Understanding pressure

Understanding Pressure

How much force are you exerting on the Earth?

To answer the question, consider the following formula:

Pressure = Force/ Density

where Force (F) = Mass (m) x Acceleration (a)

with Force expressed in Newtons (N).

where Density = Area in m2

Step 1: Finding the Force

a. Multiple your mass (in kg) by 9.8m/s/s (the gravity constant)b. The answer represents your force in Newton.

Step 2: Finding the Area.

c. On a sheet of graphing paper, trace an outline of your foot. d. Count the number of squares covered by your foot (4 squares = 1 cm2).e. Multiple the number of squares by 2 to represent both feet. f. To convert to m2 , divide the number by 10,000.

Calculating the actual force you exert on the ground:

Pascal = 1 N/1m2

For example,

Pascal = 534 N/0.11 m2

= 4,854 N/ m2

Force and surface area

• Force = Mass (in kg) x gravity (9.8m/s/s)• Surface area– 4 squares = 1 cm2

–Count how many groups of 4 you have–Divide this number by 10,000 to get from cm2 to

m2

–Multiply this number by 2 to get SA for both feet

Buildings should be designed for a distributed load of 11,500N/m2 and a point load of 11,400 N/m2

What could possibly happen to the floor if your calculated “pressure” is greater than the designed load limit?

If all 1.5 billion people in China jumped at the same time would the earth be thrown out of its

orbit?

China has a population of about: 1,330,044,544 (July 2008 est.)Average weight 86.6kg (based on US figures)

Basically the rough estimate for the weight of all those Chinese people comes out to

around 115,181,857,510.4kg. That's a pretty large amount of weight.

However the weight of the earth is approximately

6,000,000,000,000,000,000,000,000 (6E+24) kilograms; which is a significant difference. On top of all of that take into account the fact that the ground will compress absorbing a large portion of the force from the jumping, the total displacement would be negligible.

It is a cute idea; but even with all of those people nothing much would happen. In fact even if we got everyone in the world to join in (some 6.7+ billion) that total mass is still extremely insignificant when compared to the weight of the world and not to mention trying to overcome the inertia of an object that large!

Source(s):https://www.cia.gov/library/publications…http://en.wikipedia.org/wiki/Human_weigh…http://science.howstuffworks.com/questio…

Exploring Pressure

PRESSURE

• “press” = push on a surface• Pressure = Force/ surface area

P= F/ SA• Unit: pascal (Pa) =1 N/m2

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

• With backpack

Changing pressure

• Decrease surface area more pressure– Tippy toes– Can you think of opposite?

• Increase force more pressure

So Many Units!!!http://www.sensorsone.co.uk/pressure-units-conversion.html

UNITS EXAMPLESPSI (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 psi 220, 500 Pa

Air Pressure

• Why is air pressure hard to notice?

• When are times we do notice air pressure?

Activity 1:

Observations:

Explanation:

Activity 2:

Observations:

Explanation:

Activity 3:

Observations:

Explanation:

Activity 4:

Observations:

Explanation:

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 AirEmpty vs. full balloon• Full balloon is heavier

Air has mass

• 1 m3 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