Why Objects Float or Sink: Density Questions Made Easy

Quick answer: an object floats in a liquid when its average density is lower than the liquid’s density; it sinks when it is higher.

Compare densities, not just weights

A large wooden block can weigh more than a small stone but still float because its mass is spread through a larger volume. Density is mass per unit volume, so size and material both matter.

Three cases

• If object density < liquid density, the object floats.
• If object density = liquid density, it can remain suspended when fully inside.
• If object density > liquid density, it sinks.

Worked example

A block has mass 300 g and volume 500 cm³. Density = 300/500 = 0.6 g/cm³. Water is approximately 1 g/cm³, so the block floats. A 1,200 g object with volume 800 cm³ has density 1.5 g/cm³ and sinks in water.

Why does a floating object displace water?

The upward buoyant force equals the weight of the displaced liquid when the object is floating at rest. The object settles at a depth where these forces balance; it does not need to be completely under water.

Common mistakes

• Comparing mass directly with liquid density.
• Mixing grams with cubic metres without conversion.
• Assuming every metal object sinks; shape and enclosed air can change average density.

Practice

An object has mass 200 g and volume 250 cm³. Answer: density 0.8 g/cm³, so it floats in water.

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