# Programming velocity and speed in Scratch

#### Example Velocity Calculations

To begin, I'll run through a calculation with a starting speed of 2.5. A down arrow key press will set `y velocity` to `-2.5`, which is multiplied by `0.9` in the `set y velocity` block; the new `y velocity` becomes `-2.25`, which is used in the `change y by` block. The rocket moves by -2.25.

To make the rocket accelerate, the player needs to press the arrow keys continually. If the player does not press another arrow, the value of `y velocity` gradually decreases to zero. Beginning with a value of -2.25, a second pass through the `forever` block would yield a `y velocity` of -2.025 (-2.25x0.9), a third pass would yield -1.8225, and so on.

In another example, if the rocket is changing its x position by five and the player presses the left arrow key one time, then the new `x velocity` is 2.75, which means the rocket is still traveling to the right. Remember, in Scratch, positive numbers move up or right. Negative numbers move down or left.

#### Change Costume Direction

Each arrow key press also sets the orientation of the rocket, as seen in the `switch to costume ()` block. I made the additional costumes by copying the original costume. From the Costumes tab, I clicked the Edit button on each costume to open it in the image editor. Using the rotate and flip tools, I changed the image copies to face the appropriate directions.

As you see in my script, I created a costume to represent up, down, left, and right. I also changed the costume names to correspond to the direction I wanted to make them fly so that I can find the right costume when I use them in my scripts.

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