Eduard Martorell

There's a way to do what you ask.
Just create a boolean property and init it to "True". Then add a sensor of type "property" where you check if that property is equal to "True", and link 2 actuators to it:
One of type "motion" where you set the desired initial velocity, force, etc, and other of type "property" where you set that property to False.

The motion actuator will only have effect on the first frame.

Maybe there are other ways to do this, but this worked OK for me.