Diagram of forces exerted on objects

Assuming that there is an interaction between 1 and 3 (they attract each other), what are the forces between 1 and 2?

I know it is as if the force acts on a different body (1+2), but I want to know the exact forces between them.

• What does your last line mean? Sep 16, 2014 at 14:47
• I can consider 1 and 2 to be a single object with mass = mass of 1 + mass of 2. But it is not what I want. Sep 16, 2014 at 14:49
• Welcome to Physics Stack Exchange. Please review our policy on homework-like questions Sep 16, 2014 at 15:03
• It is not homework. I thought of it this way: because both of the objects should be accelerating at F/(m1+m2), the normal force between 1 and 2 should be $$\frac{F m_2}{m_1+m_2}$$. Is this correct? Sep 16, 2014 at 15:08

Your body 1 is in equilibrium.It has two forces on it -the normal force due to 2 and F due to 3.They must be equal.

So force between 1 and 2

=normal force on 1 due to 2

=normal force on 2 due to 1

=F.

EDIT

Both bodies are moving with acceleration $\frac F {m_1+m_2}$.

Now let Normal force be N.So for 1,

$$F-N=\frac {m_1F} {m_1+m_2}$$

$$N=\frac {m_2F}{m_1+m_2}$$

• Sorry, maybe my picture is misleading. Consider object 3 being the Earth, object 1 being small and object 2 being a little bigger. So bodies 1 and 2 are accelerating. Sep 16, 2014 at 14:57
• If 3 is earth both 1 and 2 will undergo acceleration g and so normal force between them would be 0. Sep 16, 2014 at 15:06
• If your question comes from a book just state the exact language with the diagram. Sep 16, 2014 at 15:07
• It does not come from a book. I compared 3 to earth just because of the mass. I thought of a better analogy. Consider 3 being a positively charged particle with considerable mass (m3>>m2>m1), 2 does not interact with 3 and 1 is a negatively charged particle. Sep 16, 2014 at 15:15
• Now i get the picture. Sep 16, 2014 at 15:34

There is a normal force on 2 due to 1, and a normal force on 1 due to 2.

• And what are they equal to? Sep 16, 2014 at 14:52