THEORY :
James Clerk Maxwell, in 186164, published his theory of electromagnetic fields and radiation, which shows that light has momentum and thus can exert pressure on objects.



SI units :
ENERGY/SPACE = [ Joul / cubic meters = Newton *meter / cubic meters = N/m^2 ]

1kg Ball VS 1kg ( 100 m^2 metal sheet )
[img]http://1.bp.blogspot.com/-a5r9GtqfrbY/Vb3ULiRMuKI/AAAAAAAACcs/EOVyEkykhcU/s1600/galileo.jpg[/img]


1kg ( wood ) and 1kg ( steel )
wood is more light so ... has got biger surface !!!

[img]http://3.bp.blogspot.com/-G8XXBln--6k/VP_2zF35VwI/AAAAAAAACRg/oFtCjI5NMtE/s1600/CIMG3703.JPG[/img]


[img]http://4.bp.blogspot.com/-RanM8z2zV1Q/Vb7rNvD4GTI/AAAAAAAACdU/dGoJ6mgUWfM/s1600/xc.JPG[/img]