Because the atoms are closely packed, liquids, like solids, resist compression an extremely large force is necessary to change the volume of a liquid. When a liquid is placed in a container with no lid, it remains in the container. That is, liquids flow (so they are a type of fluid), with the molecules held together by mutual attraction. This occurs because the atoms or molecules in a liquid are free to slide about and change neighbors. Liquids deform easily when stressed and do not spring back to their original shape once a force is removed. A gas must be held in a closed container to prevent it from expanding freely and escaping. (c) Atoms in a gas move about freely and are separated by large distances. Forces between the atoms strongly resist attempts to compress the atoms. (b) Atoms in a liquid are also in close contact but can slide over one another. Gases have neither specific shapes nor definite volumes, since their molecules move to fill the container in which they are held ( Figure 14.2).įigure 14.2 (a) Atoms in a solid are always in close contact with neighboring atoms, held in place by forces represented here by springs. Gases are not bonded to neighboring atoms and can have large separations between molecules. The distance between molecules is similar to the distances in a solid, and so liquids have definite volumes, but the shape of a liquid changes, depending on the shape of its container. The molecules in a liquid are not locked in place and can move with respect to each other. Like solids, the molecules in a liquid are bonded to neighboring molecules, but possess many fewer of these bonds. (Shearing forces are forces applied tangentially to a surface, as described in Static Equilibrium and Elasticity.) Characteristics of FluidsĬonsidered to be fluids because they yield to shearing forces, whereas solids resist them. Even a large force produces only small displacements in the atoms or molecules of the lattice, and the solid maintains its shape. The structure of this three-dimensional lattice is represented as molecules connected by rigid bonds (modeled as stiff springs), which allow limited freedom for movement. In some cases, the force between molecules can cause the molecules to organize into a lattice as shown in Figure 14.2. Although true solids are not incompressible, it nevertheless requires a large force to change the shape of a solid. Solids will take a form determined by the nature of these forces between the molecules. The atoms or molecules in a solid are in close proximity to each other, and there is a significant force between these molecules. Solids are rigid and have specific shapes and definite volumes. We will look at each of these phases in detail in this section. Matter most commonly exists as a solid, liquid, or gas these states are known as the three common phases of matter. Calculate force given pressure and area.Explain the relationship between pressure and force.Define pressure and its related SI units.Compare and contrast the densities of various substances.
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