🚀 go-pugleaf

He doesn't make thrust, he simply jumps (and then glides
when he reaches cruising altitude---that cape's gotta be a 
real "drag", though). 

The real question is why his feet don't punch through the pavement 
when he exerts his tremendous launching force. Assuming he
can jump about 1km high, and weighs 100kg, his jump imparts 
him with m g h = 100 x 10 x 1000 Joules = 1,000,000 Joules of energy
(this is about the energy content of a large meal---he must be 
hungry afterwards.) If this energy were imparted over a 1 meter
distance as he crouches and springs from the ground, the force
exerted would be around (from F . d = W) F = 1,000,000 J/ 1m = 1,000,000 N.
That's about a 250,000 lb force, and applied by the soles of his feet,
which have an area of about 100 inch^2, we get a pressure exerted by his
feet on the ground of 2,500 lb/in^2. In terms of atmosperic pressure,
thats close to 200 atmospheres of pressure! This pressure is applied
for a fraction of a second, which is long enough to damage the surface.
Certainly on soil his feet would punch right through if he tried
to take off in this manner. Even on concrete, he should punch through.

So why isn't NY covered superman's foot prints.

--
Barry Merriman
UCLA Dept. of Math
UCLA Inst. for Fusion and Plasma Research
barry@math.ucla.edu (Internet; NeXTMail is welcome)