They form understandings from an early age about these ideas and equally importantly the words used to describe them. Students tend to pay little attention to why things float or sink and may perform simple tests on objects in a tank of water without any understanding of the forces involved in why they float or sink.
These can be quite challenging to identify and understand. An object floats when the weight force on the object is balanced by the upward push of the water on the object.
The upwards push of the water increases with the volume of the object that is under water; it is not affected by the depth of the water or the amount of water. If the weight force down is larger than the upward push of the water on the object then the object will sink. If the reverse is true then the object will rise — rising is the opposite of sinking. Different objects float at different levels in the water because as most regular objects are lowered into the surface of water, the upward push of the water steadily increases until it is in balance with the weight force of the object, and the object then continues floating at this level with the two forces in balance.
Many objects that are hollow and so generally contain air float because the hollow sections increase the volume of the object and so the upwards push for very little increase in weight force down. However, it is not necessary for an object to contain air in order to float. Explore the relationships between ideas about floating and sinking in the Concept Development Maps : Laws of Motion. Students will be familiar with the idea that objects have weight and that the size of the weight force is determined by the type of material and how much of it is used to make up the object.
Molecules are very, very tiny. You can only see them with a special microscope. In some objects, like a rock, molecules are squished tightly together. In others, like wood, the molecules are more spread out. How closely molecules are crowded together in a space is called density. Objects like coins, rocks, and marbles are more dense than water.
They will sink. Objects like apples, wood, and sponges are less dense than water. They will float. Molecules can be packed in close together like in a rock or more spread out like in bubble wrap. The positioning of molecules affects the density of an object.
Objects with tightly packed molecules are more dense than those where the molecules are spread out. Density plays a part in why some things float and some sink. Objects that are more dense than water sink and those less dense float. Hollow things often float too as air is less dense than water. This is partly why huge heavy ships float. Another thing to consider is the shape of an object. Generally the more of the outside of an object that is touching the water the more buoyant it is. Water pushes back up against objects so the more surface area an object has the more water pushes back against it helping it to float.
When an object floats, it pushes water out of the way displacement. Have you ever noticed that when you climb into a bath the water level rises? This easy activity demonstrates how reducing the density of a heavy object allows it to float. Place the bubble wrapped ball on the surface of the water, you should find it now floats.
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