# Homework Help: 2 rods' interaction

1. Jun 4, 2009

### peeyush_ali

There are 2 identical rods of mass "m" and length "l" separated by a distance "l" in such a way that the 2 rods are in a straight line. Find the Gravitational force between these two rods.

2. Relevant equations

3. The attempt at a solution

I'm unable to analyse a calculus based strategy such as chosing the small elemnt of mass "dm" length "dx" and so on..

2. Jun 4, 2009

### Staff: Mentor

Just use their centers of mass, and the gravitational force equation. Write that out for us.

3. Jun 4, 2009

### peeyush_ali

so u mean to say that the interaction is nothing but...

F= Gm.m/4 l.l ??

4. Jun 4, 2009

### Staff: Mentor

Almost. What is the separation of the centers of mass in terms of l?

5. Jun 4, 2009

### peeyush_ali

the situation is like this :

|<___________________>|
_____l_____ (l+l/2+l/2) _____l_____

the separation between the centres of the 2 rods is l+l/2 +l/2 =2l

6. Jun 5, 2009

### Staff: Mentor

Two rods of length L have their centers of mass separated by L/2 or 2L? Why? Not.

7. Jun 5, 2009

### peeyush_ali

the centres are separated by a lenght of 2l

8. Jun 5, 2009

### Staff: Mentor

If each rod is length L, then I don't think so. Just use a practical example. What if each rod is 1 meter long, and you stack them end-to-end. How far apart are the centers of the rods?