# Why does spin increase during flight?

Discussion in 'Tennis Tips/Instruction' started by Claudius, Jul 8, 2009.

1. ### ClaudiusProfessional

Joined:
Jun 4, 2009
Messages:
1,031
I've heard somewhere that if you hit a ball with a certain amount of spin, the spin would actually increase when it reaches your opponent.

For example if Andy Roddick served a topspin serve and put 5000 rpms on the ball when it reaches the opponent, the ball's spin would go all the way up to 9000 rpms.

Why is this so?

2. ### ms87Rookie

Joined:
May 21, 2009
Messages:
244
it increases when it hits the court; the felt grabs the surface and the velocity of the ball is partially converted into spin.

3. ### WildVolleyLegend

Joined:
Jun 22, 2007
Messages:
6,520
MS87 is correct. The court tends to increase topspin, because it only impacts the ball on one side, much in the same way that brushing your racket under a ball tends to impart spin. In the air, spin tends to slow down due to air friction.

4. ### user92626Legend

Joined:
Jan 27, 2008
Messages:
7,419
Where did you hear this?

If this is true it would be a new physics phenomenon for me.

5. ### RaidenxNew User

Joined:
Oct 31, 2008
Messages:
34
not sure about those numbers, but assuming a ball speed of 150mph before bounce the ball would need to have topspin rpm exceeding about 19,700rpm to NOT gain rpm from bounce

its either gain speed lose spin or vice versa

6. ### wngan9447New User

Joined:
Jun 25, 2009
Messages:
65
wow... learn something everyday.. i just thought the ball moves in a certain direction due to the spin, I didn't think the spin would increase.

Subscribed to see what others thinks.

7. ### ClaudiusProfessional

Joined:
Jun 4, 2009
Messages:
1,031
http://www.popularmechanics.com/outdoors/sports/4221210.html?page=3

My numbers were slightly off. His 130 mph flat serve's spin changes from 2400 rpm to 4800 rpm. Since it's his flat serve, I'm guessing his kick serve would have a lot more rpm.

8. ### mtommerHall of Fame

Joined:
Jun 18, 2008
Messages:
2,859
What the article describes is that as the ball hits the court it can no longer spin. However, it still wants to spin because it was spinning before. The kinetic energy of the spin becomes potential energy, gathering upon itself until it is freed from the "binding" of the court as it lifts away from the court. That potential energy all of a sudden gets released and thus the ball kicks into high gear. It's not adding spin so much as making up for spin it was supposed to have if there was no interruption of the flight path.

9. ### SublimeSemi-Pro

Joined:
Jun 25, 2008
Messages:
772
It's easier to visualize if you think about it from the ball's perspective. Instead of thinking about a ball traveling through the air at 135mph, think about holding a ball over a treadmill that's spinning at 135mph.

When you drop the ball on the treadmill what's going to happen?

First it's going to spin the ball... this is the increase in spin this thread is about.

Secondly it's going to kick back, but not at the same speed the treadmill is spinning. Back to the perspective of the court, this means its going to slow down.

10. ### wngan9447New User

Joined:
Jun 25, 2009
Messages:
65
Great explanation... It makes sense

11. ### LeeDBionic Poster

Joined:
Dec 28, 2008
Messages:
44,309
Location:
East side of San Francisco Bay
...sure, whatever, I can sell you the GoldenGate Bridge.
Only thing happens is the ball slows down, allowing gravity to effect more, the spin to affect more, so the arc increases, as the ball travels the distance to your opponent.

12. ### ms87Rookie

Joined:
May 21, 2009
Messages:
244
wait, so you are suggesting that the rpm does not increase when the ball hits the court?

13. ### LeeDBionic Poster

Joined:
Dec 28, 2008
Messages:
44,309
Location:
East side of San Francisco Bay
"why does the spin increase DURING FLIGHT ? "
Not talking after the opposition hits it, not during the next point, not next week, not next year, but IN FLIGHT now.

14. ### SystemicAnomalyG.O.A.T.

