so I'm back at the University of Sydney
with rod cross hi Derek and today we're
talking about the effects of air on
projectiles we are now normally we
neglect these effects when I'm teaching
students about projectiles I tell them
forget about the air let's just talk
about gravity because it simplifies the
problem but air is real it's heavy stuff
and it affects the flight of all
projectiles alright well why don't we do
a little experiment to show the effect
that air can have this is a surprising
experiment that we've set up here first
of all I'll show you what happens to a
tennis ball when it rolls down this
inclined wrap well that falls exactly
like I would expect basically a
parabolic path as as predicted but let's
try something a little bit lighter such
as this paper cylinder
okay weighs only a gram or so the effect
of the air will be more important
watch what happens this time oh so the
paper cylinder goes backwards that
doesn't make any sense and it was
rolling forwards off that round no it
does it every time and it's because of
an effect known as the Magnus force
that's acting on a spinning cylinder
acts on a spinning ball as well and
people who play sport know about it but
I wouldn't call it a Magnus force why is
it called the Magnus force because
Magnus was the first guy who discovered
it when he was investigating why
cannonballs curved as they propagate
through the air uh-huh and through what
did he find out what he found is that
when a ball or any object is spinning
like this there's a force perpendicular
to the spin axis if it's spinning
clockwise or with topspin the forces
down if it's spinning anti clockwise
counterclockwise the forces up so how do
we get a Magnus force on a ball as the
ball is moving forward through the air
if it's spinning the air is flowing
around the ball from the front to the
back the ball is spinning in the same
direction as the air flow at the top of
the ball but in the opposite direction
at the bottom because of friction
between the air and the ball surface air
is dragged around the top of the ball
downwards towards the back but at the
bottom of the ball the air flow in the
ball opposite directions
the air comes to a screeching halt
fairly soon instead of being deflected
upwards
the net result as air is deflected
downwards due to Newton's third law the
air exerts an equal Officer force on the
ball which is upwards so how would
sports players take advantage of the
magnet force did they make the ball
curve through the air by a different
amount than that due to gravity alone so
a golfer will strike a golf ball with
backspin that exerts a vertical force a
lift force on the ball that keeps it in
the air for a longer time and therefore
troubles further a tennis player will
hit the ball with topspin that causes
the ball to curve down onto the court
after it passes the net a baseball or
cricket player will also do that but in
addition they can make a ball curve
about the vertical axis in which case
the ball will either curve to the left
or to the right away from the batter
making it much more difficult to hit the
ball that's the object of the exercise I
see so are there any other air effects
that we need to be aware of there's
quite a few actually there's the buoyant
force acting on a balloon for example or
any object there's a drag force acting
backwards that slows the ball down and
if the ball happens to have seams then
there's a side force acting on the ball
uh-huh
well that sounds like a whole nother
episode balls with seams yeah Hey
all right well stay tuned if you want to
find out how air affects bowls with
seams