The Magnus effect allows pitchers to throw curveballs, soccer players to bend kicks around defenders, and golfers to launch drives along near-triangular flight paths. This fun physics phenomenon relies on the difference in relative airspeed at various points on the surface of a round object.
Viral video captures the Magnus effect in action, 2015. Credit: YouTube channel How Ridiculous. Full footage: https://www.youtube.com/watch?v=QtP_bh2lMXc.
Picture a ball moving through the air while spinning. For the ball to experience a net force, its axis of rotation must be oriented so that some points on the ball’s surface move in the same direction as its trajectory (let’s name one of these points A) while some move the other way (we’ll call an example of this type B).
At point A, the air dragged along by the ball’s surface and the air farther away rush past each other in opposite directions; at point B, they move in the same direction, although one still moves relative to the other (unless rotational speed and flying speed are perfectly matched). This is a bit like the difference between two cars going in opposite directions on the highway and a car in the fast lane passing one in the slow lane.
Like a boat traveling through water, the ball leaves a wake behind as it cuts through the air. The differences between relative airspeeds around a rotating ball cause its wake to pull to one side, resulting in an asymmetrical imbalance of air pressure. This lopsided push bends the ball’s path as it travels.
A DIY-friendly demonstration of the Magnus effect. Unlike the basketball, which had a backspin as it fell down the dam, the paper roll’s front edge is spinning along with its trajectory. This results in a backwards arc.
The Magnus force affects cylindrical objects in a similar way. Try this at home with a sheet of paper, rolled up and taped into a cylinder. Allow the paper tube to roll off the edge of an angled surface to give it a uniform spin, and watch its path curve!
In strong supercell thunderstorms, wind moves upwards at over 90mph. Fast-moving air creates an area of low pressure, or a partial vacuum. Nature abhors a vacuum, so nearby air rushes in to join the updraft. This skyward flow can be powerful enough to suspend grapefruit-sized hail.
The tendency of a speeding air current to suck in surrounding air molecules stems from the same idea that gets airplanes off the ground: Bernoulli’s principle. The faster a fluid moves, the lower its pressure. Airplane wings have more pronounced curves on the top side than they do on the bottom, so air follows a longer path, and moves faster, above the wing than below. Higher pressure from underneath the wing translates to an upward force– that’s lift.
Aircraft and thunderstorms provide some of the most dramatic examples of this concept, but it’s also surprisingly easy to explore at home or in class. All you need is moving air and something to reveal its motion! A long, narrow bag and a working set of lungs are the perfect tools. Try to blow up eight feet of plastic bag like you would a balloon and you’ll find yourself struggling for air. Instead, hold the bag’s opening ten inches from your face, and blow into it across the gap. What’s the difference?
When your breath travels through a volume of air before reaching the bag, Bernoulli’s principle does most of the work for you! Your exhalation is a fast flow, so it creates a sink of low pressure moving into the plastic bag. Nearby air rushes in to fill the partial vacuum, and before you know it, the bag is fully inflated.
Firefighters clearing smoke from buildings take advantage of Bernoulli’s principle in almost exactly the same way. A fan placed directly in a door or window will move air through the structure. Set the same fan a short distance back from the opening, and it pulls in more air, getting the job done faster.
Imagine a torpedo in a wind tunnel. Incoming air slips around the torpedo’s nose, slides along its surface, and flies off its blunt back end. The air stream can’t navigate sharp corners, but as long as a smooth contour is available, it clings to that curve. This is called flow attachment, or the Coanda effect.
A fluid is anything that can flow freely– think water or air. Thanks to the Coanda effect, we can get a stream of fluid to go anywhere we want by giving it a smooth surface to follow. Helicopters and other VTOL aircraft use this principle to enhance lift! If a blade contour is smooth everywhere except a single edge, then that edge is where passing air slides off (just like in the image above). If the edge is angled towards the ground, then the air moves downward as it leaves the blade surface. Every action has an equal and opposite reaction; in this case, the downward momentum of the air translates to upward momentum of the aircraft. This isn’t the main way that airplanes gain altitude (that’s the Bernoulli effect), but it’s a useful way to improve performance. The Coanda effect can triple the Bernoulli lift on a blade or wing!
In this experiment, we put a cylindrical container in the path of a breath of air and attempt to blow out a candle on the other side. The air stream splits in two and follows the curved surface. There’s no sharp edge for the two halves to slide off of, so they hug the contour of the obstacle until they run into each other on the far side. The collision redirects the flow, causing a burst of air to continue on past the cylinder– almost as if it wasn’t there.
Try it for yourself with a round container and a candle! Please use adult supervision testing experiments using fire.
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