### Step 3: Simulate gravity

In this stage we’re going to start simulating gravity so that the bird falls down towards the ground.

Simulating gravity

• Make the code that displays the bird keep looping round as long as the game is being played. Make the changes on the highlighted lines below:
```from microbit import *

y = 50
while True:
led_y = int(y / 20)
display.set_pixel(1, led_y, 9)
sleep(20)```

TheÂ while True:Â line tells python to keep on repeating a block of code. The code to repeat is the code that is indented (moved across to the right). You can indent one or more line at a time by selecting it and pressing tab (which is usually to the left of the Q button on your keyboard – it looks like an arrow). You can un-indent lines (move them back to the left) by selecting them and pressing Shift + Tab.

The sleep(20) tells python to wait for 20ms every time the game loops. This stops the CPU from working too hard and slows your game down to a manageable speed. If you didn’t have this line your micro:bit would use more battery power than necessary or your simulator would crash the browser.

• At the moment, the bird just hangs in mid-air. We need to simulate gravity so that it falls towards the ground. Before your while loop (above line 6), add the line:Â speed = 1Â . Then, as the first line of the while loop (under line 7) add the line:Â  y += speedÂ . Make sure that this line is indented so that it’s lined up with the line below.

y += speed means that each time the loop goes round, the bird’s y coordinate will increase by the value of speed. As we’ve set speed to 1, this means y will increase by 1 each time the loop repeats.

If you run this code, you’ll seeÂ there’s two problems with it:

1) The bird (or dot!) does fall slowly towards the ground, but the previous position is never erased so you end up with a line rather than a dot.

Fix this by adding the line Â  display.clear()Â as the first line of the while loop (line 8). This clears the screen each time the program loops round so you can draw a fresh dot each time.

2) When the bird reaches the ground it keeps falling so the program crashes when the micro:bit attempts to draw the dot on row 6 (which doesn’t exist).

Fix this by adding these lines under y += speed:

```    if y > 99:
y = 99```

This limits the maximum y coordinate to 99.

Your code should look like this so far, with the new lines highlighted:

```from microbit import *

y = 50
speed = 1
while True:
display.clear()
y += speed
if y > 99:
y = 99
led_y = int(y / 20)
display.set_pixel(1, led_y, 9)
sleep(20)```

This isn’t particularly realistic. We’ve assumed that the bird should fall to the ground at a constant speed (which we’ve set to 1). In reality, gravity causes objects to accelerate towards the ground. So, let’s make the speed start at 0 then increase, up to a maximum of 2Â (you might have covered terminal velocity in science – that’s what this is).

Gravity and terminal velocity

Try out the code below:

Investigate the source code and spot the changes

I’ve made some small but important changes. See if you can:

1. Find the lines that are comments

Comments are part of the code that don’t actually do anything other than make it more readable for other programmers. They start with a #. You can have a whole line that’s a comment or put a # after some code and make the rest of the line into a comment.

2. Find the lines of code that stop the speed from going above 2

Notice that the line under the if statement is indented. A general rule to help you with debugging is that whenever you see a line that ends with a colon (:), the next line should be indented.

3. Find the lines of code that stop the bird from disappearing off the top of the screen.

This isn’t necessary yet, as the bird only ever falls downward, but it will be important in the next step when we start making the bird flap upwards.