Since the truck is at rest it has zero momentum. Thus, the skateboard, despite its much smaller mass has more momentum because it is moving.
Force is a measure of the rate of change of impulse.
It takes a larger force to impart a given impulse during a correspondingly shorter time. Newton s second law relates the acting force to the body s acceleration which is directly related, via multiplication by time and mass, to the body s change in momentum. To impart the largest impulse you should do both. Increasing both the time and force will increase the imparted impulse. See the solution to quiz 3 .