Physics

Unity physics engines

• Manual: Physics
• A physics engine is used to calculate accelerations, collisions, gravity and other forces
  • In 3D, Unity uses NVIDIA’s PhysX for its physics
  • In 2D, Unity uses Box2D
• When you add a physics component to a GameObject, it is then part of the physics system
  • Rigidbody, Rigidbody2D

Built-in physics engines

• Manual: 3D Physics for object-oriented projects
• Manual: Physics reference 2D

Rigidbody

• 3D
  • Manual: Rigidbody overview
  • Manual: Rigidbody class
  • Script Reference: Rigidbody
• 2D
  • Manual: Rigidbody 2D class
  • Script Reference: Rigidbody 2D
• Literally: a body (object) that does not deform, i.e., is rigid
• Takes over the movement of the GameObject it is attached to
• Note: Do not move it from a script by changing the Transform properties such as position and rotation
  • Instead, you should apply forces

The Rigidbody components

• Every rigidbody has a mass, which affects how much forces influence them
• Note: If you have problems having two non-moving Rigidbody2Ds collide, set Sleeping Mode to Never Sleep

Types of rigidbodies

• Dynamic
  • is part of the physics engine and behaves like a “regular” physics-based object
  • can be controlled indirectly (with forces)
• Kinematic
  • is not affected by the physics system
  • only queries the physics engine for collisions
  • can be controlled directly:
    • in 2D, use Rigidbody2D.MovePosition, Rigidbody2D.MoveRotation
    • in 3D, manipulate transform.position
• Static (only 2D)
  • an immovable object, can only collide with non-static rigidbodies

Interpolation

• Script Reference: Rigidbody Interpolation
• Interpolation allows you to smooth out the effect of running physics at a fixed frame rate.

Constraints

• Script Reference: Rigidbody Constraints
• “which degrees of freedom are allowed for the simulation of this Rigidbody”
• You can freeze position or rotation of a given axis

Moving dynamic Rigidbodies

• You should interact with the physics system in FixedUpdate, not Update

• Script Reference: Rigidbody.AddForce

  if (Input.GetButton("Jump"))
  {
      //Apply a force to this Rigidbody in direction of this GameObjects up axis
      m_Rigidbody.AddForce(transform.up * m_Thrust);
  }
  

• Script Reference: Rigidbody.AddTorque

  float turn = Input.GetAxis("Horizontal");
  rb.AddTorque(transform.up * torque * turn);
  

Force modes

• When applying a force, you can give the function a second argument: the mode with which the force is applied
• ScriptReference: ForceMode has a good example project showcasing all four modes
  • ForceMode.Force is the default: the resulting acceleration is dampened by the mass of the rigidbody
  • ForceMode.Acceleration gives acceleration directly, disregarding the mass of the body.
  • ForceMode.Impulse gives the body a velocity (dampened by its mass), not an acceleration, resulting in snappier movement
  • ForceMode.VelocityChange is like impulse, but disregards the mass of the body.
• Note: Rigidbody2D only supports ForceMode.Force and ForceMode.Acceleration!

Extra: More functions

• Rigidbody.AddRelativeForce
  • Adds a force to the rigidbody relative to its coordinate system.
• Rigidbody.AddRelativeTorque
  • Adds a torque to the rigidbody relative to its coordinate system.
• Rigidbody.AddForceAtPosition
  • Applies force at position. As a result this will apply a torque and a force on the object.

Changing the velocity directly

• You can also manipulate rigidbody velocity directly
  • Do this only if you don’t get desired effects with AddForce or AddTorque
• Velocity
  • Script reference: Rigidbody.velocity (Vector3)
  • Script reference: Rigidbody2D.velocity (Vector2)
• Angular velocity
  • Script reference: Rigidbody.angularVelocity (Vector3)
  • Script reference: Rigidbody2D.angularVelocity (float)

Gravity

• Note: by default, gravitational acceleration is $9.81m/s^2$
• You can change it from Unity settings
  • Edit > Project Settings > Physics (2D) > Gravity
  • for top-down 2d games, you want gravity to be zero

Extra: Raycasting

• See 2D basics

Links

• Manual: Physics HOWTOs