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Chapter 13 Forces in Fluids
13.1 Fluid Pressure
13.1.1 Describe and calculate pressure.
13.1.2 Identify appropriate SI units for measuring pressure.
13.1.3. Describe the relationship between water depth & the pressure it exerts.
13.1.4 Describe how forces from pressure are distributed at a given level in a fluid.
13.1.5 Explain how altitude affects air pressure.
- Pressure = Force / Area.
- Water pressure increases as depth increases.
- The pressure in a fluid at any given depth is constant and in exerted equally in all directions.
- Air pressure decreases as altitude increases.
pressure – the result of a force distributed over an area
pascal – the SI unit of pressure, equal to 1 newton per square meter (N/m2)
fluid – a substance or mixture that flows and has no shape of its own
Brightstorm Liquid Pressure (5:51): Liquid pressure is the increase in pressure at increasing depths in a liquid. This pressure increases because the liquid at lower depths has to support all of the water above it. We calculate liquid pressure using the equation liquid pressure = mass x acceleration due to g density x depth in fluid.
Brightstorm Atmospheric Pressure (12:43): Atmospheric pressure is the pressure caused by the mass of our gaseous atmosphere. It can be measured using mercury in the equation atmospheric pressure = density of mercury x acceleration due to gravity x height of column of mercury. Atmospheric pressure can be measured in atm, torr, mm Hg, psi, Pa, etc.
Brightstorm A Barometer (6:55): A barometer is a device used to measure pressure. Gauge pressure is the difference between a pressure and atmospheric pressure. Gauge pressure can be negative while total pressure cannot
13.2 Forces and Pressure in Fluids
13.2.1 Describe how pressure is transmitted in a fluid according to Pascal’s principle.
13.2.2 Explain how a hydraulic system works to change a force.
13.2.3 Explain how the speed and pressure of a fluid are related according to Pascal’s principle.
- According to Pascal’s principle, a change in the pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid.
- In a hydraulic lift system, an increased output force is produced by a constant fluid pressure exerted on the larger area of the output piston.
- According to Bernoulli’s principle, as the speed of a fluid increases, the pressure within the fluid decreases.
hydraulic system – a device that uses a pressurized fluid acting on pistons of different sizes to increase force
lift – an upward force due to a pressure difference between the top and bottom of a wind
Prentice Hall: Pressure in a Liquid, Principles of Fluid Pressure
Brightstorm Pascal’s Principle (7:18): According to Pascal's Principle, in a uniform fluid, pressure depends only on height. There can be no pressure gradient except for that caused by liquid pressure. This is seen in hydraulic jacks which are used to lift heavy objects.
Brightstorm Bernoulli’s Principle (7:57): Bernoulli's Principle replaces Pascal's Principle and liquid pressure for flowing fluid. It states that as you increase a fluid's speed, you decrease its the pressure that fluid exerts. Stagnant fluid exerts higher pressure than flowing fluid. Airplanes can fly because the way their wings are designed create pockets of stagnant air beneath the wings allows airplanes to fly.
Video: The Bernoulli Equation
13.3 Buoyancy
13.3.1 Explain the effect of buoyancy on the apparent weight of an object.
13.3.2 Explain the relationship between the volume of fluid displaced by an object and buoyant force acting on the object according to Archimedes’s principle.
13.3.3 Describe the relationship among object weight, buoyant force, and whether an object sinks or floats in a fluid.
- Buoyancy results in the apparent loss of weight of an object in fluid.
- If an object is less dense than the fluid it is in, it would float. If the object is more dense than the fluid it is in, it will sink.
- When the buoyant force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks.
buoyancy – the ability of a fluid to exert an upward force on an object placed in it
buoyant force – an upward force acting on an object in a fluid
Archimedes’s principle – the equivalence of the buoyant force on an object and the weight of the fluid displaced by the object
Prentice Hall: Buoyancy: Archimedes’s Principle, A Cartesian Diver
Brightstorm Archimedes Principle (6:08): Archimedes' principle states that the buoyant force on a fluid is equal to the weight of the displaced fluid. To calculate the buoyant force, we use the equation buoyant force = density of fluid x volume of displaced fluid x acceleration due to gravity. In a completely submerged object, the volume of displaced fluid equals the volume of the object. If the object is floating, the volume of the displaced fluid is less than the volume of the object but the buoyant force = the weight of the object.
Brightstorm Buoyancy (9:27): Buoyant force is the force that a fluid exerts on a object that is immersed within it. It is called buoyant force because this force is a lifting force, often making the object buoyant. Buoyant force can be calculated using Archimedes' Principle.
Brightstorm Flotation (11:17): In flotation, the buoyant force equals the weight of the floating object and the volume of the object is always greater than the volume of water displaced. Floatation can be calculated using Archimedes' Principle.
Video: The Buoyant Force
