  Then ask if anyone has ever bowled and if bowling balls are heavy.  Tell them you have a real bowling ball that floats.

Fill the aquarium with about 8 gallons of water – not too much or the water will overflow when the bowling ball is added.  Slowly lower the 16 pound (or 13 pounds and above) bowling ball into the water.  It should stay at the bottom.  Remove it and repeat for the 12 pound bowling ball.  It should also sink but its weight in the water will be nearly zero.  Remove it and lower the 8 pound (or 9 pounds or less) ball into the water.  It will float.  Discuss why this occurs.

Demo 2:

Ask if anyone has ever been swimming in salt water and if they floated.  Tell them that you will now make the 12 pound bowling ball float.

Add two pounds of salt to the water in the aquarium and stir until dissolved.  The 16 pound ball will still sink but the 12 pound ball will now float due to the increased density of the water.  Discuss why this happens.

There’s a water density calculator available.