Describe the factors which influence floating and sinking (content found…

The idea

An object floats when the upthrust on it balances its weight; it sinks when its weight exceeds the maximum upthrust the fluid can supply. Upthrust equals the weight of fluid displaced, so the outcome depends on the object's weight compared with the weight of fluid it could displace. Density is the key factor: an object less dense than the fluid floats, one more dense sinks. Shape matters because it sets the volume displaced when fully submerged, so a hollow steel hull floats while a solid steel block sinks. Overall density, not material alone, decides the result.

What earns the marks

• States that an object floats when upthrust equals its weight and sinks when its weight exceeds the maximum upthrust available.
• Identifies the density of the object relative to the density of the fluid as a deciding factor.
• Explains that upthrust equals the weight of fluid displaced, so the volume displaced sets the maximum upthrust.
• Recognises that shape or hollowness changes the volume displaced and therefore whether a given mass floats.

Common mistakes and how to put them right

• Claims that heavy objects always sink regardless of density.

  Put it right: Compare the object's density with the fluid's density instead.

• Confuses mass with density when predicting floating.

  Put it right: Use density, the mass per unit volume, as the deciding comparison.

• Assumes upthrust depends only on the object's weight.

  Put it right: Relate upthrust to the weight of fluid displaced.

Exam tips

• Compare the density of the object with the density of the fluid before deciding float or sink.
• Link upthrust to the weight of fluid displaced rather than to the object's mass alone.
• Use a hollow-hull example to show how shape changes the volume displaced.