Don't use physcs. It's easier if you use anything how this:

var move=10;
var move2=0;
var move3=1;

function armswing()
{
me=arm;
while(1)
{
my.tilt+=move*time_step;

if(move2==0)
{move-=move3*time_step;
if(my.tilt<0)
move2=1;}

if(move2==1)
{move+=move3*time_step;
if(my.tilt>=0)
move2=0;}
if(move3>0.01)
move3-=0.1*time_step;
wait(1);
}}

Now the arm swings further and further. (not tested)