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Creating drag-and-release physics with bounce mechanics in Scratch

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PA

PhysicsGamer_Alex

Posted on July 25, 2025 • Advanced

🎮 Need help with drag physics and bounce mechanics!

Hey everyone! I’m working on a physics-based game and I need help with these mechanics:

  • When I drag a sprite horizontally, it should continue moving in that direction when released
  • The sprite should fall down due to gravity
  • When it hits another sprite, it should bounce off realistically
  • The bounce should affect both speed and direction

I want to create a realistic physics system where the drag speed affects the momentum, and collisions feel natural. Any help with the physics calculations would be amazing! 🚀

Game Type: Physics-based bouncing game with scoring

PE

PhysicsEngine_Maya

Replied 42 minutes later • ⭐ Best Answer

Excellent physics question @PhysicsGamer_Alex! Let me help you build a complete drag-and-bounce physics system:

🧮 Understanding the Physics

Here’s how the physics system should work:

flowchart TD A[🖱️ Mouse Down on Sprite] --> B[Track Drag Movement] B --> C[Calculate Velocity from Drag] C --> D[🖱️ Mouse Released] D --> E[Apply Horizontal Velocity] E --> F[Apply Gravity Force] F --> G{Collision Check} G -->|No Collision| H[Continue Movement] G -->|Collision Detected| I[Calculate Bounce] I --> J[Reverse & Reduce Velocity] J --> K[Apply Bounce Physics] H --> F K --> F style A fill:#e8f5e8 style D fill:#fff3e0 style I fill:#ffe8e8 style J fill:#f0f8ff

🚀 Solution 1: Advanced Drag Physics System

First, let’s create the drag detection and velocity calculation:

    // Main physics loop
when flag clicked
set [Gravity v] to [-0.5]
set [Friction v] to [0.98]
set [Bounce Damping v] to [0.7]
set [X Velocity v] to [0]
set [Y Velocity v] to [0]
set [Is Dragging v] to [0]

forever
handle drag input
apply physics
check collisions
end

// Custom block: handle drag input
define handle drag input
if <<mouse down?> and <touching [mouse-pointer v]?>> then
if <(Is Dragging) = [0]> then
// Start dragging
set [Is Dragging v] to [1]
set [Drag Start X v] to (x position)
set [Drag Start Time v] to (timer)
set [Last Mouse X v] to (mouse x)
else
// Continue dragging
set x to (mouse x)
set [Current Mouse X v] to (mouse x)
end
else
if <(Is Dragging) = [1]> then
// Release - calculate velocity
set [Drag Time v] to ((timer) - (Drag Start Time))
if <(Drag Time) > [0.1]> then
set [X Velocity v] to (((Current Mouse X) - (Drag Start X)) / ((Drag Time) * [30]))
end
set [Is Dragging v] to [0]
end
end

⚡ Solution 2: Realistic Physics Movement

Now let’s implement gravity and momentum:

    // Custom block: apply physics
define apply physics
if <(Is Dragging) = [0]> then
// Apply horizontal velocity with friction
change x by (X Velocity)
set [X Velocity v] to ((X Velocity) * (Friction))

// Apply gravity
change [Y Velocity v] by (Gravity)
change y by (Y Velocity)

// Ground collision (simple)
if <(y position) < [-150]> then
set y to [-150]
set [Y Velocity v] to ((Y Velocity) * (-1) * (Bounce Damping))
set [X Velocity v] to ((X Velocity) * (Friction))
end

// Side boundaries
if <(x position) > [230]> then
set x to [230]
set [X Velocity v] to ((X Velocity) * (-1) * (Bounce Damping))
end
if <(x position) < [-230]> then
set x to [-230]
set [X Velocity v] to ((X Velocity) * (-1) * (Bounce Damping))
end
end

🎯 Solution 3: Advanced Collision System

For sprite-to-sprite bouncing with realistic physics:

    // Custom block: check collisions
define check collisions
if <touching [Bounce Platform v]?> then
// Get collision angle
set [Collision Angle v] to (direction to [Bounce Platform v])

// Move away from collision
repeat [5]
if <touching [Bounce Platform v]?> then
change x by ((cos of (Collision Angle)) * [-2])
change y by ((sin of (Collision Angle)) * [-2])
end
end

// Calculate bounce vector
calculate bounce physics

// Add some randomness for realism
change [X Velocity v] by (pick random [-1] to [1])

// Play bounce sound
play sound [bounce v]
end

// Custom block: calculate bounce physics
define calculate bounce physics
// Reflect velocity based on collision angle
set [Incident Angle v] to ((atan of ((Y Velocity) / (X Velocity))) - (Collision Angle))
set [Reflection Angle v] to ((Collision Angle) - (Incident Angle))

