Page 202 - Learn To Program With Scratch

This information is all we need to realistically simulate the ball’s motion,
so let’s create a Scratch program so we can see this bit of physics in action
and deepen our understanding of trajectories. The user interface of the
simulation is shown in Figure 7-33.

Set the launch speed (m/s).
(x,y) = ( –180, –140) Set the launch angle.
Fire sprite
Cannon sprite

Wheel sprite
Ball sprite

Show flight time (s) and
horizontal range (m).

Distance (m)

Figure 7-33: User interface for the projectile motion simulator
As shown, the application contains four sprites. The Wheel sprite
provides a rotation shaft for the cannon, while the Cannon sprite, which
rotates in accordance with the angle slider, provides a visual indication of
the launch angle. The Fire sprite is a button that the user clicks to fire the
ball, and the Ball sprite contains the main script for calculating the ball’s
coordinates and drawing its trajectory. The user specifies the launch angle
and the initial velocity using the two slider controls, then clicks the Fire
button. The Ball starts from point (–180, –140) on the Stage and draws the
parabolic trajectory for the specified parameters. The two monitors at the
lower-right corner of the Stage show the flight time and the horizontal
range during the flight.
The simulation starts when the green flag icon is clicked. The scripts
for the Cannon sprite (not shown here) point the cannon in the direction
specified by the angle slider control. The user can also specify the angle by
clicking and dragging the cannon. When the user clicks the Fire button, it
broadcasts a Fire message, which is received and processed by the Ball sprite
via the script shown in Figure 7-34.
To prepare to fire u, the Ball moves in front of the Cannon and the Wheel
and positions itself at the launch point. It puts its pen down and clears all
pen marks from the Stage. The script then calculates the horizontal (or x)
and vertical (or y) components of the initial velocity (named vx and vy,
respectively) and initializes the time variable (t) to 0.

180 Chapter 7
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