# Conservation of Momentum of forces

I know that momentum is conserved if there are no external forces.
But, can you tell me what external forces are ?
The site http://www.physicsclassroom.com/class/energy/Lesson-2/Internal-vs-External-Forces says that internal forces are force of gravity and force of spring.

But, I think that force of gravity causes acceleration that makes the momentum increases, so it is not conserved.
Dropping a ball from some height causes the ball accelerate and momentum changes.

So, what are external forces and internal forces ?
Are force of spring and gravity external forces ?

## Answers and Replies

Orodruin
Staff Emeritus
Science Advisor
Homework Helper
Gold Member
What is an internal or external force depends on the system you are considering. According to Newton's third law, forces come in pairs and an external force is a force from an object outside your system on an object inside your system such that the opposite force is acting on the object outside the system and therefore can be disregarded when you do the analysis of the system itself (you do still need to consider the original force).

For example, if you study a ball falling in a gravitational field due to a planet, there is a gravitational force on it from the planet. There also a force of gravity from the ball on the planet but the planet is not part of the system you are looking at (which is the ball). Thus, in this scenario, the gravitational force on the ball is an external force.

On the contrary, if you consider the Earth-Moon system as a whole (not just the Earth or just the Moon) the gravitational force between them is an internal force as both forces act on objects in the system. However, the gravity from the Sun on the Earth-Moon system is an external force as the Sun was not part of the system you were considering.

• terryds