What Is The Magnus Force?

Key Takeaways

[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