Why do some objects float on water?

If you place different objects in a bowl of water, for example, coins, pens, erasers and a ball, you will notice that some of the objects float on water, while others sink. 

Sink or float?

The three main factors that determine whether an object will float or sink are:

Weight: The weight of an object is measured in newtons and measures the gravitational force acting on that object. Heavy objects sink and lighter objects float on water. 

Density: Anything that has mass, including liquids, will have density. Density relates an objects mass to its volume. A heavy object has a higher density than a lighter object. Objects denser than water will sink and objects less dense than water will float. 

Buoyancy: Buoyancy is the ability of an object to float on water or other liquid or gas. Buoyant objects float and dense objects sink. 

Why are large ships made of steel able to float on water? The reason for this can be explained by what Archimedes noticed when he got into the bathtub. The displacement of water is what keeps ships afloat, we call this the buoyancy effect. For a ship to float on water, it needs to displace its own weight in water. The buoyant force on the boat will counteract gravity's downward pull.

Buoyant Force

Watch the next video to see an experiment that explains the upward force (thrust) exerted on objects placed in water.