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Break on Impact

Part of the Physics and Gameplay section of Coddy's Unity journey. Lesson 10 of 71.

How hard was the hit? collision.relativeVelocity is how fast the two objects were closing in on each other at the moment they met, and its magnitude is the impact speed in m/s. It is a difference of velocities: a car at 10 m/s that runs into a car going the same way at 8 m/s hits at only 2 m/s, and both cars see the same impact speed.

This egg breaks when it lands faster than crackSpeed. Dropped from 1 m it hits at about 4.4 m/s (the square root of 2 x 9.81 x 1), and from 2 m at about 6.3 m/s:

public float crackSpeed = 5f;

void OnCollisionEnter(Collision collision)
{
    if (collision.relativeVelocity.magnitude > crackSpeed)
    {
        Debug.Log("Splat!");
        Destroy(gameObject);
    }
}

Why not the egg's own rb.linearVelocity? By the time OnCollisionEnter runs, physics has already handled the hit, so the egg's velocity is what is left after it, close to zero. relativeVelocity keeps the speed of the impact itself. It is a speed, so mass plays no part: a heavy egg and a light one landing at 6 m/s report the same 6.

challenge icon

Challenge

Easy

Write Window.cs: every hit weakens the window, and it shatters at 0.

  1. In the window's OnCollisionEnter, take collision.relativeVelocity.magnitude off strength.
  2. When strength <= 0f, call Destroy(gameObject);.

Press Play, roll soft balls with the up arrow, then a hard one with Space: the window breaks. Then Check.

Try it yourself

using UnityEngine;

public class Window : MonoBehaviour
{
    public float strength = 10f;

    // Every hit takes its impact speed off strength; at 0 the window shatters
}

All lessons in Physics and Gameplay

1Rigidbody

Gravity and MassPush with AddForceJump with an ImpulseFixedUpdate for PhysicsVelocityKinematic BodiesRecap: Launch Pad

4Coroutines

InvokeInvokeRepeating, CancelInvokeYour First CoroutineLoops in CoroutinesStopping CoroutinesRecap: Countdown

7Rotation

Euler AnglesQuaternionsFace the Way You MoveTurn SmoothlyRecap: Turret

10Project: Arena Escape

The ArenaHuntersProject lessonWavesProject lessonHit PointsProject lessonThe Hunters BiteProject lessonThe ExitProject lessonOpen After the Last WaveProject lesson

2Collisions

OnCollisionEnterWhat Did I Hit?Break on ImpactStanding on the GroundBounce and FrictionCollider ShapesRecap: Breakable Wall

5Project: Coin Rush

The Rolling BallCoins to CollectProject lessonKeeping ScoreProject lessonCoins Come BackProject lessonBeat the ClockProject lessonYou WinProject lessonYou LoseProject lesson

8The Camera

Smooth FollowOrbit CameraLook AheadCamera ShakeRecap: Chase Camera

11Final Challenges

Cannon RangeMoving TargetsMarble GolfInto the CupEscape RoomThe Door

3Triggers

Trigger CollidersOnTriggerEnterPickupsCrates Keep the RestKill ZonesEnter and ExitRecap: Key and Door

6Raycasts

Physics.RaycastRaycastHitGround Check with a RayLayers and LayerMaskLine of SightOverlapSphere and SphereCastRecap: Laser Tripwire

9Enemies

Chase the PlayerChase in RangePatrol and ChaseHealth and DamageBullets That HitKnockbackRecap: Guard Dog

Practice on your own: Unity playground