# AQA A2 Physics Gravitational Fields help

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kirino1

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Im stuck on this question, any help would be greatly appreciated

The moon has a radius of 1740km and its surface gravitational field strength is 1.62Nkg^-1 to 3sf. The mass of the moon is 7.35x10^22kg.

The moons gravitational pull on the earth causes the ocean tides. show that the gravitational pull of the moon on the earth's oceans is approximately 3 millionths of the gravitational pull of the eath on its oceans. Assume the distance from the earth to the moon is 380,000km.

help

The moon has a radius of 1740km and its surface gravitational field strength is 1.62Nkg^-1 to 3sf. The mass of the moon is 7.35x10^22kg.

The moons gravitational pull on the earth causes the ocean tides. show that the gravitational pull of the moon on the earth's oceans is approximately 3 millionths of the gravitational pull of the eath on its oceans. Assume the distance from the earth to the moon is 380,000km.

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kirino1

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Pangol

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#3

I think this is best approached by working out the Moon's gravitational field strength at the surface of the Earth, and then comparing it to the Earth's gravitational field strength at the surface of the Earth. Do you know how to do that?

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PatrickD

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#4

The AQA specifications (A and B) make use of formulae booklets given to candidates. Take a look at the 'Gravitational fields and Mechanics' section of this booklet (either in the back of your textbook, in a separate booklet or on the AQA website) and compare the formulae to what information you have and what you want to calculate.

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kirino1

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#5

(Original post by

I think this is best approached by working out the Moon's gravitational field strength at the surface of the Earth, and then comparing it to the Earth's gravitational field strength at the surface of the Earth. Do you know how to do that?

**Pangol**)I think this is best approached by working out the Moon's gravitational field strength at the surface of the Earth, and then comparing it to the Earth's gravitational field strength at the surface of the Earth. Do you know how to do that?

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chrisburns123

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#6

is there any chance you could explain this? I've tried to do this by using g= GM/r^2 for both the moon, then dividing the relevant values of g into each other, but I'm getting x10^5 answers, not x10^6 :s

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Pangol

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What value do you get for the gravitational field strength of the Moon at the Earth's surface? Can you show how you get it?

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TheHaylio

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(Original post by

thanks. I got the answer

**kirino1**)thanks. I got the answer

Couldn't find a way to message you! I havent heard back from them yet no, you?

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#9

(Original post by

Couldn't find a way to message you! I havent heard back from them yet no, you?

**TheHaylio**)Couldn't find a way to message you! I havent heard back from them yet no, you?

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Chicken Bacon

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(Original post by

thanks. I got the answer

**kirino1**)thanks. I got the answer

g for moon = 13000

g for earth = 9.8

this is not 3 millionth ?

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Stonebridge

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#11

(Original post by

I'm trying the same question, care to explain how it's done ? i calculated g for the moon on the earths surface, and of course g for the earth on its own oceans is 9.8. but i get

g for moon = 13000

g for earth = 9.8

this is not 3 millionth ?

**iAre Teh Lejend**)I'm trying the same question, care to explain how it's done ? i calculated g for the moon on the earths surface, and of course g for the earth on its own oceans is 9.8. but i get

g for moon = 13000

g for earth = 9.8

this is not 3 millionth ?

Here

http://www.thestudentroom.co.uk/show....php?t=2156314

How can anyone know where you have gone wrong if you don't post your solution?

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Yorrap

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(Original post by

Same question has been asked twice

Here

http://www.thestudentroom.co.uk/show....php?t=2156314

How can anyone know where you have gone wrong if you don't post your solution?

**Stonebridge**)Same question has been asked twice

Here

http://www.thestudentroom.co.uk/show....php?t=2156314

How can anyone know where you have gone wrong if you don't post your solution?

My exam board is Edexcel but since the question is a good practice, I gave it a try;

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Stonebridge

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#13

It's correct.

You have calculated the Moon's field

If g is 10 on Earth (approx) and the value of the moon's field at the earth is 3 x 10

then the ratio of the Moon's field at Earth to the Earth's field at Earth is (3 x 10

Which is 3 x 10

You have calculated the Moon's field

**correctly.**If g is 10 on Earth (approx) and the value of the moon's field at the earth is 3 x 10

^{-5}then the ratio of the Moon's field at Earth to the Earth's field at Earth is (3 x 10

^{-5}) / 10Which is 3 x 10

^{-6}
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Yorrap

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#14

(Original post by

It's correct.

You have calculated the Moon's field

If g is 10 on Earth (approx) and the value of the moon's field at the earth is 3 x 10

then the ratio of the Moon's field at Earth to the Earth's field at Earth is (3 x 10

Which is 3 x 10

**Stonebridge**)It's correct.

You have calculated the Moon's field

**correctly.**If g is 10 on Earth (approx) and the value of the moon's field at the earth is 3 x 10

^{-5}then the ratio of the Moon's field at Earth to the Earth's field at Earth is (3 x 10

^{-5}) / 10Which is 3 x 10

^{-6}Do I understand the question wrongly?

Hmm

show that the gravitational pull of the moon on the earth's oceans is approximately 3 millionths of the gravitational pull of the eath on its oceans.

"show that the field strength of the moon on the Earth's oceans is approximately 1/3000000 of the gravitational field strength of the earth on its oceans?"

(in other words show that g

_{Moon}is approximately 9.81/3 000 000 ?)

If g of Moon on Earth's oceans = 3.36x10

^{-5}Nkg

^{-1}

and

g of Earth on its oceans = 9.81 Nkg

^{-1}

then I would do the following step and expect to get ~3 000 000 as the answer. ;

g

_{Earth}/ g

_{Moon}= ~ 3x10

^{6}

but 3.36x10

^{-5}/ 9.81 Nkg

^{-1}= 291 964.28 ~ 292 000 = 2.92x10

^{5}(or 0.292x10

^{6})

so g

_{Moon}on Earth's oceans is 0.3 millions times less than g

_{Earth}on its oceans and not 3 millions.

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Stonebridge

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#15

I've explained what the question is asking for and shown you how the calculation gives

I admit the wording of the question can be misinterpreted.

I'm certain that, in this question, the whole point being tested is that you

- know g on the Earth's surface due to the Earth is approx 10m/s (You did)

- calculate correctly the value of the Moon's field strength at the Earth (you did)

- realize this is a "ball park" figure. (do you?)

I'll say it again.

If the one is 3 x 10

So the point of the question was that you correctly get that value of approx. 3 x 10-5

Think

Let's say the first value was 3 and the second was 1,000,000

What fraction is this?

Multiply both top and bottom by 10

you get

3 x 10-5 and 10

What is this fraction?

This is the fraction you are working out.

The ratio of the Moon's field to that of the Earth at that point.

Maybe you are getting mixed up between "1 in 3 million" and "3 millionths"? These are not the same.

*the answer they want*.I admit the wording of the question can be misinterpreted.

I'm certain that, in this question, the whole point being tested is that you

- know g on the Earth's surface due to the Earth is approx 10m/s (You did)

- calculate correctly the value of the Moon's field strength at the Earth (you did)

- realize this is a "ball park" figure. (do you?)

I'll say it again.

If the one is 3 x 10

^{-5}(approx) and the other is 10 (approx) then the first one is**what the question has described as**"about 3 millionths the value of" the second. 3 x 10^{-6}So the point of the question was that you correctly get that value of approx. 3 x 10-5

Think

Let's say the first value was 3 and the second was 1,000,000

What fraction is this?

Spoiler:

3 millionths ie 3 / 1,000,000

Show

3 millionths ie 3 / 1,000,000

Multiply both top and bottom by 10

^{-5}you get

3 x 10-5 and 10

What is this fraction?

Spoiler:

It hasn't changed. The same

Show

It hasn't changed. The same

*fraction*as before.

This is the fraction you are working out.

The ratio of the Moon's field to that of the Earth at that point.

Maybe you are getting mixed up between "1 in 3 million" and "3 millionths"? These are not the same.

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Yorrap

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(Original post by

I've explained what the question is asking for and shown you how the calculation gives

I admit the wording of the question can be misinterpreted.

I'm certain that, in this question, the whole point being tested is that you

- know g on the Earth's surface due to the Earth is approx 10m/s (You did)

- calculate correctly the value of the Moon's field strength at the Earth (you did)

- realize this is a "ball park" figure. (do you?)

