# A slice causes a tennisball to spin and deviate from its normal parabolic path.

• jlyu002@ucr.e
In summary, a tennis player can hit a ball with a "slice" causing it to spin and deviate from its normal parabolic path. This is due to the Magnus effect, where the spin of the ball creates a pressure gradient that causes it to curve. This additional acceleration occurs in a horizontal direction and is caused by the difference in air flow around the ball. In the case of a backspin, there is also a small upwards acceleration.
jlyu002@ucr.e

## Homework Statement

A tennis player slices the ball to spin and deviate from its normal parabolic path. What is an explanation for this phenomenon?

I am having trouble visualizing the phenomena that is occurring with the slice and as well as the explanation give by the book.

How is the velocity changing?

## Homework Equations

On Earth, a tennis player can hit a tennis ball normally causing the ball to travel on a path that is a symmetrical parabola. A tennis player can also hit a tennis ball with a “slice” which causes the ball to spin and deviate to one side of its normal path. What is the best explanation for this deviation?
A. There is an additional acceleration on the ball. Or
B. The spin on the ball used energy so the ball could not travel in a straight line.
Here is an explanation from the book.
The sliced ball travels off to the side, so there must be an acceleration causing the velocity to change. Since gravity is always directed down toward the earth, there must be an additional acceleration from another source (other than gravity) causing the ball’s velocity to deviate to the side.

## The Attempt at a Solution

This is actually a fluid dynamics problem. The balls spins - thus, as it moves through the surrounding air the flow of air will move faster around one side of the ball and slower on the other. By Bernoulli's principle a faster flow means a lower pressure. Thus we end up with a reduced air pressure on one side of the ball and a correspondingly increased air pressure on the opposite side. This pressure gradient is what causes the ball to deviate from the parabolic trajectory.

Oh, so the pressure gradient creates a new horizontal acceleration while in the air?

It's called Magnus effect, and one explanation is that the flow on the downwind spinning side remains attached longer, causing the air to be deflected, and the ball to be curved. Wiki article (with a diagram showing a ball with top spin):

http://en.wikipedia.org/wiki/Magnus_effect

Just quickly, from the answer given to me by the textbook, it says that an additional acceleration is occurring. Is this due to the magnus effect? Where is the additional acceleration occurring? I know that there is a constant -9.80 m/s^2 downward acceleration, but I just can't seem to visualize or know where the additional acceleration is pointing in which direction.

jlyu002@ucr.e said:
Just quickly, from the answer given to me by the textbook, it says that an additional acceleration is occurring. Is this due to the magnus effect?
yes

jlyu002@ucr.e said:
Where is the additional acceleration occurring?
In the case of a ball with back spin (slice), the ball exerts a downwards force on the air, diverting the air downwards, coexistant with the air exerting an upwards force on the ball. It's relatively small in the case of a tennis ball, but a ping pong ball can be made to rise with enough speed and backspin to generate more lift (force) than the weight of the ball.

Last edited:
OOOO thanks rcgldr. There is an upward acceleration.

jlyu002@ucr.e said:
OOOO thanks rcgldr. There is an upward acceleration.
A ping pong ball can have upwards acceleration, but in the case of a tennis ball, I'm don't know if upwards acceleration is possible, versus just reduced downwards acceleration due to gravity being opposed by a upwards but weaker aerodynamic force.

woah, crazy my head hurts. lol. thanks rcgldr I'm going to try and digest this.

## 1. How does a slice cause a tennis ball to spin?

A slice occurs when the racket strikes the bottom half of the ball, causing it to rotate in a clockwise or counterclockwise direction. This rotation creates a side spin on the ball, causing it to curve in the air.

## 2. Why does a slice cause the tennis ball to deviate from its normal path?

The side spin created by a slice causes an imbalance in the air pressure on either side of the ball, resulting in an asymmetric lift force. This lift force causes the ball to curve away from its normal path.

## 3. Can a slice be used intentionally in a tennis match?

Yes, many players use a slice as a strategic shot in tennis. It can be used to hit a low, fast ball that is difficult for the opponent to return, or to create an angle that makes it challenging to return the ball.

## 4. Is a slice the only way to put spin on a tennis ball?

No, there are other types of spins that can be put on a tennis ball, such as topspin and backspin. Each type of spin creates a different trajectory and can be used for different purposes in a tennis match.

## 5. Does the type of racket affect the spin created by a slice?

Yes, the type of racket and its characteristics, such as string tension and grip, can affect the amount of spin created by a slice. A racket with more flexible strings and a larger sweet spot is more likely to produce a greater spin on the ball.

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