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The rigidbody 2D is a component that allows

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a game object to be affected by 2D physics.

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The game object must also have a collider 2D

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component attached to interact with other game objects

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using 2D physics.

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A rigidbody 2D is required when working with

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specialised 2D physics components

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such as 2D joints.

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The rigidbody 2D is also used to define important

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physical properties about the game object,

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such as it's mass, linear and angular drag,

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gravity scale and other properties.

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It is worth noting that unlike 3D physics

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the rigidbody 2D has a gravity scale

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which allows gravity to be set on a

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per-game-object basis.

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In this scene the prop_ammo_crate

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is a simple game object with a sprite

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renderer to hold and render the sprite.

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Enter play mode, and nothing happens.

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2D physics behaviour can be added to the ammo crate

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by attaching a rigidbody 2D

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with Add Component - Physics 2D - Rigidbody 2D.

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It is worth noting that there are other ways to

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attach a component to a game object.

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For more information on components and

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how to add them to game objects

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please see the information linked below.

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When in play mode, the ammo crate now falls

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under the influence of gravity.

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But it does not collide with the space ship.

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To interact with other game objects using 2D physics

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all game objects involved need collider 2d components

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and at least 1 needs to have a rigidbody 2D attached.

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The space ship already has a 2D collider.

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A collider 2D component can be added

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to the ammo crate by selecting

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Add Component - Physics 2D - Box Collider.

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The size of the 2D collider will automatically

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match the size of the sprite.

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In this case the collider needs to be adjusted

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to match the image of the ammo box.

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Now, when in play mode,

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the ammo crate collides with, and comes to rest on

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the top of the space ship.

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For more information on 2D colliders

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please see the information linked below.

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The rigidbody 2D component not only allows a

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game object to be affected by 2D physics,

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it is used to define important physical

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properties about that game object.

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Mass is the mass of the rigidbody 2D.

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2D rigidbodies with higher mass,

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like those in the real world,

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take more force to move and will have greater affect

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when colliding with other 2D rigidbodies.

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In this first case the ammo crate

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and the asteroid have the same mass.

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When the mass is changed on the asteroid

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the interaction between the two game objects

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is significantly different.

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Linear and Angular drag affect

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the velocity of 2D rigidbodies.

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Think of this as something similar to air resistance.

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Linear drag will slow the speed, or velocity

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of the rigidbody 2D.

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And angular drag will slow it's rotation

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or angular velocity.

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Gravity Scale controls how gravity affects the rigidbody 2D.

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Unlike changing the global value of gravity

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gravity scale allows for precise

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control on a per-game-object basis.

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When in play mode the ammo crate

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falls under the influence of gravity.

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Lower the gravity scale value

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and the game object falls more slowly.

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To disable gravity set this value to 0.

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When Fixed Angle is selected the rigidbody

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2D will respond to 2D physics forces

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but it will not rotate.

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Under normal conditions the asteroid will fall,

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collide with the ammo box and roll away.

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When Fixed Angle is selected

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the asteroid will continue to react to physical forces,

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such as the collision.

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But it will not rotate.

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When Is Kinematic is selected

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the rigidbody 2D is considered to be a

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kinematic rigidbody 2D

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and is not affected by 2D physics forces,

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including gravity and collisions.

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This is typically done when moving a game object

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with a 2D collider without using

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physics or physics forces.

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For example, when creating a moving platform.

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The platform could be controlled by a

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script or animation

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and be moved by it's transform,

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not by physics forces.

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However, the platform needs to interact

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with other 2D game objects.

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So the platform needs a 2D collider

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attached as well.

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This means there is now a moving game object

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with a collider 2D attached.

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When moving a collider 2D while the game is playing

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it is highly recommended that a rigidbody 2D

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be attached as well.

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Yet we don't want this platform to react

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to physics forces.

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In this case, the rigidbody will be set

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to Is Kinematic.

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This platform is moved via scripting

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with a 2D collider attached.

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The Interpolate settings can help smooth

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movement of a game object.

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If a game object appears to jitter when

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being moved by it's rigidbody 2D

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use the Interpolate settings to help

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smooth it's transform movement.

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The Interpolate setting will smooth the

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movement based on the game object's

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position in the previous frame.

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And the Extrapolate setting will use the

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predicted position in the next frame.

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Sleeping Mode controls how the rigidbody

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will sleep to save processing time.

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Never Sleep will disable sleeping

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Start Awake will make sure the rigidbody 2D

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is awake when instantiated.

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Start Asleep will initially set the

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rigidbody 2D to sleep

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but it can be woken by collisions.

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For more information on sleeping

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please see the information linked below.

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The collision detection settings control

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the type of collision detection used

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when a rigidbody 2D.

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The default is Discrete

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and it is best to use discrete unless

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there are problems.

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In this case collisions are registered if

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the game object's 2D collider

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is in contact with another during a physics update.

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Continuous is for fast moving game objects

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and collisions are registered if the game object

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appears to have collided with another between updates.

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For more information on physics 2D and the rigidbody 2D

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please see the information linked below.