Forces and Fluids

What is a fluid? A fluid is any material that can flow and take the shape of its container. Liquids - like water and blood, AND gases - like oxygen - are fluids!

Under Pressure Pressure is the amount of force pushing against an amount of area Increasing the pressure means more force is pushing in the same area Same force on smaller area. Decreasing the pressure means less force on same area. Same force on larger area. fall through ice

Why are bubbles round? Fluid exerts pressure evenly in all directions. This also explains why tires inflate evenly, not just in the spot where the air is going in!.

Pressure and Area Any object with weight exerts pressure We exert (or give) pressure against the ground below our feet (and there’s an equal force pushing back up!) If we carry extra weight, we increase the pressure under our feet because we are pushing harder against the same area BUT, if we increase the size of our shoes and spread the same force over a larger areas, the pressure decreases. That’s why we can step across deep snow in wide snowshoes!

Why did the woman sink, but not the man? Pressure = force/area A woman’s high heels sink into the soft ground, because her force is exerted over a smaller area (her heel). Force over a smaller area increases the pressure.

More Math We can calculate the amount of pressure, force or area using the formula Force = Pressure x Area Area = Force Pressure Pressure = Force Area The unit we use to measure pressure is the pascal (say pass-kal) or Pa

Pressure in the Atmosphere The layer of nitrogen, oxygen and other gases that surround the Earth. 80% of gases are found within 10km of Earth’s surface. The atmosphere is the air that stretches 150 Km above you The atmosphere is held in place by gravity. Atmospheric pressure = pressure cased by the weight of the atmosphere The force or pressure of this air is equal to about 10N on every square cm of your body!

What’s pushing on you? At sea level (like Sandy Hook) , the pressure from the atmosphere is about 101,000 Pa At Mount Everest, it is about 33,000 Pa As you go higher, air pressure decreases because there’s less air above you!

A Sea of Air? In any fluid (liquid or gas), there is more pressure the deeper you go. On Earth, a beach at sea level is “deeper” is the atmosphere than a mountain top. In the ocean, you have the pressure of the water PLUS the pressure of the air on top of that!

Pressure ONLY depends on DEPTH… not how much water there is. Where would you experience more water pressure…….. 9 feet underwater in a pool or 2 feet underwater in the ocean?

Water pressure increases with depth. Pressure and Depth Explain the difference in the water pressure from the different spouts. Water pressure increases with depth.

Working with Pressure Divers use regulators with their tanks to increase the pressure of the air coming from the tank to their lungs. This makes it easier to breathe under water (where there is a lot of pressure!)

Differences in Pressure (Go with the Flow) Fluids flow from high pressure (where there is a lot of force pushing on them) to low pressure. This is how a straw works! It is also how you breathe! Fluids flow from HIGH to LOW!

Pressure and Elevation Air Pressure decreases as elevation increases.

Pascal's Principle When a force is applied to a confined (enclosed) fluid, the increase in pressure is transmitted equally to all parts of the fluid.

Transmitting Pressure in a Fluid When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.

Hydraulic Devices In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid.

Hydraulic Devices By changing the size of the pistons, the force can be multiplied.

Hydraulic Brakes The hydraulic brake system of a car multiplies the force exerted on the brake pedal.

Buoyancy Net upward force is called the buoyant force!!! It is easier to lift a rock in water (your force PLUS the buoyant force)!!

Displacement of Water The amount of water displaced is equal to the volume of the rock.

Archimedes’ Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces. If the buoyant force on an object is greater than the force of gravity acting on the object, the object will float The apparent weight of an object in a liquid is gravitational force (weight) minus the buoyant force

Floatation A floating object displaces a weight of fluid equal to its own weight.

What’s the Difference?

What is Density? Density is the amount of matter in a specific volume If two objects have the same volume (take up the same amount of space), then the object with the greater density will weigh more.

Why Does Density Matter? Water is more dense and weighs more than the same volume of air. So air is “floating” on water. The difference in density between water and ice explains why ice floats in water. (Ice is LESS dense than water) BUT, ice is heavier than gasoline and will sink in a puddle of gasoline.

Balloons and Density Helium (used in balloons) is 8 times lighter than oxygen and will float in air. But helium is 2 times heavier than hydrogen (so it would sink in hydrogen). Hydrogen floats better in air (because it is less dense), but it’s not as safe!