Pressure

Pressure p. 416 Definition: Force exerted over a certain area. Pressure can be found by: Pressure = Force Area The units for pressure are Pascals

Calculating Pressure A woman in high heels has a force of 500 N. Her heels have a surface area of 0.005 m2. How much pressure is she putting on the ground? You know that pressure = force ÷ area 500 N ÷ .005 m2 = 100,000 Pa

Calculating Pressure The same woman is wearing work boots with a surface area of .4 m2. You know that pressure = force ÷ area You know force = 500 N 500 N ÷ .4 m2 = 1250 Pa

Increase/Decrease Pressure If you increase the area a force is applied to, the pressure decreases, and... If you decrease the area the force is applied to, the pressure increases. Ex: bed of nails, high heeled shoes vs. snow shoes, bullet, dull vs. sharp knife, etc…

Think about it Thinking back to the shoes and the formula for pressure, think about 2 ways you could increase pressure in a garden hose: Write down your ideas

Fluid Pressure Fluid (Def): a substance that can easily flow Liquids and Gases are fluids Examples of fluids: water, air, helium, oil, soda, carbon dioxide, rocket fuel, gas, etc…

Air Pressure Air takes up space Ex: balloons Molecules of air constantly move and collide with a surface, exerting a force on that surface. It is pushing on all surfaces in all directions. The air molecules in a balloon push each other and the side of the balloon to apply force = pressure

Air Has Mass 1 square meter of air = 1 kg of mass Force of gravity on this mass produces air pressure. Air pressure can change! Air wants to travel from an area of high pressure to low pressure. Ex: vacuum cleaner, breathing, heart beating, straws, ears popping, balloon popping, etc…

Crushing can demo Q: What happens when air is heated? Q: Label the area of “high pressure” and “low pressure” Q: What would happen if the can were closed instead?

Air Pressure Can Change Barometer – instrument that measures air pressure. Weather forecasters use these to predict the weather. Rapidly decreasing pressure could mean a storm is approaching Increasing pressure could indicate fair weather.

Balloon in a Flask Demo Recall that fluids move from an area of high to low pressure. Q: Explain how the balloon was sucked into the flask. Demo Explained

Air Pressure at Different Altitudes Sea Level = 101,300 Pa. 12,000 m (where airplanes fly) = 20,260 Pa. (20% the pressure of sea level) Why are airplanes pressurized? = w/out it, the pressure would be severely less than the pressure in your body = unable to breathe.

Air pressure changes Scuba divers – pressure increases with depth – must ‘regulate’ pressure in ears. So, with altitude (going higher), pressure decreases, and with depth under water (going deeper), pressure increases.

Crushing Pressure! Why does water and air pressure not ‘crush’ us? – Our body exerts pressure. Why? Body made of fluids – air, water, blood, carbon dioxide, oxygen, etc…