Kind: captions
Language: en
[Applause]
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 all right 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
ramp 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 it weighs only a gram or
so the effect of the air will be more
important Watch What Happens this time
okay wa so the paper cylinder goes
backwards that doesn't make any sense I
mean it was rolling forwards off that r
no it does it every time and it's
because of an effect known as the Magnus
Force that's acting on the spinning
cylinder acts on a spinning ball as well
and people who play sport know about it
but they 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 curve as they propagate
through the air ahuh and so what did he
find out what he found is that when a a
wall or any object is spinning like this
there's a force perpendicular to the
spin axis if it's spinning clockwise or
it Top Spin the force is down if it's
spinning anticlockwise counterclockwise
the force is up so how do we get a
Magnus force on a ball as the ball's
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 is air
is deflected downwards due to Newton's
third law the air exerts an equal
opposite force on the ball which is
upwards so how would sports players take
advantage of the Magnus Force
they make the ball curve through the air
by a different amount than that due to
gravity alone so a golfer will uh 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 it travels further MH
a tennis player will hit the ball with
Top Spin that causes the ball to curve
down onto the court after it pares the
net uh a baseball or Cricket play
will also do that but in addition they
can make a ball curve about a 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 uh 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 a sod force acting on the ball
uhuh well that sounds like a whole
another episode balls with seams yeah
that is all right well stay tuned if you
want to find out how air affects balls
with
seams