3rd edition, chapter 6, passage: Though most tennis players

[pawan.agarwal]

Though most tennis players generally strive to
strike the ball on the racket's vibration mode, more
commonly known as the "sweet spot" , many players
are unaware of the existence of a second, lesser-
known location on the racket face, the center of per-
cussion, that will also greatly diminish the strain on a
player's arm when the ball is struck.

In order to understand the physics of this sec-
ond sweet spot, it is helpful to consider what would
happen to a tennis racket in the moments after impact
with the ball if the player's hand were to vanish at the
moment of impact. The impact of the ball would cause
the racket to bounce backwards, experiencing a trans-
lational motion away from the ball. The tendency of
this motion would be to jerk all parts of the racket,
including the end of its handle, backward, or away
from the ball. Unless the ball happened to hit the rack-
et precisely at the racket's centre of mass, the racket
would additionally experience a rotational motion
around its center of mass - much as a penny that has
been struck around its edge will start to spin. Whenever
the ball hits the racket face, the effect of this rotational
motion will be to jerk the end of the handle forward,
towards the ball. Depending on where the ball strikes
the racket face, one or other of these motions will
predominate.

However , there is one point of impact, known
as the center of percussion, which causes neither
motion to predominate; if a ball were to strike this
point, the impact would not impart any motion to the
end of the handle. The reason for this lack of motion is
that the force on the upper part of the hand would be
equal and opposite to the force on the lower part of
the hand, resulting in no net force on the tennis play-
ers' hand or forearm. The center of percussion consti-
tutes a second sweet spot because a tennis player's
wrist typically is placed next to the end of the racket's
handle. When the player strikes the ball at the center
of percussion, her wrist is jerked neither forward nor
backward, and she experiences a relatively smooth,
comfortable tennis stroke.

The manner in which a tennis player can
detect the center of percussion on a given tennis rack-
et follows from the nature of this second sweet spot.
The center of percussion can be located via simple
trial and error by holding the end of a tennis racket
between your finger and thumb and throwing a ball
onto the strings. If the handle jumps out of your hand,
then the ball has missed the center of percussion.

2. According to the passage, all of the following are true of the forces acting upon a tennis racket striking a ball EXCEPT

(A) The only way to eliminate the jolt that accompanies most strokes is to hit the ball on the center of percussion
(B) The impact of the ball striking the racket can strain a tennis player's arm
(C) There are at least two different forces acting upon the racket
(D) The end of the handle of the racket will jerk forward after striking the ball unless the ball strikes the racket's center of mass
(E) The racket will rebound after it strikes the ball

OA is A. I chose D.

Question 2 asks for choosing the false option. Choice A is the official answer and is correct. However, choice D is also a correct option to choose (i.e. a non-true inference/sentence from the passage). The passage states that one motion jerks all parts of the racket, including the end of its handle, backward; the other motion jerks the end of the handle forward; and one of these predominates unless the ball hits at center of percussion. This clearly makes wrong choice D, which states that the handle will jerk forward after striking the ball unless the ball strikes racket's center of mass.
Am I getting it wrong somewhere?

[jnelson0612]

In order to understand the physics of this sec-
ond sweet spot, it is helpful to consider what would
happen to a tennis racket in the moments after impact
with the ball if the player's hand were to vanish at the
moment of impact. The impact of the ball would cause
the racket to bounce backwards, experiencing a trans-
lational motion away from the ball. The tendency of
this motion would be to jerk all parts of the racket,
including the end of its handle, backward, or away
from the ball. Unless the ball happened to hit the rack-
et precisely at the racket's centre of mass, the racket
would additionally experience a rotational motion
around its center of mass - much as a penny that has
been struck around its edge will start to spin. Whenever
the ball hits the racket face, the effect of this rotational
motion will be to jerk the end of the handle forward,
towards the ball. Depending on where the ball strikes
the racket face, one or other of these motions will
predominate.

However , there is one point of impact, known
as the center of percussion, which causes neither
motion to predominate; if a ball were to strike this
point, the impact would not impart any motion to the
end of the handle. The reason for this lack of motion is
that the force on the upper part of the hand would be
equal and opposite to the force on the lower part of
the hand, resulting in no net force on the tennis play-
ers' hand or forearm. The center of percussion consti-
tutes a second sweet spot because a tennis player's
wrist typically is placed next to the end of the racket's
handle. When the player strikes the ball at the center
of percussion, her wrist is jerked neither forward nor
backward, and she experiences a relatively smooth,
comfortable tennis stroke.

