i was geting error of mass notdefiened
for pendulum motion u , i guess u want the object to be oscillating .
go to Motion study/click motor/select the face u want to oscillate/
Under the motion tab , "constant speed" will be selected by default , select oscillating there., then u can define degrees of oscillation, frequency , amplitude(deg).
hope this would work for u ..
Mass definition is not required in this place , i think ..
By this , it will come into motion but not due to gravity effect it will oscillate due to applied external device