Rigidbody 2D - Official Unity Tutorial

The rigidbody 2D is a component that allows

a game object to be affected by 2D physics.

The game object must also have a collider 2D

component attached to interact with other game objects

using 2D physics.

A rigidbody 2D is required when working with

specialised 2D physics components

such as 2D joints.

The rigidbody 2D is also used to define important

physical properties about the game object,

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

gravity scale and other properties.

It is worth noting that unlike 3D physics

the rigidbody 2D has a gravity scale

which allows gravity to be set on a

per-game-object basis.

In this scene the prop_ammo_crate

is a simple game object with a sprite

renderer to hold and render the sprite.

Enter play mode, and nothing happens.

2D physics behaviour can be added to the ammo crate

by attaching a rigidbody 2D

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

It is worth noting that there are other ways to

attach a component to a game object.

For more information on components and

how to add them to game objects

please see the information linked below.

When in play mode, the ammo crate now falls

under the influence of gravity.

But it does not collide with the space ship.

To interact with other game objects using 2D physics

all game objects involved need collider 2d components

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

The space ship already has a 2D collider.

A collider 2D component can be added

to the ammo crate by selecting

Add Component - Physics 2D - Box Collider.

The size of the 2D collider will automatically

match the size of the sprite.

In this case the collider needs to be adjusted

to match the image of the ammo box.

Now, when in play mode,

the ammo crate collides with, and comes to rest on

the top of the space ship.

For more information on 2D colliders

please see the information linked below.

The rigidbody 2D component not only allows a

game object to be affected by 2D physics,

it is used to define important physical

properties about that game object.

Mass is the mass of the rigidbody 2D.

2D rigidbodies with higher mass,

like those in the real world,

take more force to move and will have greater affect

when colliding with other 2D rigidbodies.

In this first case the ammo crate

and the asteroid have the same mass.

When the mass is changed on the asteroid

the interaction between the two game objects

is significantly different.

Linear and Angular drag affect

the velocity of 2D rigidbodies.

Think of this as something similar to air resistance.

Linear drag will slow the speed, or velocity

of the rigidbody 2D.

And angular drag will slow it's rotation

or angular velocity.

Gravity Scale controls how gravity affects the rigidbody 2D.

Unlike changing the global value of gravity

gravity scale allows for precise

control on a per-game-object basis.

When in play mode the ammo crate

falls under the influence of gravity.

Lower the gravity scale value

and the game object falls more slowly.

To disable gravity set this value to 0.

When Fixed Angle is selected the rigidbody

2D will respond to 2D physics forces

but it will not rotate.

Under normal conditions the asteroid will fall,

collide with the ammo box and roll away.

When Fixed Angle is selected

the asteroid will continue to react to physical forces,

such as the collision.

But it will not rotate.

When Is Kinematic is selected

the rigidbody 2D is considered to be a

kinematic rigidbody 2D

and is not affected by 2D physics forces,

including gravity and collisions.

This is typically done when moving a game object

with a 2D collider without using

physics or physics forces.

For example, when creating a moving platform.

The platform could be controlled by a

script or animation

and be moved by it's transform,

not by physics forces.

However, the platform needs to interact

with other 2D game objects.

So the platform needs a 2D collider

attached as well.

This means there is now a moving game object

with a collider 2D attached.

When moving a collider 2D while the game is playing

it is highly recommended that a rigidbody 2D

be attached as well.

Yet we don't want this platform to react

to physics forces.

In this case, the rigidbody will be set

to Is Kinematic.

This platform is moved via scripting

with a 2D collider attached.

The Interpolate settings can help smooth

movement of a game object.

If a game object appears to jitter when

being moved by it's rigidbody 2D

use the Interpolate settings to help

smooth it's transform movement.

The Interpolate setting will smooth the

movement based on the game object's

position in the previous frame.

And the Extrapolate setting will use the

predicted position in the next frame.

Sleeping Mode controls how the rigidbody

will sleep to save processing time.

Never Sleep will disable sleeping

Start Awake will make sure the rigidbody 2D

is awake when instantiated.

Start Asleep will initially set the

rigidbody 2D to sleep

but it can be woken by collisions.

For more information on sleeping

please see the information linked below.

The collision detection settings control

the type of collision detection used

when a rigidbody 2D.

The default is Discrete

and it is best to use discrete unless

there are problems.

In this case collisions are registered if

the game object's 2D collider

is in contact with another during a physics update.

Continuous is for fast moving game objects

and collisions are registered if the game object

appears to have collided with another between updates.

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

please see the information linked below.