@dwayne I wud like to correct you...its not as u r thinking.....say for eg. you apply some force on wall...but it does not moves....it doesn't mean that u haven't applied any force.......

Now for the first quesetion.....guys it so simple yaar what u ppl r doing!!!

Force on each side of the frame will be........f = TL...

so force due to four sides........f' = 4Tl........

But the film has the other side also.......so........F = 8TL.....

 hope everyones clear now........