Instantiate with force?

Hello. This is the code I am using to spawn some things. How can I make it so that after the objects are spawned, they travel in a certain direction at a constant speed?

var source = objs[(Random.Range(0, 2))];
var position = new Vector3(Random.Range(481.5735, 559.3441), 
                           -791.811, 
                           Random.Range(380.1254, 420.0663));
Instantiate(source, position, Quaternion.identity);

Thanks

Given that your objects in your array objs all have a rigid body;

Rigidbody clone = Instantiate(source, position, Quaternion.identity) as Rigidbody;
if (clone)
{
    if (!clone.constantForce)
    {
        clone.gameObject.AddComponent("ConstantForce");
    }
    else
    {
        Debug.LogWarning("Clone already have constantForce.", clone);
    }

    clone.constantForce.force = new Vector3(0, 10, 0); // upward force!
}
else
{
    Debug.LogWarning("Have no rigid body. No force added.", this);
}

I have a similar problem but I'd like to add a force that launches the projectile at an angle rather than straight up, down, left, right, forward, back. So something like 30* off of straight ahead z.

Maybe at the local coordinate level if possible.