How to make a central point of gravity

Question

I want to make planets that have central points of gravity, but I cannot figure out how to. All I need is to be able to make round game objects have gravity like planets. Please help

Edit:

I am wondering how to make gravity that can put objects in orbit.

Answer 1

Suppose, you have 2 types of objects : Big object & small objects.

Small objects revolves around big object. Add below script on small objects.

alt text

 using UnityEngine;
 using System.Collections;
 
 public class CircularGravity : MonoBehaviour {
     
     public Transform target; // Big object
     Vector3 targetDirection;
 
     public int radius = 5;
     public int forceAmount = 100;
     public float gravity = 0;
     private Rigidbody rb;
 
     private float distance;
 
     void OnDrawGizmos() {
         Gizmos.color = Color.yellow;
         Gizmos.DrawWireSphere(transform.position, radius);
     }
 
     // Use this for initialization
     void Start () {
         Physics.gravity = new Vector3 (0, gravity, 0);
         rb = GetComponent<Rigidbody>();
     }
     
     // Update is called once per frame
     void Update () {
 
         targetDirection = target.position - transform.position; // Save direction
         distance = targetDirection.magnitude; // Find distance between this object and target object
         targetDirection = targetDirection.normalized; // Normalize target direction vector
 
         if(distance < radius) {
             rb.AddForce (targetDirection * forceAmount * Time.deltaTime);
         }
 
 
     }
 }

Answer 2

Answer by sys12 · Sep 16, 2015 at 12:11 PM

Check this out: http://docs.unity3d.com/ScriptReference/Rigidbody-centerOfMass.html

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sys12 · Sep 16, 2015 at 01:12 PM 0

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http://forum.unity3d.com/threads/finding-the-center-of-mass-of-a-collection-of-rigidbodies.87578/

Answer 3

Answer by Suddoha · Sep 16, 2015 at 12:37 PM

I'm not sure how well sys12's suggestion works for this purpose, but you can do some funny stuff with the physics system. The following is just quickly made out of nowhere, but it's quite cool for orbit-like effects. You can tweak it way more to have your desired behaviour.

You can also code something similar from scratch which doesn't invole the physics system, but just play around with it, maybe it inspires you a little bit:

(disable useGravity on the rigidbody)

 using UnityEngine;
 using System.Collections;
 
 public class Gravity : MonoBehaviour
 {
 
     public GameObject planet;
     private Rigidbody rb;

     [Range(0, 5)]
     public float factor = 1;
     public ForceMode mode;
 
     void Awake()
     {
         rb = GetComponent<Rigidbody>();
     }
 
     void FixedUpdate()
     {
         Vector3 g = (planet.transform.position - transform.position) * factor;
         rb.AddForce(g, mode);
     }
 }

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ItsKhanny · Sep 16, 2015 at 12:55 PM 0

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putting in the exact code other than the class name, i got the following errors

error CS0178: Invalid rank specifier: expected ,' or ]'

error CS1519: Unexpected symbol `float' in class, struct, or interface member declaration

Suddoha ItsKhanny · Sep 16, 2015 at 01:54 PM 0

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Oh sorry, probably deleted that by accident as I tested the script so it was definitely there. $$anonymous$$y bad. I'll fix the original post.

ItsKhanny · Sep 16, 2015 at 01:02 PM 0

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It was because you did private RigidBody not private RigidBody = rb

edit: What does the script do exactly

Suddoha ItsKhanny · Sep 16, 2015 at 01:59 PM 0

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Oh btw, don't forget to disable the rigidbodies gravity.

The script takes an object as a gravity target (planet variable). It then calculates the direction from the current objects position to the target and applies a force in this direction via the rigidbody.

The mode determines how the force is applied (acceleration suits well i think), the factor variable can be used to adjust the amount of acceleration.

Without adjustements, the script will probably not work for something like a walk on the planet. That has to be improved but it's just to get you started anyway. ;)

ItsKhanny · Sep 16, 2015 at 04:05 PM 0

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does this script work on making stuff orbit and crash into objects/planets

Suddoha ItsKhanny · Sep 16, 2015 at 06:02 PM 0

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Have you tried to use it? Play around with it, as i said you may have to adjust the one or other thing. i cannot provide a script that totally fits your needs, as I 1) don't know which exact behaviour you're looking for and 2) it's probably too much to ask for on a Q/A site.

Answer 4

Answer by remi07 · Jan 13 at 03:03 PM

Try the following:

 public class ObjectOnPlanet: MonoBehaviour
 {
 
     private Rigidbody rigid;
    
     void Start()
     {
         rigid = GetComponent<Rigidbody>();
     }
 
     Vector3 planetGravity;
 
     private void FixedUpdate()
     {
         planetGravity = Quaternion.FromToRotation(Vector3.up,transform.position) * Physics.gravity;
         rigid.AddForce(planetGravity,ForceMode.Force);
     }
 
 }

Basically we take the gravity vector and rotate it to align it to the vector that goes from the origin of the scene to the object position (basically transform.position). That particular rotation is deduced by using the Quaternion.FromToRotation(Vector3.up, transform.position) because physics.gravity is by default aligned along Vector3.up in world space.

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How to make a central point of gravity

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