Homework Help: Gravitational Force

1. Feb 26, 2005

mark9159

hey...heres a quesiton thats been bugging me

"The earth and the moon attract each other with a gravitational force. How does the force acting on the more massive earth compare to the force acting on the moon?"

Choices: a.) there is a greater force acting on the earth. b.)the same force is acting on both the earth and the moon. c.)there is a greater force acting on the moon. d.) not enough information exists to determine the force.

my answer: the same force is acting on both the earth and the moon.

but im not sure..according to Newton's Third Law of Motion, the gravitational forces on each object are equal. Gravitational forces act between any two objects and always pull one towards the other.

buut..gravitational force depends on the mass of each object and how far apart they are...so im confused...is there more force acting on the earth or is it equal?

2. Feb 26, 2005

misskitty

Thats an interesting questions to think about...nice change from the Thermal stuff I'm doing.

Anyway, you're right by looking at Newton's third law. But I don't think that the forces would be the same even though the law states that the magnitudes of the two objects acting on one another are equal. I think the key might be in that the magnitudes of force are equal and OPPOSITE.

To determind which force is greater did you try to work it out mathermatically by solving for the force of the moon and the force of the earth and then comparing the two?

MissKitty

3. Feb 26, 2005

arildno

The gravitational attraction between two objects depends on the PRODUCT of the two objects' masses, not on either mass singly.

The magnitudes of the two forces are then equal, the directions of the forces opposite to each other, according to Newton's 3.law.

Last edited: Feb 26, 2005
4. Feb 26, 2005

misskitty

I did say that the magnitudes were EQUAL and OPPOSITE according to Newton's third law. But that's not the question. The question is how does the force acting on the Earth compare to the force acting on the Moon?

5. Feb 26, 2005

arildno

A magnitude is a positive number.

6. Feb 26, 2005

misskitty

Not necessarily...It doesn't have to be a positve number.

7. Feb 26, 2005

Kerbox

yes it does... that is what magnitude means

8. Feb 26, 2005

arildno

Either it's zero, or it's positive (i.e, non-negative).
The concept of "opposite magnitude" is meaningless.

9. Feb 26, 2005

misskitty

Ok fine, but thats still not the question. How do the forces compare?

10. Feb 26, 2005

arildno

I've already answered it.

11. Feb 26, 2005

HallsofIvy

You have been repeatedly told that the magnitudes of the forces are the same- the directions are opposite. Why do you not believe it?

The gravitational force of one object on another depends on the PRODUCT of the two masses. That's why the magnitudes are the same.

12. Feb 26, 2005

VietDao29

Hi,
$$F_{g} = G\frac{m_{1}m_{2}}{r^{2}}$$
So the force acting on both moon and earth is the same. They both attract each other.
Viet Dao,

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