PRESSURE “press” = push on a surface Pressure = Force/ surface area P= F/ SA Unit: pascal (Pa) =1 N/m 2
Who exerts more pressure on the ground an elephant or a woman in spike heels? 65kg on a surface of 2 cm 2 (e.g., high heel shoes) will result in a pressure of 3 250 000 Pa (beneath the high heels, if the person is standing on the surface of planet Earth). A four ton elephant, on the other hand, standing on one foot will cause a pressure of only 250 000 Pa under that foot. Woman wins! Stiletto heels have an area of about 1/16 of a square inch. Elephants, unlike humans, walk with two feet on the ground at a time. Each foot is about 40 square inches. Thus, the woman "wins" by far more than 1,500 psi versus 75 psi. This explains why people with wood floors don't want women walking on them in high heels. Source: http://hypertextbook.com/facts/2003/JackGreen.shtmlhttp://hypertextbook.com/facts/2003/JackGreen.shtml
Changing pressure Do a push up with whole hands vs. finger-tips –Which one feels harder? –What did we change- force or surface area? –Whole foot versus tip-toes Now try push ups with your whole foot and then on your tip-toes. Now do your push-ups with a backpack. - What did we change - force or surface area?
Changing pressure Decrease surface area more pressure –Tippy toes –Can you think of opposite? Increase force more pressure
Buildings should be designed for a distributed load of 11,500N/m 2 and a point load of 11,400 N/m 2 What could possibly happen to the floor if your calculated “pressure” is greater than the designed load limit?
So Many Units!!! http://www.sensorsone.co.uk/pressure-units-conversion.html http://www.sensorsone.co.uk/pressure-units-conversion.html UNITSEXAMPLES PSI (pounds per square inch) Pumps for balls, tires mmHg (Millimeter of Mercury) Blood pressure, the weather Bar and Atm (atmosphere)Scuba diving Surface (1 atm)101,000 Pa 10 meters below (2 atm)202,000 Pa Tires 32 psi220, 500 Pa
Air Pressure Why is air pressure hard to notice? When are times we do notice air pressure?
Evidence for Air Pressure Properties of Air 1. Cup and paper towel demo Predict- what will happen to the paper towel when you submerge the cup in water? Why is the paper towel able to stay dry?
Evidence for Air Pressure Air takes up space (has volume)
Evidence for Air Pressure Properties of Air 2. Empty vs. full balloon Predict how the full balloon’s mass will compare to the empty balloon
Evidence for Air Pressure Properties of Air Empty vs. full balloon Full balloon is heavier Air has mass 1 m 3 of air = 1 kg In palm, weight of air = force of washing machine (why don’t we feel it?)
Movement of Air Pressure 3. Syringes –Describe the movement of air pressure –How strong is air pressure?
Evidence for Air Pressure Properties of Air 4. Balloon lift demo Air can exert pressure –See how much you can lift Can you think of other ways to demonstrate that air takes up space and exerts pressure? http://www.youtube.com/watch?v=08ezkSrQ5lc&feature=related
Air Pressure Air Pressure is a measure of the force of the air pressing on any surface
Movement of Air Pressure Nature wants to balance air pressure Rockets