Why Things Float or Sink

Looking Around 2 — Water. Understanding why some things float and others sink builds on the unit's ideas about water and its uses.

• Objects that are light for their size usually float.
• Objects that are heavy and solid usually sink.
• The shape of an object can also help it float, even if it is heavy.

Verbal: Light things like wood, cork, and plastic usually float. Heavy solid things like iron and stone usually sink. A wide, hollow shape can help even a heavy object float.

Symbolic: Light + right shape -> float. Heavy + compact solid -> sink.

Level 1 — Light things float

Wood, cork, dry leaves, and plastic are light and usually float.

Level 1 — Heavy solid things sink

Stones, iron nails, and steel spoons are heavy and solid, so they sink.

Level 2 — Shape can help heavy things float

A heavy iron ship floats because its wide, hollow shape holds a lot of air and spreads its weight out.

Level 2 — India

A small stone sinks in the Ganga river, but a big wooden or metal boat with people and goods still floats because of its shape.

A heavy iron ship floats on the sea, but a small iron nail sinks in a bucket. Why?

Step 1 — The nail is small, solid, and compact — no shape to help it.
Step 2 — The ship is wide and hollow, spreading its weight over a large area.
Step 3 — The ship's shape lets it stay on top of the water, even though it is heavy.
Answer: Shape, not just weight, decides whether an object floats.

• Name one light object that floats.
• Name one heavy solid object that sinks.
• Why does a big iron ship float even though iron nails sink?

Stretch: Take a flat piece of aluminium foil and shape it into a small bowl. Does it float better than a scrunched-up ball of the same foil? Try it with an elder.

Revision tip: Remember: light things usually float, heavy solid things usually sink, and a good wide shape can help heavy things float too.

Open the Practice tab for graded questions on Why Things Float or Sink.