Question from a TBR practice test

[Ari1584]

1.) If a pendulum were placed over a large iron ore deposit under the Earth’s surface, what change would be observed in its period?

a) The period of the pendulum would be shorter, because the density of the iron ore beneath it would slightly decrease the fraction of its acceleration that is due to gravity.
b)The period of the pendulum would be longer, because the density of the iron ore beneath it would slightly increase the fraction of its acceleration that is due to gravity
c) The period of the pendulum would be shorter, because the density of the iron ore beneath it would slightly increase the fraction of its acceleration that is due to gravity
d)The period of the pendulum would be longer, because the density of the iron ore beneath it would slightly decrease the fraction of its acceleration that is due to gravity

*I got this question right from P.O.E, but i'm not sure i really understand the answer as to why this would happen. The answer is C, and their reasoning is:
The density of the iron ore deposit beneath the pendulum is greater than the average density of the Earth. Thus, there will be more mass hence, more gravitational force pulling downward) in the vicinity of the pendulum, which increases the denominator in the fraction of the periodicity equation (acceleration due to gravity, g). That is, substituting a slightly larger value for g in the equation produces a slightly smaller fraction in the periodicity equation, which in turn means a shorter period for the pendulum. We can eliminate choices B and D, because they predict a longer period, rather than a shorter one; and we can eliminate choice A, because it incorrectly explains the shorter period as resulting from a decrease (rather than an increase) in acceleration due to gravity.

I guess what im saying is that, when i saw this question with iron ore, the first thing that came to mind was magnetism, not an increase in mass of the earth?? Did anyone automatically think of an increase in mass of the earth? I just didn't really understand the question i guess. Can someone explain their thought process through it please.

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[0Complications]

1.) If a pendulum were placed over a large iron ore deposit under the Earth's surface, what change would be observed in its period?

a) The period of the pendulum would be shorter, because the density of the iron ore beneath it would slightly decrease the fraction of its acceleration that is due to gravity.
b)The period of the pendulum would be longer, because the density of the iron ore beneath it would slightly increase the fraction of its acceleration that is due to gravity
c) The period of the pendulum would be shorter, because the density of the iron ore beneath it would slightly increase the fraction of its acceleration that is due to gravity
d)The period of the pendulum would be longer, because the density of the iron ore beneath it would slightly decrease the fraction of its acceleration that is due to gravity

*I got this question right from P.O.E, but i'm not sure i really understand the answer as to why this would happen. The answer is C, and their reasoning is:
The density of the iron ore deposit beneath the pendulum is greater than the average density of the Earth. Thus, there will be more mass hence, more gravitational force pulling downward) in the vicinity of the pendulum, which increases the denominator in the fraction of the periodicity equation (acceleration due to gravity, g). That is, substituting a slightly larger value for g in the equation produces a slightly smaller fraction in the periodicity equation, which in turn means a shorter period for the pendulum. We can eliminate choices B and D, because they predict a longer period, rather than a shorter one; and we can eliminate choice A, because it incorrectly explains the shorter period as resulting from a decrease (rather than an increase) in acceleration due to gravity.

I guess what im saying is that, when i saw this question with iron ore, the first thing that came to mind was magnetism, not an increase in mass of the earth?? Did anyone automatically think of an increase in mass of the earth? I just didn't really understand the question i guess. Can someone explain their thought process through it please.

As I was reading the question my thoughts initially went to magnetism as well, but they never stated the composition of the pendulum, so when I glanced at the answer choices I realized it wasn't going to deal with magnetism and immediately switched my thinking to and increased mass and the effects it would have on gravity thus effecting the pendulum.

Once I started thinking grav/acceleration I eliminated the longer periods, then read the second part of the answers for shorter periods (A & C). Then I crossed off A as a possible answer choice because it is not true, so that left me with C. Did a quick read of the secondary part of C and called that the answer.

So, you definitely aren't alone in your initial thoughts for magnetism on this question.

 

[Ari1584]

As I was reading the question my thoughts initially went to magnetism as well, but they never stated the composition of the pendulum, so when I glanced at the answer choices I realized it wasn't going to deal with magnetism and immediately switched my thinking to and increased mass and the effects it would have on gravity thus effecting the pendulum.

Once I started thinking grav/acceleration I eliminated the longer periods, then read the second part of the answers for shorter periods (A & C). Then I crossed off A as a possible answer choice because it is not true, so that left me with C. Did a quick read of the secondary part of C and called that the answer.

So, you definitely aren't alone in your initial thoughts for magnetism on this question.

I'm glad im not alone on that one! So is the iron ore increasing the mass of the EARTH? and an increase in mass of the earth causes a decrease in gravitational acc?

 

[0Complications]

I'm glad im not alone on that one! So is the iron ore increasing the mass of the EARTH? and an increase in mass of the earth causes a decrease in gravitational acc?

No. It would not increase the mass of the earth, but the density and implicit in that the mass of the earth in the location of the the pendulum is greater. As a result of the greater mass the gravitational force exhibited by the earth in that location would be greater causing the acceleration to be larger than 9.8 m/s*s. The larger acceleration is what is going to cause the pendulum to have a shorter period.

have a g = 9.8 is an averaged value, so depending where you're at on the earth that value can either increase or decrease.

 

[rocketbooster]

I first eliminated A and B because the 1st and last parts of each answer choice did not go together intuitively.

I ended up being D because I assumed it had something to do with magnetism. I assumed the iron was blocking part of the magnetism or something, so less g.

But yea, it doesn't tell you the composition of the pendulum so you can't assume anything on magnetism. thus, just go to C by default haha. on the real MCAT would you know the exact reasoning behind it that is explained in the answer explanation? probably not, but you only need to eliminate 3/4 answers.

how would you know the density of the iron ore is greater than the average density of the Earth? no idea.