I'll say it again.

If the one is 3 x 10

So the point of the question was that you correctly get that value of approx. 3 x 10-5

Think

Let's say the first value was 3 and the second was 1,000,000

What fraction is this?

Multiply both top and bottom by 10

you get

3 x 10-5 and 10

What is this fraction?

This is the fraction you are working out.

The ratio of the Moon's field to that of the Earth at that point.

Maybe you are getting mixed up between "1 in 3 million" and "3 millionths"? These are not the same.

**Stonebridge**)I've explained what the question is asking for and shown you how the calculation gives

*the answer they want*.I admit the wording of the question can be misinterpreted.

I'm certain that, in this question, the whole point being tested is that you

- know g on the Earth's surface due to the Earth is approx 10m/s (You did)

- calculate correctly the value of the Moon's field strength at the Earth (you did)

- realize this is a "ball park" figure. (do you?)

I'll say it again.

If the one is 3 x 10

^{-5}(approx) and the other is 10 (approx) then the first one is**what the question has described as**"about 3 millionths the value of" the second. 3 x 10^{-6}So the point of the question was that you correctly get that value of approx. 3 x 10-5

Think

Let's say the first value was 3 and the second was 1,000,000

What fraction is this?

Spoiler:

3 millionths ie 3 / 1,000,000

Show

3 millionths ie 3 / 1,000,000

Multiply both top and bottom by 10

^{-5}you get

3 x 10-5 and 10

What is this fraction?

Spoiler:

It hasn't changed. The same

Show

It hasn't changed. The same

*fraction*as before.

This is the fraction you are working out.

The ratio of the Moon's field to that of the Earth at that point.

Maybe you are getting mixed up between "1 in 3 million" and "3 millionths"? These are not the same.

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JessyJoy

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#17

Moon's pull on oceans:

g=(6.67x10^-11)(7.35x10^22)/(380000x10^3)^2

g=3.4x10^-5

Earth's pull on oceans:

g=(6.67x10^-11)(5.98x10^24)/(6.37x10^6)^2

g=9.8

3.4x10^-5/9.8=3.5x10^-6

Which means 3.4x10^-5 is approximately 3 millionths of 9.8 because 3 millionths = 3/1x10^6 = 3x10^-6

g=(6.67x10^-11)(7.35x10^22)/(380000x10^3)^2

g=3.4x10^-5

Earth's pull on oceans:

g=(6.67x10^-11)(5.98x10^24)/(6.37x10^6)^2

g=9.8

3.4x10^-5/9.8=3.5x10^-6

Which means 3.4x10^-5 is approximately 3 millionths of 9.8 because 3 millionths = 3/1x10^6 = 3x10^-6

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CurrentDude

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(Original post by

Moon's pull on oceans:

g=(6.67x10^-11)(7.35x10^22)/(380000x10^3)^2

g=3.4x10^-5

Earth's pull on oceans:

g=(6.67x10^-11)(5.98x10^24)/(6.37x10^6)^2

g=9.8

3.4x10^-5/9.8=3.5x10^-6

Which means 3.4x10^-5 is approximately 3 millionths of 9.8 because 3 millionths = 3/1x10^6 = 3x10^-6

**JessyJoy**)Moon's pull on oceans:

g=(6.67x10^-11)(7.35x10^22)/(380000x10^3)^2

g=3.4x10^-5

Earth's pull on oceans:

g=(6.67x10^-11)(5.98x10^24)/(6.37x10^6)^2

g=9.8

3.4x10^-5/9.8=3.5x10^-6

Which means 3.4x10^-5 is approximately 3 millionths of 9.8 because 3 millionths = 3/1x10^6 = 3x10^-6

Instead of saying "Moon's pull on ocean" I'd use, Gravitation field strength of the moon at 38x10^7 is

Moon's g = 6,67x10^-11xmass of moon / (38x10^7)^2 and then follow the steps you've used.

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Absent Agent

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(Original post by

eassssyyyyyyyyy u serious u look dumb

**zzxxDash53xxzz**)eassssyyyyyyyyy u serious u look dumb

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