gravity guide + solution

Author: dklein00

app/lib/game/core/levels.ts

@@ -453,6 +453,7 @@ export const LEVELS = {
     availableNodes: [
       "Display",
       "Value",
+      "Time",
       "Math",
       "Switch",
       "KeyPress",

content/docs/level-guides/gravity.mdx

@@ -3,3 +3,49 @@ title: Gravity
 ---
 
 # Level 8: Gravity
+
+## Objective
+
+In this level the player is tasked with creating a simple physics system to move two gameobjects:
+- The trashcan needs to be assigned a **downward velocity**.
+- The raccoon needs to be given a **custom velocity handle** to handle gravity and a jump input.
+- The raccoons Y axis position needs to be calculated for every frame.
+When all these mechanics are implemented the player has to make the raccoon jump intro the trashcan.
+
+## Scenario
+
+The player encounters a floating trashcan.
+The raccoon wants to be able to jump into the trashcan, but it has no ability to jump yet and is unable to reach the trashcan.
+With the help of the player the raccoon wants to make the trashcan fall from the sky and jump into it.
+
+## Key Concepts Introduced
+
+### Velocity-based movement
+
+In this level we are using velocity-based movement to further expand on the movement we already added in the "Move" level. 
+Velocity describes how fast an object is moving and in what direction.
+So instead of the movement keys directly affeting the raccoons position, they will only affect the velocity of the raccoon.
+This means that each frame the raccoons position needs to be calculated using its last position and it's current velocity.
+
+For simplicity we are only applying this concept to the Y axis in this level.
+
+### Custom handles
+
+Custom handles can be added using the `exportToGameobject` node. They help in tracking more data over from the last frame.
+
+In this level the player only has the raccoons Y axis position available as import and export.
+A custom velocity handle needs to be added to keep track of the raccoons momentum in each frame.
+
+## Strategic Goals
+
+- Assign a value to the trashcans Y axis velocity
+- Create a custom velocity handle and calculate the raccoons Y axis position for each frame using the velocity
+- Create a constant gravity force using `deltaTime` and add it to the velocity each frame
+    - Make sure the velocity doesn't scale infinitely while the raccoon is standing on the ground
+- Add a `KeyPress` node, which triggers a jump force being added to the velocity
+
+
+## Relevant Documentation
+
+- [Physically Based Animation](/docs/physically-based-animation/dynamics-of-a-point-mass)
+- [Export Node](docs/nodes/export)