Gravity and Falling Systems

# CHAPTER 4

Gravity and Falling Systems #

1. Introduction #

2. Learning Objectives #

• Differentiate between constant falling speed and gravitational acceleration.

• Implement a basic gravity system using vertical velocity.

• Code a responsive jump mechanic based on an initial impulse.

• Implement Terminal Velocity to prevent game-breaking physics glitches.

• Understand "Arcade" gravity tuning for platformer games.

3. Gravitational Acceleration (The Math) #

• Standard Earth gravity is approximately -9.81 m/s².

• In code, we must continually subtract this value from our vertical velocity (velocity.y).

float gravity = -9.81f;
Vector3 velocity;

void Update()
{
    // Apply gravity to the vertical velocity over time
    velocity.y += gravity * Time.deltaTime;

    // Apply the resulting velocity to the actual position
    transform.position += velocity * Time.deltaTime;
}

4. The Jump Mechanic (The Impulse) #

• When the player presses 'Jump', you instantly set velocity.y to a high positive number.

• In the following frames, gravity will automatically chip away at that positive number, slowing the player down at the apex of the jump, and eventually pulling them back down.

float jumpForce = 5.0f;

if (Input.GetKeyDown("space") && isGrounded)
{
    // Instantly overwrite the vertical velocity
    velocity.y = jumpForce; 
}

5. Terminal Velocity (The Speed Limit) #

• You must enforce Terminal Velocity: a maximum falling speed.

float terminalVelocity = -50.0f;

// Apply gravity
velocity.y += gravity * Time.deltaTime;

// Clamp the falling speed to prevent tunneling
if (velocity.y < terminalVelocity)
{
    velocity.y = terminalVelocity;
}

6. Arcade Tuning (The "Mario" Jump) #

1. 1. High Gravity, High Jump: Double the gravity to -20f so the player falls fast, and double the jump force to compensate. This makes the jump feel "snappy."

1. 2. Variable Jump Height: If the player lets go of the jump button early, artificially multiply the gravity or cut the velocity.y in half. This allows for short hops versus high jumps.

7. Visual Learning: The Jump Arc #

      (Apex: velocity.y = 0)
            ____
          /      \
(v.y = 5)/        \ (v.y = -5)
        /          \
[ JUMP ]            [ LAND ]
(v.y = 10)          (v.y = -10, reset to 0)

8. Best Practices #

• Resetting Velocity on Landing: The moment your character touches the ground, you MUST set velocity.y = 0 (or a very small negative number like -2f to keep them stuck to the floor). If you let gravity build up while standing on the floor, the moment the player walks off a ledge, they will shoot downward at Mach 3!

9. Common Mistakes #

• Grounded Checks: The hardest part of jumping is writing the isGrounded boolean. If your code thinks the player is slightly in the air because of a bumpy floor, they won't be able to jump. Always use a generous Raycast or a SphereCast pointing downward from the player's feet to reliably detect the floor.

10. Mini Project: Build a Custom Gravity Controller #

class GravityController
{
    public float gravity = -20.0f; // Snappy arcade gravity
    public float jumpVelocity = 8.0f;
    Vector3 currentVelocity;
    bool isGrounded = true;

    void Update()
    {
        // 1. Check if on ground (Conceptual method)
        isGrounded = CheckGround(); 

        if (isGrounded && currentVelocity.y < 0)
        {
            // Reset gravity buildup when standing on the floor!
            currentVelocity.y = -2f; 
        }

        // 2. Jump input
        if (Input.GetKeyDown("space") && isGrounded)
        {
            currentVelocity.y = jumpVelocity;
        }

        // 3. Apply Gravity Acceleration
        currentVelocity.y += gravity * Time.deltaTime;

        // 4. Apply Final Movement
        transform.position += currentVelocity * Time.deltaTime;
    }

    bool CheckGround() { /* Perform Raycast downward */ return true; }
}

11. Practice Exercises #

1. 1. Why must gravity be added to velocity over time, rather than added directly to position?

1. 2. What is Terminal Velocity, and what critical physics bug does it prevent?

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the mathematical difference between linear movement (speed) and gravitational movement (acceleration). How is this represented in code?

• Q: What is "Variable Jump Height" (where holding the jump button goes higher than tapping it)? Explain the logic used to achieve this arcade physics mechanic.

• Q: Describe the concept of "Tunneling" in physics engines. How does enforcing a Terminal Velocity mitigate this issue for fast-falling objects?

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #