Physics Objects (Rigidbodies) - Unity Official Tutorials

As a general rule, if you're going to have

moving game objects in your game

you should make sure that they are

rigidbody objects.

Rigidbodies are components that allow

a game object to be effected by physics.

They allow the object to fall under gravity,

and have physics properties such as mass,

drag and velocity.

When we add a rigidbody component

to a game object we often then refer to it

as a rigidbody object.

A rigidbody component is required for any physics

based interaction, and the game object

must also have a collider attached

in order to interact with other physics objects.

Without a rigidbody our power cube will

simply hover in mid air.

But let's see what happens when we add one.

Like any other component it can be added

using the Add Component button at

at the bottom of the inspector, or from

the 'Component' top menu.

You will find it under the Physics section.

Now our object falls under gravity

and can be controlled by the physics engine

and any forces that are applied to it.

Rigidbodies have numerous options.

Firstly there are settings to control the mass,

drag and angular drag of the game object.

The mass of the object effects how collisions

are treated with the object.

Game objects with a higher mass will react

less when collided with a lower mass

game object.

The drag of a game object effects how

quickly it will slow down without

other interactions.

Think of it like air resistance.

It's used to determine the rate of a loss

of linear velocity.

Similarly, angular drag effects how

quickly the game object will slow it's

angular velocity, i.e. how

fast it is rotating.

So for example if you're adding torque

to the object to rotate it,

the angular drag will create resistance

to this force. The next option is

whether or not the game object is

effected by gravity.

By enabling this checkbox we use gravity.

Settings for gravity can be seen in the

Edit - Project Settings - Physics area of Unity.

As you can see it's a 3 dimensional vector

which by default has a real world

value of -9.81.

Because you can customise it globally here

you could also create interesting effects

Such as low gravity for a platformer

or even setting it to a different axis

as part of a puzzle game.

For example, let's add gravity to the

Z axis by a value of 5.

And now the power cube is pulled towards

towards the global Z axis.

The Is Kinematic setting effects whether

or not a rigid body will react to physics.

Ordinarily when a scene begins, all static

geometry, meaning any non-rigidbody objects

are checked once by the physics engine

and not checked again for efficiency.

However when you move a static object

the physics engine must re-check all other

static objects for the sake of accuracy,

and this can be expensive to performance.

To avoid this, Kinematic rigidbody objects can be used

and moved via their transform

by using the Translate function.

This means that you can have physics objects

that effect others but are not effected themselves.

An obvious example of this would be the

paddle in a Pong or Breakout style game.

In this example our rigidbody power cube

has Use Gravity checked.

When we press play, the object falls to the ground.

We also have our round prop samoflange ball

object, which has a similar component setup.

If the power cube does not have gravity

then it will not fall under it, but it will

be effected by other objects.

If we don't want it to be effected by other

objects we can use Is Kinematic.

And as stated we can also move the object

via it's transform. So we'll make use of

this simple script, which uses the

translate function to move it via it's

forward direction every frame.

And as you can see, the object still

interacts with the others but remains a

rigidbody, so is constantly informing

the physics engine of it's location

and not forcing the physics engine

to re-evaluate the entire scene.

The Interpolate and Extrapolate settings

are there to solve jittering.

If you experience slight movement of your

object when moving it via it's rigidbody,

make use of the interpolate setting in order to

smooth the transform movement based on the

previous frame. And the extrapolate setting

to smooth based on a predicted

location in the next frame.

The next setting is for the type of

collision detection. We have Discrete,

Continuous and Continuous Dynamic.

The default is discrete and unless you

have any problems you should use discrete.

Continuous is for fast moving objects

that are interacting with static geometry.

And continuous dynamic is for fast moving

objects that are interacting with other

dynamic objects.

Finally the constraints section of the

rigidbody component allow you to

constrain movement or rotation of the object

by physics. For example, if you

had a Tetris style game you might not

want the cubes of your game to rotate

as they fell in to place. You could constrain

this using the rotation constraints here.

In this example our power cube is

falling on to the workbench. It's a rigid

body that has Use Gravity checked.

And as standard it falls like this.

If we didn't want it to rotate as it falls

we can freeze the rotation within the constraints.

And now when it falls, no rotation.