ElastoLab A Physics Playground for Kids

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ElastoLabA Physics Playground for Kids

• David Buck
• Simberon Inc.
• www.simberon.com

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History

• Started as a Masters thesis 1990 Smalltalk
• Re-wrote in C (1993)
• Close to release (1996)
• Rewrote in Smalltalk (1998)
• Released for sale (2001)
• Physics re-written in Smalltalk (2002)

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Demo
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Physics

• How do you simulate physics?
• Easy
• Calculate the forces on each particle.
• a F/m
• Add a bit of the acceleration to the velocity
• Add a bit of the velocity to the position
• Repeat
• Eulers method

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Problem

• Falling particles work great
• Springs explode when the strength gets too high

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Solution Runge Kutta

• Calculate forces 4 times, combine results.

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Better

• But really strong springs still explode.
• Solution Adaptive Runge Kutta
• Adapt step size according to error between single
  and double steps.

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Barriers

• Easy you just reflect the particles position
  and velocity by the barrier

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Umm Problem

• Springs explode and rigid bodies dont spin.
• Solution Step forward to the time of collision
  then apply an impulse to the particle

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What time is the collision?

• How do we know what time the particle hit?
• What if many particles hit around the same time
  which hit first?

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Solution

• Use Bisection Method to narrow down time of first
  collision.

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Resting Contact?

• What happens if the particle is sitting on the
  barrier? Collision time is almost zero.

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Dynamic Constraints

• Detect resting contacts and add a dynamic
  constraint force.
• The constraint force is a force just strong
  enough to prevent the particle from penetrating
  the barrier.

Constraint Force
Net Force
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Multiple resting contacts

• What happens if a particle is in resting contact
  with two or more barriers?

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Solution

• Use Quadratic Programming or Baraff 94
• ltunclegt
• Ok, just stop the particle and hope nobody
  notices.

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User Interface

• Every physics object has a corresponding
  Display object.
• A Camera defines the transformation from world
  to display and back.
• Clipping implemented for lines and polygons.

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Custom Widgets

• Custom widgets enhance the appearance of
  ElastoLab.

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Interfacing to Sound

• Smalltalk calls a C DLL I wrote
• The C DLL calls the COM interface to access the
  DirectX 8 sound facilities.

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Help Facilities

• Three kinds of help
• Fly by help
• Help mode Windows help files
• Guided tour

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Guided Tour
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Source Code Management

• For C, I just made copies of the source files.
• Smalltalk StORE using a Postgres database.
• ElastoLab consists of 2 bundles, 15 packages, 300
  classes.

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Making it Fast

• Displaying bitmaps works much faster if you use
  convertForGraphicsDevice and cache the result.
• High level optimizations better than low level
  optimizations

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Packaging

• Runtime Packager used for stripping image.
• ResHacker used to create one executable.
• InstallShield Express used to install.

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Message Board

• Online message board runs on Web Toolkit.
• SSL pages link to Smalltalk servlets
• Small app (4 classes)

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Wrap Up

• Smalltalk is quite fast enough to handle
  ElastoLab.
• Flexibility of Smalltalk is a big advantage.
• Packaging can create a single executable that
  fits into the Windows environment.
• SSP pages and Smalltalk servlets can be used to
  build community with a message board.