Forces and Fluids

What is a fluid? A fluid is any material that can flow and take the shape of its container. A fluid can flow because its particles easily move past each other. Examples of fluids are Liquids - like water and blood, AND gases - like oxygen

Why do fluids exert pressure? Why does the air in a balloon exert pressure on the balloon? Copyright © Houghton Mifflin Harcourt Publishing Company

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 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!

Under Pressure Pressure is the amount of force pushing against an area A greater force results in greater pressure. Pressure also depends on the area over which the force is exerted. A greater area results in less pressure.

A woman’s high heels put more her weight over a small area - that’s a lot of pressure! The man’s weight is spread out. Pressure = force/area Why did the woman sink in the soft ground, but not the man?

Do the 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 atmosphere is the air that stretches 150 Km above you 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 Quite a difference!

Air Pressure decreases as elevation increases.

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, the air is also pushing down on the water! Pressure ONLY depends on depth!

Water pressure increases with depth.

Pressure in a Fluid Pressure increases at greater depth.

When a force is applied to a confined 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.

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! Air also moves from areas of higher pressure to areas of lower pressure. This movement of the air is known as wind. It is also how you breathe!

Every Breath You Take Fluids (air) going from high to low pressure explains how you breathe Fluids (air) going from high to low pressure explains how you breathe Take a breath in and your chest cavity expands (more area and less pressure) Take a breath in and your chest cavity expands (more area and less pressure) Breathe out and reverse the flow! Breathe out and reverse the flow!

Bernoulli’s Principle When the speed of a fluid increases, pressure in the fluid decreases. When the speed of a fluid increases, pressure in the fluid decreases.

Bernoulli’s Principle Flow is faster when the pipe is narrower Flow is faster when the pipe is narrower This is why water flows faster when you put your thumb over the end of a garden hose This is why water flows faster when you put your thumb over the end of a garden hose So pressure is lower in a fluid that is moving faster! So pressure is lower in a fluid that is moving faster!

Bernoulli’s Principle Can you explain what will happen and why?

Bernoulli’s Principle and Curveballs Slower air (where the air and direction of the ball are opposite) has higher pressure and pushes the ball.