How do you complete push tutorial on Star Wars force unleashed when pushed the barrels dont do any damage to the troopers...

You dont have to kill them just blow everything in the room around. Try it.
That is a bit of a lame answer... The thing to do is crash the objects into the stormtroopers, each other or the walls so that they are destroyed.
You know they're wrecked because they turn black instead of dull metal. I couldn't get this one until I lined up the chunks of metal with the stormtroopers, using the "gentle" force push to get them in position, and then blasted them with the REAL force push (for which you hold down the push button, charging it up) sending the 'troopers flying and destroying the objects at the same time. Do this three times and it should be over. If its not, just blast each thing in turn, against the wall - as they tend to get stuck there anyway - until you've gotten everything. Then you can do the challenges!
It's a good idea to try the challenges when you've upgraded your force powers and talents to quite a high level. That way you're prepared.
It can be quite frustrating otherwise! You get totally wiped out as there are so many of them firing at you - make sure you've got your mechanical thumbs on to keep moving and to get a few force points in the deflecting laser fire area... You'll need it!