Objects float if they are light for their size and sink if they are heavy for their size.

An object can be light for its size if it contains air, such as a hollow ball.

Materials with a boat shape will float because they effectively contain air.

Water pushes up on objects with an up-thrust force.

Objects float if the up-thrust force from the water can balance their weight (gravity force).

Objects float depending on their density compared to water; for an object to float its density needs to be less than that of water.

In the olden days, accused witches were dragged to the nearest body of water, stripped to their undergarments, bound and then tossed in to to see if they would sink or float. An innocent person would sink; a witch would float.

The up-thrust depends on the amount of water displaced.

Objects float better in pure water than salt water.

Water surfaces have a cohesive force (surface tension) that makes them act like a ‘skin’.

Small, dense objects (e.g. a pin; a water spider) can ‘float’ on the surface of water without breaking it, due to surface tension effects.

Definition: a push or a pull

Definition: amount of mass per unit mass of an object (i.e. the concentration of mass, or how ‘heavy for its size’ an object is). The *__ of water is 1 kg per liter.

Definition: amount of force applied per unit area. At a given *__, twice the area will experience twice the force.

Definition: A floating object will experience an upthrust force from water, equal to the weight of water displaced (pushed aside). It will sink into the water until it reaches the point where the weight of the water pushed aside equals its own weight.

Will it float: A fresh egg.

Will it float: A non-fresh or rotten egg.

Will it float: A dead body.

Will it float: A person wearing a life-jacket.