Re: why cant Mary Poppins fly using her umbrella? (faulty physics in film)

Look at the things needed to fly.

Firstly you need a force upwards that equals your weight downwards.  How is
this to be produced.

MP flies using her umbrella.  I suppose it is assumed the umbrella is
somehow providing lift AND is strong enough to support the person underneath.  

Two immediate methods of providing the lift spring to mind.  

Either 

1)there is an updraft of air pushing on the underside of the umbrella
or
2)the umbrella is somehow providing lift much like a wing.

For 1) - 
where does the updraft come from and how much updraft do you need?

The updraft would have to be causing a pressure on the underside of the
umbrella to counteract her weight.

Pressure is Force / Area.

the force downwards is from her weight (her mass and gravity)

If her umbrella has a diameter of 2m (pretty big umbrella), it has a
surface area of pi.1.1 (pi by the radius squared) or about 3.14 metres
squared.  Let's say she weighs about 50kg.  That is a pressure of about 16
kg per metre squared or about 156 Newtons per metre squared.

(kg is usually the unit of mass.  When we talk about kg and force we are
really talking weight.  On earth one kg provides a force downwards of 1kg .
9.8 m/s/s i.e about 9.8 newtons))

Now according to this site http://www.knmi.nl/samenw/hydra/faq/press.html

You would need an updraft speed of around 15 m/s or about 29 knots or about
54 km/hr.  That is a mighty big updraft - not one you would normally see
outside of a thunderstorm.

For 2)
lift is given by L= Cl . 1/2 . rho . velocity squared . surface area
rho is density, CL is coefficient of lift
 http://www.grc.nasa.gov/WWW/K-12/airplane/lifteq.html

Here the hard bit is knowing what the coefficient of lift is of an umbrella

This site
 http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

gives various parameters for airfoil shapes.  A typical type of wing with a
surface area of about 3.14 metres squared would need an airflow of around
60mph to lift 50 kg.  Now an umbrella would not be nearly as good as the
standard airfoild shape used in that simulation, so that figure would be a
lower limit.

Finally we come to structure.  Can an umbrella hold a 50 kg person up? 
Think of it this way - in a strong wind does an umbrella drag people along
or do they turn inside out and collapse?

Martin Smith (hoping I got all my conversions correct)