<p>Guys… for the ball and weights one, how on Earth can it increase frequency and not period? Period is the reciprocal of frequency!</p>
<p>Also, the question asked the force pair for “a book resting on a table”. The answer is obviously table on the book and nothing else, why does the doc say otherwise?</p>
<p>“Consider a box resting on the floor. It experiences two forces: Its weight mg downward and the normal contact force N by the floor acting on it upwards. Are these two forces action-reaction pair according to the Newton’s third law of motion?
The weight mg is the force of gravity by the Earth on the box. Its reaction force is thus the force of gravity by the box on Earth acting on the centre of the Earth, also denoted by mg.
• The normal contact force N acts upwards on the box by the floor. Its reaction pair, N, is the force the box acts downwards on the floor.
• mg and N are not action-reaction pair since they both act on the same body, the box.”
from h2physics.org</p>
<p>For the ball and weights…
The weights on the string act as the centripetal force, let M = mass of weights and m = mass of bob
Mg = m<em>v^2/r
If M increases, m stays the same and r stays the same, thus v MUST increase.
Period T = 2pi</em>r/v
Since v is increasing and r stays the same, T MUST decrease.
f = 1/T
since T is decreasing, f MUST increase.</p>
<p>v increases, f increases, T decreases</p>
<p>If the mass of the bob increases, going back to Mg = m<em>v^2/r , then v MUST decrease since M and r stay the same.
T = 2pi</em>r / v , since v decreases, T MUST increase.
f = 1/T , since T increases, f must decrease</p>
<p>T increases, f decreases, v decreases</p>
<p>what is the curve for the may test?
Do you guys think this test is more difficult than the blue book test? so can we still get 59/75 equals 800 curve?</p>
<p>@thesoxpride10 How is it not the table on the book? If there’s no counterforce then the book will float through the table.</p>
<p>And you just agreed with me on the ball and weight one. The answer that everyone rallied behind on the doc is wrong. </p>
<p>@BassGuitar There’s no point in arguing. Collegeboard will determine who’s wrong and right. </p>
<p>@BassGuitar
Its not the table on the book. The question asked which of the following is the resultant force to the force on the book by the earth because of newton’s 3rd law. Newton’s third law states that every force has an equal and opposite force. These pairs cannot be forces acting on the same object. The normal force is a force by the table on the book and the force of gravity is the force by the earth on the book. These ARE NOT action reaction pairs since they act on the same object. The correct pairing of the action-reaction forces is
Force of gravity by the earth on the book - force of gravity on the earth by the book
Force of repulsion between electrons by the table on the book - force of repulsion between electrons by the book on the table.
Another way to look at it is that the normal force results from Newton’s First Law. The book is at rest thus there MUST be a second force on the book to counteract the force of gravity, this force is the normal force.</p>
<p>The answers on the google doc regarding the two ball and weights questions completely agree with my explanation above, so I don’t know what you are referring to. Perhaps I misunderstood your post above. What do you think the answers to the question about adding weights at the bottom question about increasing the mass of the bob/ball should be?</p>
<p>@BassGuitar
@thesoxpride10 is absolutely right on both of those. His explanations couldn’t have been better. I’m a little confused though. You said yourself that frequency is the reciprocal of period. If f=1/T, then f*T=1. If frequency goes up then T must go down because the product of f and T is always 1. Say f=2. Then T=1/2. If f goes up to 4, then T must go down to 1/4. </p>
<p>@blahblah97 I do think this test was definitely harder than the blue book test, but for some reason I have a feeling the curve won’t be better than the blue book one. That one is already fairly lenient, but we can hope. </p>
<p>@thesoxpride10 </p>
<p>On the google doc there are people saying that it affects period and frequency separately without mentioning the other, which is not correct. @calistudent07 the question asked which things will change at all IIRC, which direction it changes does not matter. I have a feeling I either forgot the question or misread the question now though.</p>
<p>Who says that an action/reaction pair do not have to act on the same object? In fact, Newton’s third law states the opposite: for every force there is a counterforce in the opposite direction on the object that exerted the first force. How is it not the table acting on the book? You even contradicted yourself by stating correctly at the end of your post: “Another way to look at it is that the normal force results from Newton’s First Law. The book is at rest thus there MUST be a second force on the book to counteract the force of gravity, this force is the normal force.” which perfectly corroborates my argument. </p>
<p>The book was not resting on the earth, it was resting on a table. Newton’s third law prevents the book from floating through the table by pointing out the force by the table on the book, which is the correct answer.</p>
<p>And the book does not exert a force on the Earth at all, the table that it is resting on does. Is it that fallacy which is confusing people?</p>
<p>@BassGuitar I’m almost positive the question asked for which of the following increased, so that’s why period cannot be an answer if frequency is one. I think what @thesoxpride10 is saying that one way to look at the normal force in this situation is that it arises from Newton’s First Law. If the object is at rest on the table, there must be no net force. Thus the normal force counteracts the force of gravity on the book. However, this is not the same as Newton’s Third Law. The Third Law states that if object A acts with a force F on B, B acts with a force -F on A. The Earth acts on the book, so the book must exert a force on the Earth. The effect is negligible because of the Earth’s mass in comparison to the book, but the book still exerts a force on the Earth equal in magnitude to the force of the Earth on the book. The “action” force in the problem was the force of gravity on the book (by the Earth). By Newton’s Third Law, the reaction force is the force of gravity on the Earth by the book. Had the question listed the action force as the force of the book on the table (whose magnitude is equal to mg even though it is NOT gravity), then the reaction would be the normal force of the table on the book. However, this was not the case.</p>
