Pressure

What happens to a whale’s lungs when it dives underwater?

Learning Objectives

After completing this lesson, the student will be able to describe similarities and differences in loads on a body based on fluid properties.

Standards

  • NGSS HS-ETS1-3
  • CCSS.Math.Practice.MP1 
  • CCSS.Math.Practice.MP2
  • CCSS.Math.Practice.MP4

Supplies

  • Balloon
  • Tub of water
  • Tape measure

Units Used

  • Mass: kilogram (kg)
  • Length: centimeter (cm)
  • Length: meter (m)
  • Temperature: Celsius (C)
  • Time: second (s)
  • Force: Newton (N) (1 N=1 kg m/s2)
  • Pressure: kilopascal (kPa=1000 N/m2)

Pressure

Fill a balloon with air and estimate its radius.  (If you’re having trouble measuring the radius, you can use the relationship between the radius and the circumference of a sphere: circumference=2π radius).  Radius in air =_________ cm

Now, put your air-filled balloon under water.  What is its radius? Radius in water = _________cm.  

Did the radii in air and under water differ?  How? Why do you think that is?

Pressure! Pressure (in pascals) is measured in terms of force (newtons) per area (meters squared).  1,000 pascals are in a kilopascal (kPa).  The standard atmospheric pressure at sea level is 101.325 kPa of pressure (also referred to as 1 ATM, 1 atmosphere) – there’s a lot of air weighing down on top of you every day!  As you rise in elevation, the pressure decreases.  On top of Mt. Everest, 8,848 m above sea level, atmospheric pressure is about 30 kPa.  What do you think would happen to your balloon if you carried it up to the top of Mt. Everest?  

The ideal gas law tells us PV=nRT where P is pressure, V denotes volume, n is equal to the number of gas molecules, R is the ideal gas constant, and T is the temperature of the gas.  If we were able to keep the temperature of the air inside the balloon constant (tuck it into your jacket as you climb Everest), and assuming no air escapes the balloon, we can estimate how much the balloon will expand by setting PV for the balloon at sea level equal to PV for the balloon atop Everest.  If the pressure atop Everest is about 0.3 (30/101.325) times the pressure at sea level, what would be the volume of the balloon atop Everest?  

Balloons are used to study earth and space from high altitudes, heights of 30,000m (30km) are not uncommon!  If you were filling a balloon with helium, standing outside your school, to launch for high altitude research, would you want to fill it fully?  Why or why not?

Coming back to the whale of a question we opened with, what do you think happens to a whale’s lungs when it dives deep underwater? Whales are uniquely adapted to the pressures of the deep ocean. If you’re interested to lean more, check out this website from the University of Hawaii.

Next Steps

Another important fluids concept, tied closely to pressure, is density. Check out this density lesson next.

A Day in the Life
Form and Function
Balance
Movement
The Literate Engineer

Last updated: November 23, 2022.