Rigidbody.velocity

Description

The velocity vector of the rigidbody.

In most cases you should not modify the velocity directly, as this can result in unrealistic behaviour. Don't set the velocity of an object every physics step, this will lead to unrealistic physics simulation. A typical example where you would change the velocity is when jumping in a first person shooter, because you want an immediate change in velocity.

using UnityEngine;
using System.Collections;

// The velocity in y is 10 units per second.  If the GameObject starts at (0,0,0) then
// it will reach (0,100,0) units after 10 seconds.

public class ExampleClass : MonoBehaviour
{
    public Rigidbody rb;

    private float t = 0.0f;
    private bool moving = false;

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

    void FixedUpdate()
    {
        if (Input.GetButtonDown("Jump"))
        {
            // the cube is going to move upwards in 10 units per second
            rb.velocity = new Vector3(0, 10, 0);
            moving = true;
            Debug.Log("jump");
        }

        if (moving)
        {
            // when the cube has moved over 1 second report it's position
            t = t + Time.deltaTime;
            if (t > 1.0f)
            {
                Debug.Log(gameObject.transform.position.y + " : " + t);
                t = 0.0f;
            }
        }
    }
}

Note: A velocity in Unity is units per second. The units are often thought of as metres but could be millimetres or light years. Unity velocity also has the speed in X, Y, and Z defining the direction.