A6.5-cm-diameter ball has a terminal speed of 18 m/s. what is the ball's mass? use 1.2 kg/m3 for the density of air at room temperature.

The terminal velocity is the highest velocity that an object  can reach when falling through a medium opposing a drag force to the motion, and it is given by

where 
m is the mass
g is the gravitational acceleration
is the density of the medium
is the projected area of the object
is the drag coefficient

We can consider the ball as a perfect sphere, so its drag coefficient is . The projected area is the cross-sectional area of the ball, so

the radius of the ball is half the diameter:

so the area is


And so by using the data of the problem and re-arranging the equation, we can calculate m (the mass) from the terminal velocity: