Page 52 - Learn To Program With Scratch

try it out 2-2
Find the rocket’s final (x,y) position when it executes each of the two scripts shown
below . What mathematical theorem can you use to prove that the two scripts are
equivalent?

y
 100

50

x
50 100


Other Motion Commands
TennisBallChaser There are just four motion commands left to explore: point towards; a sec-
.sb2 ond type of go to block; if on edge, bounce; and set rotation style.
You’ve already learned about rotation styles, and you saw the if on edge,
bounce command in action in Chapter 1 (see Figure 1-13 on page 12). To
see the other two commands in action, let’s create a simple application of
a cat chasing a tennis ball, as illustrated in Figure 2-8.

Script for the Ball sprite

Script for the Cat sprite

.
Figure 2-8: Programming a cat to run after a tennis ball

As shown, the application contains two sprites, named Cat and Ball,
and two scripts. When you click the green flag icon, the Ball sprite follows
the mouse pointer. The Cat sprite continuously points towards the Ball and
moves toward it using the glide command. Go ahead and build this appli-
cation to see how it works. You can find the forever block in the Control
palette and the mouse x and mouse y blocks in the Sensing palette. You
can find the complete application in the file TennisBallChaser.sb2.
In the next section, we’ll look at the Pen palette and learn how to make
a sprite leave a visual trace of its motion.

30 Chapter 2
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