Jump with an Impulse

Part of the Physics and Gameplay section of Coddy's Unity journey. Lesson 3 of 63.

AddForce takes a second argument, a ForceMode, that says how to use the vector. Without it you get ForceMode.Force, a push that lasts one physics step, so it has to be added step after step. A kick, a jump or an explosion happens all at once, and that is ForceMode.Impulse: the whole push in one go, changing the speed by the impulse divided by the mass.

This football (0.5 kg) is kicked on a click. Its speed jumps by (0, 2, 6) / 0.5 = (0, 4, 12) m/s at the next physics step, and gravity and the grass take over from there:

void Update()
{
    if (Input.GetMouseButtonDown(0))
    {
        rb.AddForce(new Vector3(0, 2, 6), ForceMode.Impulse);
    }
}

A one-off push can be called from Update(), on the frame of the click: Unity keeps it and applies it at the next physics step. The four modes differ in two ways, all at once or over a step, and with or without the mass:

Force: over a step, divided by mass (engines, wind).
Acceleration: over a step, mass ignored.
Impulse: all at once, divided by mass (kicks, jumps).
VelocityChange: all at once, mass ignored: the vector is added straight to the speed.

Challenge

Easy

Make the frog jump. Each press of Space gives it one upward kick of jumpImpulse (10 in the Inspector). The frog weighs 2 kg, and the jump must take its mass into account: a heavier frog jumps lower.

Holding Space down is still one jump. Press Play and hop, then Check.

Try it yourself

using UnityEngine;

public class FrogJump : MonoBehaviour
{
    public float jumpImpulse = 10f;
    private Rigidbody rb;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {
        // Space: one upward kick of jumpImpulse
    }
}