Joined:
Feb 25, 2006
Messages:
14,440
Location:
Stuck in the Matrix somewhere in Santa Clara CA
No laws in physics are violated when a ball's RPMs increase sometime after being struck by racquet. The change in spin rate can happen while the ball is in flight or when the ball interacts with a court surface. I'll address the former situation first. I have observed numerous balls hit with a moderate linear speed and a very slow spin rate noticeably change its spin while in flight (prior to a bounce). I have noticed moderately spinning balls suddenly stop spinning for a short time while in flight. I believe that I have also noticed other balls that had almost no spin early in its flight, start to spin faster as the ball speed slowed during its flight.

I have a vague idea about why these apparent anomalies happen. However, I am not very fluent in fluid dynamics to really explain these phenomenon. I do know that balls or projectiles with very low spin rates will be susceptible to very small changes in pressure that it encounters during its flight. Witness the significant effects of these slight changes in air pressure for a knuckelball in baseball or a floater serve in volleyball.

When a ball has sufficient topspin, a high pressure system on the upper part of the ball and a low pressure system on the lower part of the ball will be created. This will give the heavily topspun ball its flight characteristics that we've all seen. This is the so-called Magnus effect in action. Now when the spin rate is very low, the Magnus forces will also be very low. If these forces are low, the spin and the flight of the ball will be more suspectible to the whim of the wind and more suspectible to slight changes in air pressure that the ball encounters during its flight.

15. ### SystemicAnomalyG.O.A.T.

Joined:
Feb 25, 2006
Messages:
14,440
Location:
Stuck in the Matrix somewhere in Santa Clara CA
^ Note that a ball with topspin will usually decrease a bit in spin rate during its flight (prior to the bounce). The anomalies that I spoke of (above) are for moderate or slow moving balls that are hit fairly flat (fairly low spin rates).

Let me now address the 2nd situation -- changes in spin rate when the ball interacts with the court (ie, when it bounces). Usually, but not always, the spin rate increases when the ball bounces. I believe that we are all aware that the ball loses quite a bit of linear speed on the bounce. Many of us have also noticed that the ball often spins faster after a bounce. Take a look at both of the following pages for more about speed and spin changes:

Ball Bounce: The Role of the Court in Generating Spin

Let us now consider 2 types of collisions (the ball interacting with the court is a collision). For elastic collisions, both momentum and energy are conserved. However, an inelastic collision, like our ball bounce, will result in a conservation of momentum but not a conservation of energy. Some of the energy lost in the ball bounce is manifested as heat -- heat due to the compression of the ball during its impact with the court. The ball will often deform quite a bit on most shots and will lose quite a bit of its speed as a result of this.

While some of the kinetic energy present before the bounce will be lost due to compression, some of it will be converted. The kinetic energy will be a combination of it linear component as well as its rotational component. Since the linear speed is reduced by the bounce, this will allow some of the kinetic energy to be manifested in its rotation speed after the bounce. However, this is not always the case.

16. ### SystemicAnomalyG.O.A.T.

Joined:
Feb 25, 2006
Messages:
14,440
Location:
Stuck in the Matrix somewhere in Santa Clara CA
Try the following simple experiments.

Toss a ball a few meters away from you with a low rate of topspin. It should be fairly easy to see that the spin rate of the ball is greater after the bounce than it is before the bounce. How much the spin rate increases will depend on a number of factors -- forward speed of the ball, trajectory of the ball (and the angle at which the ball hits the court), the amount of topspin prior to the bounce.

Now try throwing the ball with underpsin (backspin). First try it with a low trajectory toss. If the ball with underspin hits the ground at an angle that is less than 45 degrees, the ball will tend to skid and stay low. Notice what happens to the spin -- it changes spin direction -- underspin beomes topspin!!! Most balls hit, regardless of the type of spin it had pre-bounce will have forward spin (topspin) after the boounce.

