# Conservation of Momentum of forces

• terryds
In summary, external forces are forces that come from outside the system being studied and can be disregarded in the analysis of the system. Internal forces are forces between objects within the system. Examples of external forces include the force of gravity from an object outside the system, while examples of internal forces include the force of gravity between objects within the system.

#### terryds

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 ?

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

## 1. What is the law of conservation of momentum?

The law of conservation of momentum states that the total momentum of a closed system remains constant over time, regardless of any external forces acting on the system. This means that the total momentum before a collision or interaction is equal to the total momentum after the collision or interaction.

## 2. How is momentum defined and calculated?

Momentum is defined as the product of an object's mass and velocity. It is calculated by multiplying an object's mass (in kilograms) by its velocity (in meters per second).

## 3. Can momentum be lost or gained?

No, according to the law of conservation of momentum, the total momentum of a closed system remains constant. This means that momentum cannot be lost or gained, it can only be transferred or redistributed between objects within the system.

## 4. Does the conservation of momentum apply to all types of forces?

Yes, the law of conservation of momentum applies to all types of forces, including gravitational, electromagnetic, and nuclear forces. As long as the system is closed and there are no external forces acting on it, the total momentum will remain constant.

## 5. What are some real-life examples of the conservation of momentum?

One example of the conservation of momentum is a billiard game, where the momentum of the cue ball is transferred to the other balls upon impact. Another example is a rocket launching into space, where the momentum of the fuel and gases being expelled propels the rocket forward. Additionally, car collisions and sports such as ice skating and diving also demonstrate the conservation of momentum in action.