Manhattan GMAT. Book 7: P.104 Passage F Help!

[OneTwoThreeFour]

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

5. Which of the following can be inferred from the passage?
(A) If a player holds the tennis racket anywhere other than the end of the handle, the player will experience a jolting sensation.
(B) The primary sweet spot is more effective at damping vibration than the secondary sweet spot.
(C) Striking a tennis ball at a spot other than the center of percussion can result in a jarring feeling.
(D) Striking a tennis ball repeatedly at spots other than a sweet spot leads to "tennis elbow."
(E) If a player lets go of the racket at the moment of impact, the simultaneous forward and backward impetus causes the racket to drop straight to the ground.

Spoiler:

[spoiler] The correct answer is A for 2 and C for 5. I understand why A is the right answer because the first paragraph mentions the racket also has a vibration node, which is the first "sweet spot." But I don't understand why the answer is C for 5. Wouldn't striking the the tennis ball at the vibration node would also have the same effect as striking at the center of percussion? The passage never mentioned that hitting at the first sweet spot would result in a jarring feeling. In fact, from answer choice 2 we know that hitting at the first spot can eliminate the jolt. If hitting at the first spot can eliminate the jolt, than how can it also cause a jarring feeling? I don't play tennis, maybe that is why I am having such a hard time distinguishing between these two. [/spoiler]

[Adam@Knewton]

You're going to be pleased at how simple this explanation is!

Your analysis of the passage and of the right answer ideas for both #s 2 and 5 is perfect. You're just mis-reading choice (C) in #5. It doesn't day "any spot other than..." -- it just says "a spot other than...." So choice (C) does NOT mean, as you originally thought, that the center of percussion is the only way to avoid the jarring feeling; it only means that there exists some spot somewhere, other than the center of percussion, where you would get a jarring feeling, and this is obviously inferable from the passage.

To clarify a little better, take this similar sentence: "Going to a place other than New York can cause you to be in Italy."

It would be false if we said that going to ANY place other than New York WILL result in you being in Italy -- however, it's true that, if you go to some unknown non-NY place, it's possible you're in Italy. Especially if you smell delicious food when you arrive.

[OneTwoThreeFour]

Haha, thank you so much Adam! For Data Sufficiency questions, a choice that is possible counts as a no. But I guess it is different for reading comprehension. Man, I really need to rewire my brain for reading comprehension!

[lunarpower]

it appears that adam has already addressed your post, but i should comment here:

Haha, thank you so much Adam! For Data Sufficiency questions, a choice that is possible counts as a no. But I guess it is different for reading comprehension.

... not really. the issue is not data sufficiency vs. reading comprehension; the issue is the word "can" in the statement. if there is even a single affirmative possibility, then "can" is true.
for instance, if a data sufficiency question asks, "can the integer N be written as the product of two positive integers x and y, where 1 < x < y?" then a single affirmative possibility, in this case, means "yes".
for instance, if N = 16 then the answer is "yes", because you can have x = 2 and y = 8. because a single possibility is enough for "can" to be true, the existence of other possibilities that don't satisfy the condition is irrelevant.

[OneTwoThreeFour]

Thanks for clearing that up for me Ron. Its just that for all the DS questions I have encountered so far, a choice that could or could not answer the question is always wrong. Because of that, I subconsciously assumed all the choices that could maybe answer the question given is always wrong. I will definitely pay more attention to the wording from now on though, and watch out for any DS questions where there are choices that could answer the question.

[mundasingh123]

Dont the 2 statements in Bold contradict each other

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.

First statementsays handle will be jerked backward and the 2nd statement says the handle will be jerked forward

[mundasingh123]

Hi Adam , Ron

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.

Doesnt this mean A is true for Question 2.

[Adam@Knewton]

Hi Adam , Ron

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.

Doesnt this mean A is true for Question 2.

All this means is that, if you drop the ball on the "sweet spot" while holding the racket as described, the handle will in fact jump out of your hand -- but that doesn't imply that, when holding it properly, you'd feel a "jolt" as described in (A) to #2. In fact, we're told that the center of percussion "will also greatly diminish the strain on a player's arm when the ball is struck." Thus, we can infer properly that there is at least a greatly diminished "jolting" sensation can be achieved through the "sweet spot" and not through the "center of percussion," thus eliminating choice (A).

[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!

[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!