// Calculate new velocity components
set [Speed v] to (sqrt of (((X Velocity) * (X Velocity)) + ((Y Velocity) * (Y Velocity))))
set [New Speed v] to ((Speed) * (Bounce Damping))

set [X Velocity v] to ((cos of (Reflection Angle)) * (New Speed))
set [Y Velocity v] to ((sin of (Reflection Angle)) * (New Speed))

🎨 Solution 4: Enhanced Visual Feedback

Add visual indicators for better user experience:

    // Visual feedback system
when flag clicked
forever
clear

// Draw velocity vector while dragging
if <(Is Dragging) = [1]> then
pen down
set pen color to [red]
set pen size to [3]
go to x: (x position) y: (y position)
go to x: (mouse x) y: (mouse y)
pen up

// Show predicted trajectory
draw trajectory preview
end

// Draw velocity indicator during flight
if <(Is Dragging) = [0]> then
if <((abs of (X Velocity)) + (abs of (Y Velocity))) > [1]> then
pen down
set pen color to [blue]
set pen size to [2]
go to x: (x position) y: (y position)
go to x: ((x position) + ((X Velocity) * [5])) y: ((y position) + ((Y Velocity) * [5]))
pen up
end
end
end

// Custom block: draw trajectory preview
define draw trajectory preview
set [Preview X v] to (x position)
set [Preview Y v] to (y position)
set [Preview VX v] to (((mouse x) - (x position)) / [10])
set [Preview VY v] to [0]

pen down
set pen color to [yellow]
set pen size to [1]
repeat [20]
change [Preview X v] by (Preview VX)
change [Preview Y v] by (Preview VY)
change [Preview VY v] by [-0.5]  // Gravity preview
go to x: (Preview X) y: (Preview Y)
end
pen up

🎮 Solution 5: Game Integration System

Complete system with scoring and multiple bounce targets:

    // Game management
when flag clicked
set [Score v] to [0]
set [Bounces v] to [0]
set [Combo v] to [0]
set [Last Bounce Time v] to [0]

// Enhanced collision with scoring
define check collisions
if <touching [Bounce Platform v]?> then
// Existing bounce physics code...
calculate bounce physics

// Scoring system
change [Bounces v] by [1]
set [Time Since Last v] to ((timer) - (Last Bounce Time))

if <(Time Since Last) < [2]> then
// Combo bonus
change [Combo v] by [1]
change [Score v] by ((10) * (Combo))
say (join [Combo x] (Combo)) for [1] seconds
else
// Reset combo
set [Combo v] to [1]
change [Score v] by [10]
end

set [Last Bounce Time v] to (timer)

// Special effects for high combos
if <(Combo) > [5]> then
set [ghost v] effect to [50]
wait [0.2] seconds
clear graphic effects
end
end

💡 Pro Physics Tips

  • 🎯 Velocity capping: Limit max speed to prevent glitches
  • ⚡ Frame-rate independence: Use timer-based calculations
  • 🔄 Energy conservation: Gradually reduce velocity over time
  • 🎨 Visual feedback: Show trajectory and velocity vectors
  • 🎵 Audio cues: Different sounds for different bounce types
  • 🎲 Realistic variation: Add slight randomness to bounces

This creates a complete physics system with realistic drag, gravity, and bouncing! 🚀⚽

PA

PhysicsGamer_Alex

Replied 1 hour later

@PhysicsEngine_Maya This is incredible! 🤯

I implemented the drag physics system and the bouncing works perfectly now! The trajectory preview is such a nice touch - it really helps players understand the physics.

One question: How can I make different bounce platforms have different bounce strengths? Like trampolines vs. walls?

MaterialPhysics_Jake

Replied 2 hours later

@PhysicsGamer_Alex Great follow-up! Here’s how to create different material properties:

    // Material system
define check material collision
if <touching [Trampoline v]?> then
set [Bounce Damping v] to [1.2]  // Amplify bounce
set [Material Sound v] to [boing]
else
if <touching [Rubber Wall v]?> then
set [Bounce Damping v] to [0.9]  // High bounce
set [Material Sound v] to [bounce]
else
if <touching [Concrete Wall v]?> then
set [Bounce Damping v] to [0.3]  // Low bounce
set [Material Sound v] to [thud]
end
end
end

play sound (Material Sound)
calculate bounce physics

You can also add material-specific effects like particle systems or screen shake! 🎮

VB

Vibelf_Community

Pinned Message • Moderator

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Outstanding discussion on physics systems! For developers looking to create even more advanced physics engines, our community offers expertise in:

  • ⚡ Advanced collision detection algorithms
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