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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
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It doesn't work like that dave13.

When an object is dropped from rest it starts gaining speed and covers more and more distance with increasing time. So your assumption of equal distance covered in equal time intervals is wrong.

But this discussion is taking us far from GMAT PS so let's get back to using the formula given explicitly in the problem :-D

Best,
Gladi
dave13 wrote:


how abou this method :)


\(d = \frac{1}{2} * 32.2*5^2 = 805\) feet distance for 5 seconds


805/5 = 161 distance for one second



\(161*2 = 322\) :lol: closest to D
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
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HKD1710 wrote:
When an object is dropped, the number of feet N that it falls is given by the formula \(N=\frac{1}{2} gt^2\), where t is the time in seconds since it was dropped and g is 32.2, if it takes 5 seconds for the object to reach the ground, how many feet does it fall during the last 2 seconds?

(A) 64.4
(B) 96.6
(C) 161.0
(D) 257.6
(E) 402.5

Project PS Butler : Question #15


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So N feet in last 2 seconds can be found

if we subtract N in last 5 seconds - N in last 3 seconds
N = 1/2 * 32.2 [ 16]

N = 257.6

D
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
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Hi Gmatprep550,

The question does not "require" a physics background but knowing a bit of physics helps. This is very much similar to solving RC passages from particular fields. For ex. a science passage will favour someone who has a degree in science, even if any non-science test-taker can read it still get all questions correct. However, a good GMAT question will always have all the necessary info given in the question itself.

All the info needed to solve this questions is given in the question itself. Notice how the question specifically mentions "dropped" and also "last two seconds"... an attentive test-taker would realize that the formula given cannot be applied as it is with t = 2.

Hope this helps.

Best,
Gladi

Gmatprep550 wrote:

I believe it requires Physics bg to solve such questions, Do we still receive such type of question in GMAT?
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
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Quote:

Thanks for kind response Gladiator59, but from my past experience of GMAT question one would have simply used T =2 in formula or followed as stated by dave13 that it will travel distance of X in 5 min hence divide it by and multiply by 2 as they have not specified speed will increase or decrease at any point of time.

Please correct me if I am missing something.


Note that it is an official question so definitely fair play. It is like a function question.
You are given the relation between Distance covered and time. All you can use is the given relation. Until and unless you know the relation is linear, you cannot assume that distance covered is same every second. You don't need to know physics for this. Otherwise, you didn't need the given relation. You already know that Distance = Speed*Time.
The relation is given to you because it is specifically defined.
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
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HKD1710 wrote:
When an object is dropped, the number of feet N that it falls is given by the formula \(N=\frac{1}{2} gt^2\), where t is the time in seconds since it was dropped and g is 32.2, if it takes 5 seconds for the object to reach the ground, how many feet does it fall during the last 2 seconds?

(A) 64.4
(B) 96.6
(C) 161.0
(D) 257.6
(E) 402.5

Project PS Butler : Question #15


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Distance traveled in last 2 seconds = Distance traveled in 5 seconds - Distance traveled in 3 seconds

Distance traveled in 5 seconds \(= 1/2*g*t^2 = 1/2*32.2*25 = 16.1*25\)

Distance traveled in 3 seconds \(= 1/2*g*t^2 = 1/2*32.2*9 = 16.1*9\)

Distance traveled in last 2 seconds \(= 16.1*25 - 16.1*9 = 16.1*16 = 257.6\)

Hence, D.
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
Gladiator59 wrote:
A different question, with a physics background. :-)

I feel it should be mentioned explicitly that the formula is for an object dropped from rest but the word "dropped" kind of implies that.

So, the formula given is when the object is dropped from rest. So the distance it covers in the last two seconds will be distance covered from start till 5 secs minus
distance covered from start to 5-2 = 3 secs

This step is essential as the object already has a non zero velocity when it is 2 secs before it reaches the ground.

Let distance covered in last two sec be d.

\(d = 0.5gt^2 - 0.5gt'^2\)
\(d = 0.5g( t^2 - t'^2)\) where t is 5 sec and t' is 3 sec
Hence,
\(d = 0.5*32.2*(25 - 9)\)
\(d = 16.1*16\)
\(d > 256\) only slightly greater

Hence Option (D) is correct.

Best,
Gladi


HKD1710 wrote:
When an object is dropped, the number of feet N that it falls is given by the formula \(N=\frac{1}{2} gt^2\), where t is the time in seconds since it was dropped and g is 32.2, if it takes 5 seconds for the object to reach the ground, how many feet does it fall during the last 2 seconds?