Now throw an underspin ball at a high trajectory so that it hits the gound at a very steep angle (as one one do very a drop shot). This bill will tend to sit up, not skid, afte the bounce and not travel forward very much. This time the spin change will probably be much different. If the ball achieves tospin post-bounce, it will often/usually be at a very low spin rate. In some cases, the ball will have almost no spin after the bounce. The extreme case: if you can get the ball to bounce backward, you will notice that the ball still has backspin. This latter case does not happen very often in actual play (but it can!).

You might also discover that side spin (vertical axis) will have no effect on the bounce. While the Magnus forces created by side spin will cuase the ball to deflect (trajectory curves left or right), the vertical axis spin will have no effect on the bounce itself. You can verify this with a little bit of experimentation. Try dropping a ball straight down with vertcal axis spin. If you do this successfully (cleanly), you should observe that the ball bounces straight up -- it won't bounce left or right (or forward/backward).

On the other hand, spiral spin will not cause the flight of the ball to bend but will have an effect on the bounce. The ball will bounce left or right depending on which way the spiral spin is imparted -- clockwise spiral will casue the ball to bounce to the right (from the thower's/hitter's perpsective). Spiral spin does not generate any Magnus forces so it does not cause a deviation in the flight of the ball. In fact, spiral spin tends to stabilize the flight (minimize deviation). This is why spiral spin is imparted to bullets (and sometimes arrows).

If you drop a ball straight down imparting spiral spin, you should be able to see that the ball bounces in the direction of the spin as I stated in the previous paragraph.

17. ### Steady EddyHall of Fame

Joined:
Jul 19, 2007
Messages:
3,110
Location:
Arizona
In "Match Play and the Spin of the Ball" by Bill Tilden, Mr. Tilden asserts that sometimes (when you put your weight into a shot) the ball increases speed as it crosses then net. This isn't another one of Mr. Tilden's ideas, is it?

18. ### SystemicAnomalyG.O.A.T.

Joined:
Feb 25, 2006
Messages:
14,440
Location:
Stuck in the Matrix somewhere in Santa Clara CA
^ Tilden was on crack at the time (or perhaps an opium derivative).

19. ### LeeDBionic Poster

Joined:
Dec 28, 2008
Messages:
44,309
Location:
East side of San Francisco Bay
While I'll admit, taken literally, what Tilden says suggests he's on crack for sure....
But taken out of direct context, I suspect what he means is "PERCEIVED spin and skid", where the returner finds the ball "heavy", and the ball is harder to play than the shot appears. Maybe even meaning Tilden thinks his shots are heavy and tough for this opponent's to read and get into position in time and prep for.... maybe?
I know a very tiny bit about science and kinetics. After you hit the ball, and no one else hits it yet, the ball does not accelerate, the ball does not gain more spin..... literally.
But perception by the returner might be a different story. The old HEAVY BALL ding......

20. ### ramseszergProfessional

Joined:
Feb 23, 2006
Messages:
1,076

Have you read anything in this thread? Spin can increase when the ball hits the court, translating kinetic energy into rotational.

21. ### ramseszergProfessional

Joined:
Feb 23, 2006
Messages:
1,076
Do this.. take a tennis ball, and throw it down at concrete at an angle. The ball will bite the court and when it comes off the bounce it will have more spin than what you originally imparted on it.

22. ### SystemicAnomalyG.O.A.T.

Joined:
Feb 25, 2006
Messages:
14,440
Location:
Stuck in the Matrix somewhere in Santa Clara CA
While the ball is in flight, it does not accelerate overall (altho' the vertical component does decelerate and accelerate due to gravity). It will not accelerate on the bounce either. However, you are absolutely wrong about the spin rate. Try the experiments, I suggested in post #16 -- it should become apparent that it is real, not an illusion.

23. ### ms87Rookie

Joined:
May 21, 2009
Messages:
244
actually, there is acceleration on the ball during its entire flight, but that is probably way over your head.
anyway, the ball DOES increase spin when it hits the court, assuming its velocity with respect to the court is greater than its angular velocity.

24. ### ms87Rookie

Joined:
May 21, 2009
Messages:
244
actually, if the angular velocity is greater than the linear velocity, the ball will accelerate. we've all seen these balls - usually framed shanks that land and jump in an unpredictable direction.