physics
posted by paro .
Many cultures around the world still use a simple weapon called a blowgun, a tube with a dart that fits tightly inside. A sharp breath into the end of the tube launches the dart. When exhaling forcefully, a healthy person can supply air at a gauge pressure of 6.0 kPa.
What force does this pressure exert on a dart in a 1.4cmdiameter tube?

Multiply the gauge pressure in Pascals (6000) by the tube's circular inner area cross section in square meters (pi D^2/4) to get the force in Newtons.
pi D^2/4 = 1.54*104 m^2
I get about 1 Newton, which is about 1/4 pound force. 
Use the equation Pressure = Force / Area. Solve the equation for Force to get Force = Pressure * Area. Now, just plug in the values. Force = 0.006 Pa * (pi)0.008^2. Notice you need to divide the radius (0.008 m), not the diameter, in the equation for the area of a circle. The answer is 1.21 N.