Density is an important physical characteristic of matter. All objects have density and that density can increase or decrease as the result of actions taken on the object. The effects of density are important for the workings of the universe and for our daily lives. It is simple to find the density of an object and see the effect of density.
Identification
Density is defined as the ratio of an object's mass to its volume. You can express this mathematically as density (abbreviated with the Greek symbol rho) is equal to mass (m) divided by volume (V).
Size
Density increases either with increasing mass or with decreasing volume. For example, if you have two balls of the same mass and you compress one of the balls to a smaller size, the compressed ball will have a higher density than the other. Similarly, if you have two balls of equal volume but different masses, the ball with the higher mass has a higher density. It is for this reason that a bowling ball has a higher density than a volleyball, even though both are similar in volume.
Features
Objects with higher density are invariably heavier than low-density objects of similar appearance. This increased weight has some benefits: you would not want to bowl with a low-density volleyball. Density is most noticeable in the case of buoyancy. If you want something to float, it needs to have a lower density than the liquid. This is why a canon ball will sink in water but a metal ship floats. The density of the ship is less, because it is filled with air and cargo, while the canon ball is pure, high-density metal.
Considerations
You can change the density of a complex object (an object made of more than one substance) by increasing the mass or by decreasing the volume. In practice, you increase the mass by adding to the object. Filling a bowl with water or adding people to a boat are examples of increasing mass while keeping volume constant. To decrease the volume, you need to compact the object. Crushing an empty soft drink can or shrinking the open space of a bag decrease the volume and increase the density.
Gravity Effects
Gravity is dependent on both the mass and the volume of an object, so density is important. A high density body, like a neutron star or a black hole, has a much higher density than a normal, high volume star. A neutron star, with a diameter of only about 20 kilometers (12.4 miles), has 100,000,000,000 times the gravity of the Earth.
Buoyancy Effects
When water freezes, the molecules crystallize, with the space between each molecule increasing. If the mass of water does not change, this has the effect of decreasing the density of frozen water. It is for this reason that ice floats on top of water, as seen in the frozen surfaces of lakes and in icebergs. When the ice melts, the density increases again. This is an important effect for roof stability: when ice and snow collect on a roof and melt, the increased density of the resulting water can be enough to collapse the roof.
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