<p>@calistudent07 Okay, so I see where you are coming from. Earth acts on book, so book must act on Earth from Newton’s Third Law. Right? Well I remember the the question asking something like “A book is resting on a table. What is the counterforce for this?” In which case it would be book --> table, and the answer would be table --> book. OR, you can look at the question as Earth --> book, thus book --> Earth. I think it can be interpreted multiple ways depending on how much you want to abstract it. But in real life it would be Earth --> book, and table --> Earth. I still think it’s FAR more likely that they were look for the book --> table, table --> book interpretation. They want to make sure you understand why the book doesn’t float through the table. </p>
<p>And I guess you’re right on that one about the weights. I hope I just forgot and didn’t misread the question!</p>
<p>@BassGuitar
Im 100% confident the question mentioned Newton’s Third Law, which means the answer has to be the book on the earth</p>
<p>No it’s not, I’m telling you. YES, the book does exert a gravitational force on the Earth equal to the Earth’s pull on it, but that’s not what they were looking for. They wanted the normal force.</p>
<p>I’m not sure why Newton’s 3rd Law is giving you confidence in your answer. Because it also corroborates my claim that it’s the table on the book.</p>
<p>@BassGuitar You can check out Thompson Peterson SAT Physics book, question 60 on practice test 4 seems similar</p>
<p>@BassGuitar
Perhaps a physics textbook will convince you.
From Giancoli’s Physics 9th Edition</p>
<p>Figure 4-14a is a bust (the statue of a persons shoulders and head) lying on the table, the Normal Force and the Force of Gravity are labeled)
“The two forces shown in Fig. 4-14a are both acting on the statue, which remains at rest, so the vector sum of these two forces must be zero (Newton’s Second Law). Hence Fg and Fn must be of equal magnitude and opposite directions. But they are NOT the equal and opposite forces spoken of in Newton’s third law. The action and reaction forces of Newton’s third law act on DIFFERENT OBJECTS, whereas the two force shown in FIg. 4-14a act on the same object.”
“WARNING: Weight and normal force are NOT action-reaction pairs”.</p>
<p>@thesoxpride10 Check my post again:</p>
<p>“I still think it’s FAR more likely that they were look for the book --> table, table --> book interpretation”</p>
<p>That’s a perfect example of Newton’s Third Law, and they’re both on different objects, in accordance with your post. But it can be convoluted a little. When I say “the weight of the book” I am referring to Earth —> book, or book —> table?</p>
<p>I think we’ll just continue spinning around circles unless someone can reproduce the question exactly, and even then it will be misinterpreted probably. </p>
<p>never did the question mention book–>table. It was always Earth–>book</p>
<p>The question was asking for the normal force right? The normal force on the book must be table --> book or else the book will just fall through the table. Yes, Weight and normal forces are not action-reaction pairs. You can see it on a incline plane problem. However, on a flat surface, it is. Which is why you would put on object on a flat surface so they remain motionless. </p>
<p>The question was, paraphrased “A book lies on a table on Earth. The book experiences the force of gravity exerted by the Earth on the book, which of the following is the reactionary force due to Newton’s Third Law?”
The answer is simply “The force on the Earth by the book”. If you put anything else other than this I can 100% guarantee you got it wrong. </p>
<p>@idkName It doesn’t matter if the book is on an incline plane or on a flat surface. Normal Force and Weight aren’t and will NEVER be action-reaction forces. Yes, on a flat surface Normal Force and Weight are numerically equal and directed in opposite directions, but this is because of Newton’s SECOND (or First, depending how you interpret it). Newton’s Second Law states that Fnet = m*a. If the object is at rest, a is 0. Fnet = 0. Fnet = Fgravity + Fnormal, -Fgravity = Fnormal. Or if you wish, Newton’s First Law states that the sum of all forces on an object in motion at constant velocity or at rest is 0. Thus -Fgravity = Fnormal. Newton’s Third Law states that every force has an equal and opposite force. This means if object A exerts a force on B, then B exerts a force of the same magnitude on A, in the opposite direction. If we apply this to our book/table problem, this means that if object E (Earth) exerts a force on object B (book), object B must exert an equal force in the opposite direction. Nowhere in this statement do you see the appearance of object T (table).
Had the question called upon the force of the table on the book (instead of earth on book), the correct answer would have been the force of the book on the table. However the question explicitly stated “Force of Earth on the book” and “Newton’s Third Law” which means that the answer HAS to be, with no doubt, The force of the book on the table</p>
<p>Yep, your right. Norma force is never reaction force, but rather the force the book exert on the earth is. If thats what the question asks, then its book -> earth. However, if I recall, the question ask something different.</p>
<p>Less than a week until scores. Hoping the curve is decent. At least -10 raw for 800. I found this test tough, but then again Physics C isn’t the best prep for it. </p>