# Applying physics to a node

To apply forces to a physical object, collect them into a ForceNode and then apply them to the object. The ForceNode is a node that specifies the “context” of the force; i.e. the local coordinate transform that determines the direction of the force. Because ForceNodes are separate from ActorNodes, a ForceNode can be placed in a different portion of the model tree from the ActorNode to which the forces applies. This allows for forces to be applied indirectly to a model (such as wind sweeping across the scene, or a mechanical impulse from an appendage of the model) without having to do the calculations necessary to transform from the ActorNode’s coordinates to the coordinates of the force’s source.

To add a force to a physical object, add the force using either the addLinearForce method (for translational forces) or the addAngularForce method (for rotational forces):

```actorNode.addLinearForce(pusherForce)
```

Conversely, forces can be removed using the corresponding remove calls:

```actorNode.removeLinearForce(pusherForce)
actorNode.removeAngularForce(spinnerForce)
```

By default, linear forces don’t factor in the mass of the object upon which they act (meaning they are more like accelerations). To factor in the mass of the object when applying the linear force, use the following call to enable mass-dependent calculations:

```pusherForce.setMassDependent(1)
```

## Example 1: Gravity

To apply a gravitational pull to the “jetpack guy” from the previous example:

```gravityFN=ForceNode('world-forces')
gravityFNP=render.attachNewNode(gravityFN)
gravityForce=LinearVectorForce(0,0,-9.81) #gravity acceleration

```

Since the gravitational force is relative to the entire world (and shouldn’t change if, for example, the jetpack guy tumbles head-over-heels), the gravityForce vector was added to a ForceNode attached to render. So regardless of the orienation of the NodePath controlled by an, the force will always pull towards the bottom of the scene.

Since all objects in the scene should be affected by gravity, the force was added to the set of forces managed by the PhysicsManager itself. Since forces ignore the mass of the objects they act upon by default, this force will pull all objects towards the ground at standard gravitational acceleration. The next example shows how to apply a force to a single object.

## Example 2: Rotary Thruster

Here is another example of applying forces to objects and the way in which the ForceNode alters the effect:

```thruster=NodePath("thruster") # make a thruster for the jetpack
thruster.reparentTo(jetpackGuy)
thruster.setPos(0,-2,3)

thrusterFN=ForceNode('jetpackGuy-thruster') # Attach a thruster force
thrusterFNP=thruster.attachNewNode(thrusterFN)
thrusterForce=LinearVectorForce(0,0,4000)
thrusterForce.setMassDependent(1)