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Deceleration on Impacts

Discussion in 'Physics' started by Nanako, Feb 19, 2015.

  1. Nanako

    Nanako

    Joined:
    Sep 24, 2014
    Posts:
    1,047
    How does unity handle the deceleratikon of rigidbodies on impacts? I did some testing.
    So far all ive done is crank the fixed timestep to 0.01, and i fired a gravityless-ball v ery slowly at a kinematic wall at a velocity of Vector3.forward (so the magnitude of velocity is 1.0)

    , and tracked the magnitude of its velocity during the impact:



    in these debug messages, the leftmost number is the magnitude of velocity, and the right one is the current frame number (number of fixedupdate calls since start of running). note also i added a conditional to not display message when the velocity was >=1 or = 0

    As can be seen here, it takes about 7 frames to lose most of its speed, but then gets stuck at 8.642675E-06 until frame 393 (15 frames)

    I need some help making sense of this. How is unity doing this gradual deceleration, is there a deterministic pattern to it ? And why does it get stuck at some very low value for 15 frames (0.15 seconds) before finally coming to rest?
     
  2. Nanako

    Nanako

    Joined:
    Sep 24, 2014
    Posts:
    1,047
    i've been doing some testing, i made this simple test harness script
    Code (CSharp):
    1. using UnityEngine;
    2. using System.Collections;
    3. using System.Diagnostics;
    4.  
    5. public class VelocityMonitor : MonoBehaviour
    6. {
    7.     int i = 0;
    8.     public float j;
    9.     float mag =10f;
    10.     Stopwatch timer = new Stopwatch();
    11.    
    12.     void Start()
    13.     {
    14.         rigidbody.velocity = Vector3.forward*mag;
    15.     }
    16.     void FixedUpdate()
    17.     {
    18.         j = rigidbody.velocity.magnitude;
    19.         if (j < mag && j > 0.0001f)
    20.         {
    21.             timer.Start();
    22.             print(j + "  num " + i);
    23.         }
    24.         else if (timer.IsRunning)
    25.         {
    26.             timer.Stop();
    27.             print("Total Time Taken to stop " + timer.Elapsed.TotalSeconds);
    28.         }
    29.         i++;
    30.     }
    31. }
    32.  
    I also cranked up the fixedTimestep to 0.005 (200 ticks per second!)
    with this value, i was able to get readouts for speeds up to about 8 M/s. The faster the ball is going, the less time it takes to decelerate to a stop, it seems. I maybe be wrong, but that doesn't seem to quite gel with my intuition about physics,

    at about 10 m/s it just goes from fullspeed to zero in a single frame, and i have to crank up the timestep some more to see anything in there