Okay I don't know how much you need but here is one way to work around the problem.
First of all put a fall method in your act()
public void act()
{
if (notOnGround())
{
fall();
}
}
The notOnGround() would have to detect wether the hero/player what ever is on the ground (please tell me if you need help with that too). The fall code could be very simple and look like this:
public void fall()
{
setLocation(getX(), getY() + 1);
}
Though I have seen some people on this site using acceleration and stuff to make the fall look more realistic.
For the collision part I would like to know what you mean exactly. Is it platforms, the world edges or something else you want to check collision with?