ok, I got this moving, and stuff...

But I need some more help...

I've added code for a jump:
if (key_w)
{
Player_2.pos_y -=30*time_step;
}
else
{
Player_2.pos_y +=30*time_step;
}
I get him to move up and down, but here lies the problem, how can I get him to move to pos_y = 400, and then fall back to 450? As it is now, he goes further up as long as I hold the button.