Ramapriya wrote:
I know that a car's engine needs to be mated to the transmission for
propulsion. This I know isn't the case with an aircraft with a jet
engine; if the engine is turned on without the brakes applied, the
jet
of air leaving the engine will hurl the craft forward.

My doubt is, why does this forward motion effect not occur during
push-back, when the engines are normally turned on? Is it because at
low revs the engine would lack the punch to initiate the forward
motion
of a heavy aircraft? If not, I'd imagine the push-back becomes a bit
of
a tiresome affair by the time it's over...

As others have said, the tug can certainly over power it. However, I
have read accounts of pilots from the 727 days saying that at idle, the
engines did put out a lot of thrust and that on landings it added a lot
of float. Will most jets roll on the idle engines if the brake is not
set?

-Robert