Identifying The Magnus Effect In Table Tennis
As we all wait for the elite professional table tennis scene to return in early May, in today’s post we will look at one of the more fundamental physical phenomena in table tennis that not all viewers are aware of: the Magnus Effect.
The Magnus Effect is a physical phenomenon that explains how the spin of a ball modifies its trajectory while the ball is in the air. Unlike other aspects of table tennis mechanics, the Magnus Effect is not the result of gravity and friction, forces that we are familiar with and experience every day, but the result of fluid dynamics. This makes it difficult to visualize and form an intuitive understanding of the Magnus Effect.
However, although it is difficult to create an intuition for the cause of the Magnus Effect, once you understand its results (which are extremely simple), then you can see how the Magnus Effect is present in several common table tennis contexts.
What is the Magnus Effect?
A very non-rigorous explanation of the Magnus Effect (courtesy of Wikipedia) is as follows. When a spinning ball moves through a fluid (such as air), the ball “pushes” the air in one direction, and as a result of Newton’s Third Law (every action has an equal and opposite reaction), the air pushes back on the ball in the opposite direction. The visualization below shows a ball with topspin traveling to the right, which kicks up the air behinds it. As a result, the air exerts a reactive downward force on the ball.
If this is difficult to form an intuition around, that is fine; from a table tennis perspective what matters is not the cause but the effect of the Magnus Effect. The most important takeaway is that the Magnus Effect means that the spin of the ball affects its trajectory while the ball is still in the air and before it even hits the table.
What makes the Magnus Effect counterintuitive is that for topspin and underspin, the direction of the force is opposite of what we usually associate for each spin. When the ball bounces on the table or off the raquet, we typically associate a topspin ball with jumping upward and forward (e.g. for a kicker serve) and an underspin ball will jump downward and backward (e.g. Ma Lin’s famous ghost serve).
However, by contrast, the Magnus Effect exerts a downward force (and a smaller backward force when the ball is falling) on topspin balls and an upward force (and a smaller forward force when the ball is falling) on underspin balls. We can better visualize both the direction of the Magnus Effect and how strongly it can influence the ball’s trajectory in the below video, where a basketball falling downward with backspin floats very far forward. If you tilt your neck sideways while watching the video, then you can see how a ball with backspin traveling horizontally across a table will feel an upward force due to the Magnus Effect (of course, in a table tennis scenario, this force is still weaker than gravity, so the ball still falls down onto the table).
Identifying the Magnus Effect in Table Tennis
At its essence, the Magnus Effect may be slightly counterintuitive but it is extremely simple: topspin is dragged down when the ball is in the air, and underspin is lifted up when the ball is in the air. We look at its consequences in two common scenarios: the fast topspin rally and the counter against a spinny opening loop.
The Magnus Effect In Fast Topspin Rallies
One of the biggest results of the Magnus Effect is that it is more desirable to add topspin to loops regardless of the speed. Spin vs speed is often viewed as a tradeoff where one must lose one to gain the other, which can be the case when the amount of spin/speed generated is limited by the player’s physical abilities. However, in the certain in-game contexts, increasing topspin may actually enable more speed. Watch the following winner by Ding Ning below.
The radar on the net measures the speed of the winner to be a fast 70 km/hr, but notice how the ball actually does not bounce that deep on the table. It only bounces roughly at the halfway point between the edge and the net both on Ding’s game-winner and the shot immediately before that. This is because the ball is loaded with spin (watch the ball roll on the floor when the point ends), so a heavy Magnus force drags the ball down faster. Hence, Ding Ning could have hit even faster, and the ball would still have landed within the table.
In general, adding more topspin counterintuitively makes the ball land shallower due to the Magnus Effect. This gives players a wider margin of error to hit the ball hard and fast without having to worry about it going off the table. As long as the player brushes the ball sufficiently and adds enough spin, the player can hit as hard as he or she wants and the ball will still drop down onto the table because of the increased Magnus force.
Although it is usually desirable to land serves, pushes, and opening deep onto the table, in the fast rally the ball is so fast and the opponent has so little time to react that the shallow depth appears not to matter much anyway. Moreover, with the heavy amount of spin involved and high downward velocity due to the Magnus Effect, the ball gets a serious kick once it bounces off the table. Looking at the slow-motion replay you can see how much pressure Ding Ning applies to Mima Ito during that point. On the first block, the ball is already up to near her face-level (granted, Mima Ito is short and also bending down). On the second shot, Ito is a bit late, and the ball jumps over her paddle.
Leveraging the Magnus Effect Against Slow Spinny Loops
Many amateur players have had the experience where, even if an opening loop is slow, if it is spinny it can often be quite challenging to block because the spin causes the ball to bounce off the racquet so that it flies off the edge of the table. One solution is to close the racquet and essentially hit the ball downward in order to counter the heavy topspin..
Alternatively, a player can simply add his or her own topspin to the ball, so even if the ball bounces off the racquet at a higher angle than expected, the Magnus Effect will forgive minor errors and drag the ball down. In the clip below, Ma Long opens his angle and counters Fan Zhendong’s slow spinny opening with a heavy topspin counter in which his racquet goes almost straight up instead of down or even forward.
Bonus: Around-the-net Shots
The Magnus Effect carries the most in-game implication for top-spin balls and topspin is usually the heaviest spin in the match, but it also affects heavy underspin and sidespin balls. For underspin balls, the Magnus Effect gives heavy chops their floating effect. While the distances involved in table tennis are too small for us to see some of the extreme bends that we do in soccer/football on sidespin balls, the curve induced by the Magnus Effect can clearly be seen in some of the more ambitious around-the-net shots that Youtube stars like Adam Bobrow take.
Unfortunately, ITTF has killed ITTV, meaning that past matches are no longer publicly available to watch. Hence, no analysis blog posts are scheduled for the immediate future. You can check out past analysis posts here.