Gravity and Mass

Part of the Physics and Gameplay section of Coddy's Unity journey. Lesson 1 of 63.

A Rigidbody hands its object to Unity's physics, as the coconut showed when it fell. From then on gravity speeds the object up by 9.81 m/s every second, and the Rigidbody has the settings that decide how. Two of them are mass, in kilograms (1 for a new Rigidbody), and useGravity (on for a new Rigidbody).

Mass does not change how fast things fall. Gravity pulls a heavy body harder, and the heavy body is harder to speed up by exactly as much, so a 1 kg crate and a 50 kg anvil dropped from the same height land on the same frame. Mass matters when bodies push each other: the anvil knocks the crate aside, and the crate hardly moves the anvil. This party balloon gets a light mass and is left alone by gravity, so it hangs where the scene put it:

void Start()
{
    Rigidbody rb = GetComponent<Rigidbody>();
    rb.mass = 0.2f;
    rb.useGravity = false;
}

With useGravity off, the balloon still has its mass and still moves when something bumps into it: only gravity stops pulling. Set it back to true and gravity pulls again from wherever the object is. There is no air in Unity's physics either, so a feather falls like a brick unless you give its Rigidbody some Linear Damping.

Challenge

Easy

The crane holds two crates over the dock: a heavy Safe and a light Box. Both have a Rigidbody and a CraneLoad script whose weight the Inspector sets (80 on the Safe, 2 on the Box).

When the game starts, set the Rigidbody's mass to weight and stop gravity pulling, so the crate hangs where it is.
When Space is pressed, let gravity pull it again, so it drops.

Press Play, press Space and watch which crate lands first, then Check.

Try it yourself

using UnityEngine;

public class CraneLoad : MonoBehaviour
{
    public float weight = 1f;

    void Start()
    {
        // Set the mass to weight and stop gravity pulling
    }

    void Update()
    {
        // Space: let gravity pull again
    }
}