Finding Gravitational Force b/w 2 People I just need some clarification. The question is:

What is the gravitational force between two 100 kg people standing 1 m apart, and how does this compare with the gravitational force of either of them relative to Earth?

I'm guessing the formula to use here is Newton's Law of Gravitation. So, I'd use $M_1, M_2$ = 100 kg, and use $1$m as $r$, right? And to find one of the people's force relative to the Earth, what value do I use for $r$? Isn't the distance between the Earth and any person almost negligible? Thank you.
 A: For a spherical object the gravitational field is the same as if all the mass was at the centre of the object. This is known as Newton's shell theorem. So when we talk about the distance $r$ from the Earth in the law of gravitation we mean the distance from the centre of the Earth.
Strictly speaking this isn't true since the Earth is an oblate spheroid not a sphere, but in practice the Earth is close enough to spherical for this to be an excellent approximation.
For two people around $2$m tall and $1$m apart the assumption that the people are spherical is obviously a poor one and will not give the correct gravitational field. However doing this calculation properly is something you aren't taught until you start doing a physics degree, so I would guess that you are also supposed to assume the people are spherical and use the distance from their centre of mass. This will give you an order of magnitude estimate of the force, which is probably close enough to show how small it is compared to the force between the people and the Earth.
A: $$F = \frac{G m_1 m_2}{ r^2}$$
where  $r$  is the distance between the center of gravity of the individuals.
$$
F = \frac{6.67\cdot10^{-11} \text N \cdot { \text m^2/ \text k\text g^2}\times 100~\text k \text g \times 100~\text k\text g}{1~\text m^2}=6.67\cdot10^{-7} \text N
$$
The gravitation force of one of them with the earth would be about 1000 N.  
