How does force work in Unity?

Questions & Answers
1

If I do AddRelativeForce(Vector3.up * 5) one time on a stationary object with a mass of 1 (no drag or gravity), it's velocity will be (0,.1,0).

Why/How does it get that value (.1)?

2

By default, the functions that Joshua has mentioned uses ForceMode.Force, which adds a continuous force to the rigidbody, using its mass. In this mode, the unit of the force parameter is applied to the rigidbody as mass*distance/time^2.

If you're intending to apply a force once-off, you should probably be using ForceMode.Impulse, which adds an instant force impulse to the rigidbody, using its mass. In this mode, the unit of the force parameter is applied to the rigidbody as mass*distance/time.

3

Marowi posted what I was going to say. Here's the way I think about it:

ForceMode.Force is not designed to be used instantaneously; it's designed to be used every FixedUpdate. 

AddRelativeForce(force)

The idea with that is, you'll have applied the full force after a period of one second. So every fixed timestep, you only apply force x Time.fixedDeltaTime. Or, in your case, (0, .1, 0), provided you have Fixed Timestep set to .02 in the Time Manager.