A certain system consists of a number of interacting objects. Our Solar System is an example. What is the sum of these internal forces?

Question

1. A certain system consists of a number of interacting objects. Our Solar System is an example. What is the sum of these internal forces?
2. Can these internal forces cause the system to accelerate?

I am NOT understanding this. How do I find the sum of the internal forces of our solar system?

Answer 1

Consider a simplified problem of three masses interacting by gravitational forces.

Bode 1 exerts a force on body 2 and body 2 exerts a force on body 1. The forces on each body are equal and opposite in direction so they cancel each other.

Similarly, body 1 exerts a force on body 3 and body 3 exerts an equal and opposite force on body 1 so they again cancel.

This holds for body 2 and body 3. This means the internal forces cancel in pairs, and so there is no net force on the system. This means that there will be no acceleration of the system.

This reasoning can be applied to the solar system.