The applet is on Page 2. Under Useful Items, in the lower left corner of the window, the following activities are suggested for the applet:

• A1: The Effect of Mass on Projectile Motion.
• A2: One Ball Dropped From Rest, One Ball Launched Horizontally.
• A3: Intersecting Trajectories
• A4: Two Balls Launched Horizontally From The Same Height But Different Horizontal Positions And With Different Speeds
• A5: Two Balls Launched From The Same Height With Equal Horizontal And Different Vertical Velocity Components.

Detailed information on how the applet functions and a lesson on projectile motion are available under Help in the top left corner of the applet.

The applet shows the motions of two balls performing projectile motion simultaneously. One ball (red) always moves without air resistance and is subject only to gravity. The other ball (yellow) moves with an acceleration consisting of two parts: one due to gravity and one due to air resistance. The former is constant, the latter has magnitude proportional to the square of the ball's speed and direction opposite to the ball's velocity.

Expressed in symbols, the x and y components of the yellow ball's acceleration, a_x and a_y, are equal to:

a_x = -(C/m)vv_x and a_y = -g - (C/m)vv_y,

where C is the drag coefficient (adjustable), m the ball's mass (adjustable), v the ball's speed, v_x and v_y the x and y-components of the ball's velocity (initial velocity adjustable), and g the magnitude of the acceleration due to gravity (adjustable).

The x-axis is horizontal and points to the right, and the y-axis is vertical and points upward.

These two equations can be expressed by the one vector equation

= - (C/m) v .