(A) 64.4
(B) 96.6
(C) 161.0
(D) 257.6
(E) 402.5

Project PS Butler : Question #15


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I believe it requires Physics bg to solve such questions, Do we still receive such type of question in GMAT?
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
Gladiator59 wrote:
Hi Gmatprep550,

The question does not "require" a physics background but knowing a bit of physics helps. This is very much similar to solving RC passages from particular fields. For ex. a science passage will favour someone who has a degree in science, even if any non-science test-taker can read it still get all questions correct. However, a good GMAT question will always have all the necessary info given in the question itself.

All the info needed to solve this questions is given in the question itself. Notice how the question specifically mentions "dropped" and also "last two seconds"... an attentive test-taker would realize that the formula given cannot be applied as it is with t = 2.

Hope this helps.

Best,
Gladi

Gmatprep550 wrote:

I believe it requires Physics bg to solve such questions, Do we still receive such type of question in GMAT?


Thanks for kind response Gladiator59, but from my past experience of GMAT question one would have simply used T =2 in formula or followed as stated by dave13 that it will travel distance of X in 5 min hence divide it by and multiply by 2 as they have not specified speed will increase or decrease at any point of time.

Please correct me if I am missing something.
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
Hi chetan2u, Bunuel, VeritasKarishma, Gladiator59, generis

Could you please help me with regard to below mentioned issue.

Gmatprep550 wrote:
Gladiator59 wrote:
Hi Gmatprep550,

The question does not "require" a physics background but knowing a bit of physics helps. This is very much similar to solving RC passages from particular fields. For ex. a science passage will favour someone who has a degree in science, even if any non-science test-taker can read it still get all questions correct. However, a good GMAT question will always have all the necessary info given in the question itself.

All the info needed to solve this questions is given in the question itself. Notice how the question specifically mentions "dropped" and also "last two seconds"... an attentive test-taker would realize that the formula given cannot be applied as it is with t = 2.

Hope this helps.

Best,
Gladi

Gmatprep550 wrote:

I believe it requires Physics bg to solve such questions, Do we still receive such type of question in GMAT?


Thanks for kind response Gladiator59, but from my past experience of GMAT question one would have simply used T =2 in formula or followed as stated by dave13 that it will travel distance of X in 5 min hence divide it by and multiply by 2 as they have not specified speed will increase or decrease at any point of time.

Please correct me if I am missing something.
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
Hi generis,

What a beautiful explanation, I really liked it the way you have explained it. :inlove:

Still have one question :-D

How I will realize that I need to use different method like this. Just one point I can guess is out of 5 sec how much in 2 sec, Anything apart form it?
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
Hi VeritasKarishma,

I already understood all that part but just wanted to know if I receive such kind of question in future what part from question will advice me to follow such procedure. for the same reason I asked generis

"How I will realize that I need to use different method like this. Just one point I can guess in this question is out of 5 sec how much in 2 sec, Anything apart form it? "

PS- I have added red part in it now as I missed that to specify in previous comments.
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
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Gmatprep550 wrote:
Hi VeritasKarishma,

I already understood all that part but just wanted to know if I receive such kind of question in future what part from question will advice me to follow such procedure. for the same reason I asked generis

"How I will realize that I need to use different method like this. Just one point I can guess in this question is out of 5 sec how much in 2 sec, Anything apart form it? "

PS- I have added red part in it now as I missed that to specify in previous comments.


The hint is that they have given you the formula to calculate distance so that is what you must use in each case. Also, observe the formula. Distance depends on t^2 so it is not a linear relation. Hence dividing into 5 equal parts is incorrect.
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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
The key to the question that makes it solvable without physics lies in the words "t is the time in seconds since it was dropped".

Using 2 directly into the formula treats it as if it has been dropping for 2 seconds SINCE it was initially released.

No, it has been dropping for 5-2=3 seconds since being released.

The formula only returns total distance dropped for a time period beginning when it was initially at rest.

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Re: When an object is dropped, the number of feet N that it falls is given [#permalink]
generis

Hi, I understood your solution. But the answer marked here is option D. So the CA is C or D? I’m confused.

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When an object is dropped, the number of feet N that it falls is given [#permalink]
Since it takes 5 seconds to hit ground

N = 1/2gt^2

= 32.2 x 25/2

= 805/2

= 402.5

Distance covered in last two seconds will be.

We need to find distance covered after 3 seconds then we subtract fhat from 402.5

9 x 32.2/2

= 289.8/2
= 144.9

402.5 - 144.9

= 257.6
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