The following simulation models the motion of a ball falling in air that exerts a frictional force on the ball that is in the opposite direction as the ball's velocity and has a magnitude proportional to the ball's speed.  The proportionality constant is k.

Notice that a ball dropped from rest eventually reaches a constant speed called the terminal speed.  This is equal to g/k where g is the acceleration due to gravity and k is a constant that is proportional to the force of air resistance at a certain speed.  You can set a value of k before clicking on a link in order to observe its effect on the motion of the projectile.

Answer the following questions:

1.  Suppose a skydiver is falling at her terminal speed when she pulls her 'chute.  During the time interval that the chute opens, what direction is her acceleration?

2.  If a projectile is projected downward with an initial speed greater than g/k, the object's terminal speed, is its instantaneous acceleration, at any instant before it reaches terminal speed, positive or negative?

 
Position vs. Time

Velocity vs. Time

Acceleration vs. Time

Ball dropped from rest

Ball projected upward from y=0

Ball projected downward with initial speed greater than the terminal speed (vterm=g/k)

Drag Coefficient: 

k= s-1