Liquids

Pressure The pressure that a block exerts on a table is simply the weight of the block divided by its area of contact.

Pressure in Liquid Liquid pressure = weight density X depth Note: Total pressure would also include atmospheric pressure (101.3kPa). Water pressure depends on depth and not on volume. At any point within a liquid, the forces that produce pressure are exerted equally in all directions.

Pressure in Liquid Liquid pressure = weight density X depth

Pressure in Liquid Liquid pressure = weight density X depth 3 m 6 m The surface that must withstand the greater pressure is the surface with the deepest water behind it, not the most water.

Aquaducts

Liquid Pressure

Liquid Pressure The forces in a liquid that produce pressure against a surface add up to a net force that is perpendicular to the surface.

Liquid Pressure Liquid escaping through a hole initially moves perpendicular to the surface.

Buoyancy Buoyant Force = weight of the water displaced (net upward force, opposite in direction to gravity) The submerged object’s volume, not its weight determines buoyant force.

Buoyancy 3kg 0kg 1kg 2kg Buoyant Force = weight of the water displaced What is the buoyant force in Newtons? Recall that the mass of 1 L of water =1kg 1 kg = 9.8 N

Displacement Determining Volume

Displacement Determining Volume

Displacement Determining Volume Water Displaced

Density What makes an object sink or float? What about the density of the fluid?

Does it sink, or does it float? An object more dense than the fluid in which it is immersed sinks. An object less dense than the fluid in which it is immersed floats. An object equal to the density of the fluid in which it is immersed neither sinks nor floats. (Think of fish.)

Archimedes’ Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces The volume of displaced water is the same at any depth. Note: Any object with a volume of 1 L will experience a buoyant force of 9.8 N when fully submerged in water.

Archimedes’ Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces What about the density of the liquid? Principle of Flotation—A floating object displaces a weight of fluid equal to its own weight.

Pascal’s Principle A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.

Pascal’s Principle A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid. 500kg 10kg 50 times the area = 50 times the force

Pascal’s Principle A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid. 500kg 10kg Input force X distance = Output force X distance

Pascal’s Principle

Pascal’s Principle

Surface Tension The contractive tendency of the surface of liquids

Surface Tension Liquid surfaces tend to contract to the shape with the least surface area

Capillarity Adhesion—attraction between unlike substances Cohesion—attraction between like substances