In this example we have fixed joints

and spring joints.

This example forms a basic wrecking ball

that does this.

Firstly the fixed joint. Fixed joints

work in a similar way to parenting objects

in the hierarchy.

The joint simply locks a game object

to the point in the world, or to a

connected rigidbody.

In this example we simply have placed the

first capsule in the chain up near

the ceiling and attached a fixed joint.

This locks it in place.

The other parameters of this fixed joint

are Break Force and Break Torque.

This is the minimum amount of force required

to break the joint.

When the joint is broken the game object

can once again move freely.

The other capsules in the chain have

spring joints. Spring joints work on the

principle that the game object is trying

to reach a target position. The target

position that it's trying to reach is the

position that it's set to in the scene view

and any rigid bodies that are attached to it

will pull it away from this position

on an invisible spring attached

to the anchor point.

If the spring joint has a connected rigidbody

then the target point that the spring

is trying to reach is relative to that

game object rather than to world space.

So for example, the second capsule here

is trying to reach capsule 1, and capsule 3

is trying to reach capsule 2.

As these move they will no longer be heading

towards the original positions, rather

they will be heading towards the anchor

point of wherever those capsules happen to be.

The anchor is the pivot point of the joint.

You should consider it as where the spring is

attached to the game object.

This is shown in the scene view by

an orange dot or box.

The spring parameter is a measure of how

strong the spring is. The higher the

value the tighter or stronger the

spring becomes. The damper parameter

is how much the joint will slow down

under motion. The higher the number, the

less the spring will overshoot.

The min and max distances are for setting

up a sort of dead zone for where the

spring isn't active. They effectively restrict

the length of the spring.

Similar to the fixed joint, the spring

also has a break force and a break torque,

which you can set to a certain amount in order

to find the minimum force required to

break that joint.

Finally, Hinge Joints.

Hinge joints are ideal for things like

doors and share some properties with

spring joints. The axis of the hinge

is the one that it rotates around.

In this example our physics door

has the hinge joint attached. The anchor

is set to 1 in the X axis,

which moves it from it's center to it's edge.

Again the joint is represented in

orange in Unity and you can see a small line

showing you where the hinge is currently located.

The axis then allows us to specify

around which axis the hinge will pivot.

We've set this to the Y axis so it will

react like a real hinge with a simple script

we're applying a force to the door

when we click on it, which pushes it

backwards in it's Z axis

using the shortcut minus transform.forward.

In the Add Force lesson you may have seen

that this fires the door straight out of the frame.

But now that we have a hinge,

when we apply the force to the door

it simply pivots around that point.

and no matter how many times we add force

to it, it doesn't break.

In a similar manner to our spring joint

we can apply spring-like characterises

by using the Use Spring checkbox

and using the values underneath the spring settings.

The other differences between hinge joints

and spring joints are motors and limits.

Motors are for introducing a velocity

to your joint, for example a revolving door.

The Target Velocity is the velocity the

joint is trying to rotate at and the

force is used to try and attain that velocity.

If Free Spin is checked then the force will

only be used to try and accelerate

the joint and not to decelerate it.

Limits can be used if you wish to constrain

the motion of your joint, like with the

normal door. The minimum and maximum

are the angles between which the joint

can move. Min and Max Bounce are the amount

by which the joint will bounce when it reaches

one of it's limits. And similar to our

spring joint, yet again we have break

forces and torques.

For example, if we add in a reasonably low

number into the break force property,

we can break the door off of the hinge

when adding a force. You should note that

when doing this the door doesn't fly directly

backwards but is instead pulled away from it's hinge.