A fluid is a substance that flows. Both gases and liquids are fluids.

• A gas is compressible. The molecules move freely with few interactions.

  

• A liquid is incompressible. The molecules are weakly bound to one another.

  

Fluids exert forces on the walls of their containers. Pressure is the force-to-area ratio F/A.

• Pressure in liquids, called hydrostatic pressure, is due to gravity. Pressure increases with depth.

• Pressure in gases is primarily thermal.

  Pressure is constant in a container.

Buoyancy is the upward force a fluid exerts on an object.

• Archimedes’ principle says that the buoyant force equals the weight of the displaced fluid.

• An object floats if the buoyant force is sufficient to balance the object’s weight.

《 LOOKING BACK Sections 6.1 Equilibrium

An ideal fluid—an incompressible, nonviscous fluid flowing smoothly—flows along streamlines. Bernoulli’s equation, a statement of energy conservation, relates the pressures, speeds, and heights at two points on a streamline. Flow of a viscous fluid requires a pressure gradient and is described by Poiseuille’s equation.

Elasticity describes how objects deform under stress. An object’s

• Young’s modulus characterizes how it stretches when pulled.

• Bulk modulus tells us how much it is compressed by pressure.

《 LOOKING BACK Sections 9.4 Restoring forces

Gases and liquids are two of the three common states of matter. Scientists study atmospheres and oceans, while engineers use the controlled flow of fluids in a vast number of applications. This chapter will let you see how Newton’s laws can be applied to systems that can flow or deform.