Is there a way to code this diagonal force?

Quick and simple Q for those who know:

A projectile is being fired from a weapon (think: grenade launcher for planting trees).

Writing these two lines is just not working? If I remove the other, the other works fine, but they do not play nice together. How can I combine the Add Force lines in to one? Or at the very least make them work simultaneously.

 p.AddForce(transform.forward * pSpeed, ForceMode.VelocityChange);
 p.AddForce(transform.up * pSpeed, ForceMode.VelocityChange);

Thanks!