Gravity plays a key role in atmospheric pressure by holding gases close to planetary surfaces. Larger planets with stronger gravity can support thicker atmospheres than smaller planets. Atmospheric pressure on Earth results from the weight of nitrogen, oxygen, and other gases in the air above. Pressure decreases with increasing altitude as there is less air above. Temperature and humidity also impact air pressure, with warmer or more humid air having lower density and pressure than cooler or drier air at the same location. Molecular motion principles like Bernoulli's principle explain how moving air has lower pressure than still air.

1. AIR PRESSURE

2. Keeping an Atmosphere
• Atmosphere is kept by the world’s gravity
▫ Low mass (small) worlds= low gravity
=almost no atm.
▫ High mass (large) worlds = high gravity
= thick atm.
• Gravity and pressure
▫ Air pressure depends on how much gas there is.
▫ i.e. The atmospheric thickness.
▫ Air pressure makes air near Earth’s surface dense.

3. Gravity and Atmospheric Pressure
• The stronger the gravity, the more gas is held by the world
and the greater the weight of atm. on a point

4. Earth’s Atmosphere
• About 10 km thick
• Consists mostly of
nitrogen (78% N2)
and oxygen (21%
O2)
• Air has mass
because it is made
of atoms,
molecules, and
compounds.

5. The air is made up of molecules
that are in constant motion.

6. • Gravity pulls the air
molecules toward the earth,
giving them weight.
• The weight of the air
molecules pulling down and
all around us is called the
air pressure.
• Sea level Air Pressure =
14.7 lbs./in2

7. DEMONSTRATION - Suction Cup

8. DEMONSTRATION Kamikaze
Water Glass Trick
• Fill a cup with water.
• Place an index card on
top.
• Invert the glass and
card
• Watch the Air
Pressure at work!

9. • Air pressure is the
weight of air
molecules from
above.
• Air pressure is equal
in all directions.
• As you go up in
altitude, pressure
decreases.
Pressure = force per unit area
Air Pressure

10. DEMONSTRATION - Hercules Spheres
• Can you pull these
hemisphere’s apart?

11. Balloon Auto-Inflate

12. The Force Mat

13. Milk Jug Mess

14. • Altitude – As Altitude , Pressure Decreases 
• Temperature - Temperature , Density , Air Pressure 
• Humidity - Humid Air Weighs Less , Air Pressure 
Influences of Air Pressure

15. Influences of Air Pressure - ALTITUDE

16. High altitudes = lower pressure
Low altitudes = higher pressure
Influences of Air Pressure - ALTITUDE

17. As
elevation
goes up
Barometric
pressure
goes
down.
This is an inverse relationship.

18. Influences air pressure - Temperature
• Heat and Temperature are different.
1. Heat = Total amount of energy in a substance.
2. Temperature = Measure of Molecular Motion.

19. • Warm Air = Lower Pressure
• As temperature increases molecules spread further apart, become less dense,
and produce less pressure.
• Density = Mass / Volume
Slow moving
Very Dense
Warming up
Moving faster
Very Warm air Moving really fast
Spreading out Becoming less Dense
50°
70°
90°
Molecules
move
faster
Air becomes
less dense.
Less dense air
rises (displaced).
Influences air pressure - Temperature

20. Balloon Charmer

21. Cork Shot

22. Cooler air Slow
moving Closer
Together Becoming
more dense
Cooling down
Moving slowerVery Warm air Moving
really fast Spread out
Less Dense
50°
70°
90°
Molecules start
to move slower.
Air becomes
more dense. More dense
air sinks.
Influences air pressure - Temperature
• Cooler Air = More Pressure
• As temperature decreases molecules move slower and
group together, become more dense, and produce higher
pressure.

23. Molecular Motion
• Molecules in motion will create a lower pressure
area.
• High pressure will rush in to even out the air
pressure.
• The faster molecules move, the lower the air
pressure will get.
• BERNULLI’s PRINCIPLE:
▫ Molecules in motion have lower air pressure than
those not moving.

24. Bernoulli's Wind Bag

25. Ping Pong Ball Funnel

26. End with a CRUSH – THE CAN
CRUSHER!