2. According to the passage, all of the following are true of the forces acting upon a tennis racket striking a ball EXCEPT

(A) The only way to eliminate the jolt that accompanies most strokes is to hit the ball on the center of percussion
(B) The impact of the ball striking the racket can strain a tennis player's arm
(C) There are at least two different forces acting upon the racket
(D) The end of the handle of the racket will jerk forward after striking the ball unless the ball strikes the racket's center of mass
(E) The racket will rebound after it strikes the ball

OA is A. I chose D.

Question 2 asks for choosing the false option. Choice A is the official answer and is correct. However, choice D is also a correct option to choose (i.e. a non-true inference/sentence from the passage). The passage states that one motion jerks all parts of the racket, including the end of its handle, backward; the other motion jerks the end of the handle forward; and one of these predominates unless the ball hits at center of percussion. This clearly makes wrong choice D, which states that the handle will jerk forward after striking the ball unless the ball strikes racket's center of mass.
Am I getting it wrong somewhere?

Pawan, I agree. You have identified why D is wrong. D says that the end of the handle will jerk forward, whereas the passages states that the end of the handle could jerk forward or backward. I've bolded the relevant portions in paragraphs two and three to help us reach this conclusion.

Also, notice that answer choice A has extreme language ("the only way" . . .). Extreme answers are easier to disprove, so those should get extra consideration when searching for an untrue answer choice.

Thank you,

[asth678]

The answer I had picked was B and I understand why its incorrect but I am not sure as to why is answer C the correct one-It says that striking the ball at a spot other than the center of percussion will result in a jarring feeling.This would not be true if your ball hit the first sweet spot you would not experince any jarring feeling either.

I understand that your are supposed to choose the best answer not the most perfect one but keeping that in mind option B also fits as the passage says
"many players
are unaware of the existence of a second, lesser-
known location on the racket face, the center of per-
cussion, that will also greatly diminish the strain on a
player's arm when the ball is struck. "

Since the passage says "it will greatly diminish"- isn't that a degree of comparison between the first and the second sweet spot and the impact they have?

[jnelson0612]

The answer I had picked was B and I understand why its incorrect but I am not sure as to why is answer C the correct one-It says that striking the ball at a spot other than the center of percussion will result in a jarring feeling.This would not be true if your ball hit the first sweet spot you would not experince any jarring feeling either.

I understand that your are supposed to choose the best answer not the most perfect one but keeping that in mind option B also fits as the passage says
"many players
are unaware of the existence of a second, lesser-
known location on the racket face, the center of per-
cussion, that will also greatly diminish the strain on a
player's arm when the ball is struck. "

Since the passage says "it will greatly diminish"- isn't that a degree of comparison between the first and the second sweet spot and the impact they have?

Actually it appears that A, not C, is the correct answer. Does this help?

[stephenjmurph]

I vociferously disagree with the answer to question 2:

Answer A: "The only way to ELIMINATE the jolt ... on the center of percussion."
First, "eliminate" is a STRONG word! The first paragraph states "location.... that will also greatly diminish the strain..." THEN we get to the first sentence of paragraph three which states "there is ONE POINT of impact, known as the center of percussion, which causes neither motion to predominate... "would not impart any motion to the end of the handle."
So, although both spots DIMINISH the joint, the center of percussion is the ONLY spot that eliminates! THIS IS MENTIONED IN THE PASSAGE AND THEREFOR CAN'T BE THE ANSWER.

Answer C"
Second, and in my opinion, more importantly, THE PASSAGE ACTUALLY DOESN'T MENTION TWO FORCES EXPLICITLY ACTING ON THE RACKET... SERIOUSLY, TRY TO FIND IT.
The whole second paragraph mentions "MOTIONS" albeit two different kinds of motions. BUT (and I know from my physics major, even though you don't need to take this into account), two motions do NOT necessarily mean two forces. In fact, in this exact case, there is only one force on the racket which creates two motions.
The only time FORCE is actually used, is when it specifically refers to the "FORCES ON THE HAND" - NOT the racket. Yes, motion of the racket can create multiple forces on the hand, but again, if you can only use in the information in the passage, you don't know that there are more than one force ON THE RACKET.

Because Answer A is actually true, and because C was not mentioned, the Answer should be C!

[RonPurewal]

the passage is about the moment of impact, so, the second force comes from the ball that's hitting the racket.