Sinking and Floating

Most people, without experimenting, think that the heavier an object is the more likely it is to sink. Weight does not determine the answer to whether an object will sink or float. An object's density determines whether an object sinks or floats.

Everything on earth is made of molecules. If the molecules are packed closely together, the object is called dense. The more spread apart the molecules are, the less dense an object is. The objects which have molecules packed more tightly together will sink. Examples are a paper clip and a penny. Those whose molecules are packed more loosely will float. Cork and wood will float.

The density of liquids is not always the same. Some are denser than others. In a container, corn syrup will sink to the bottom below water. Oil will remain on top of both of these. It is the least dense of the three. Corn syrup is the densest.

Boats do not sink into the water unless they are damaged or have a hole. A boat will not sink if the water is heavier than it is. Much of a large boat is air. An object floats if the water weighs more than the object. If the object weighs more than the water, it sinks. If the weight pushing down on the water is more than the upward push of the water, an object floats. If it is greater, the object sinks.

Freshwater is less dense than salt water. Things float better in salt water. People always have an easier time floating in the ocean.

All objects do not float at the same level. As an object falls down into the water, the upward push of the water strikes a balance with the object. The object then continues to float at this level.

Objects do not have to contain air in order to float. Some objects are hollow and contain air. A plastic cup and a hollow ball are examples. These hollow places allow the water to exert a greater push upward and cause the object to rise. An object does not need to have air to float, however.

When an object floats, some part of that object is still below water. When an object is in the water, it displaces the weight of the object. That is, it takes the place of the object in the water, and the water level rises. Even if an object floats, it displaces a small amount of water. That is, the water rises a small amount above its previous level because the object has taken up some of its space. If an object sinks, the water level rises the same amount as the amount of water the object displaces. The object takes the place of the water so the water has to rise.

The shape of an object determines if it floats or sinks. A piece of clay which is flat and long will not sink in water. If the clay is molded into a hard ball, it will sink.

The upward push of the water against an object can be felt when anyone tries to push down into the water something which is floating. An example would be a large beach ball. Water still exerts some upward push on objects which appear to have sunk. Floating is the balance of the weight of the object put into the water with the upward push on that object in the water.

An experiment to show how the change in weight can determine the level at which an object floats can be done by using an empty plastic container. When empty, it floats on the water. By adding different amounts of sand or something similar, the level of floating or sinking will change.




A: When an object floats, the upward push of the water is greater than the downward push of the object.
B: Shape does not have anything to do with whether an object sinks or floats.
C: Density is related to how big an object is.
D: All of the above

A: A beach ball will float.
B: Pennies will sink.
C: Wood will float.
D: Paper clips will float.

A: An object which sinks displaces water.
B: If a boat is too heavy, it will sink.
C: When an object floats, no part of it is below the water.
D: None of the above

A: A paper clip
B: A twig
C: A hollow ball
D: A bottle cork

A: Small objects always float.
B: Large objects always sink.
C: Many hollow objects float.
D: Salt water is less dense than fresh water.

A: Five pennies
B: Five paper clips
C: Two beach balls
D: Both a and b








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