Physical Harm from an Expelliarmus

[Jim Ferer]

No: HPFGUIDX 44182

Cindy:"So, then.  Maybe the impact of Expelliarmus relates to the 
power of the thing on the receiving end of the spell.  Voldemort and 
Wormtail are strong, so they are not physically harmed by the spell, 
perhaps.  The spider is strong (and is able to throw off stunning 
spells), so it is not blasted backward by Harry's spell."

I've always believed we can understand magic and spells the same way 
everything else works. Some magicians are stronger than others both 
on the giving and receiving ends. IOW, in our Muggle world, who would 
you rather take a punch from, Daniel Radcliffe or Robbie Coltrane?  
Which one would you be more likely to hurt if you took a swing? 
Magical strength and constitution (to borrow D&D terms) is the same 
for ordinary forces and for magic.

Cindy:"-- unsure why Harry's Expelliarmus spell was successful 
against the spider when Harry's and Cedric's solo stunning spells 
were not"

I'm not sure, either, but it seems intuitive it would be easier to 
get somebody to drop something than to knock 'em out; and the 
Stunning spells were hitting the spider's exoskeleton. Harry and 
Cedric made it when they hit the spider's underbelly.