# Momentum conservation and Newton's 3rd

• mrcotton
In summary, a soldier holding a machine gun will experience a force from the gun while it is firing bullets due to the conservation of momentum in closed systems. This force is equal and opposite to the force of the gun on the soldier, as stated by Newton's Third Law. While it is possible to explain the force with the conservation of momentum, it is important to fully demonstrate an understanding of the physics involved in this scenario.
mrcotton

## Homework Statement

Explain why a soldier holding the machine gun will experience a force from the gun
whilst it is firing bullets

## Homework Equations

Momentum is always conserved in closed systems

F= Δmv/Δt

## The Attempt at a Solution

The system has zero momentum then when the bullet is fired we must have an equal but opposite momentum.

The force on the gun is the bullets change in momentum divided by the time that the momentum change occurs in.

So why can't we just say according to Newtons 3rd that there must be an equal and opposite force of the gun on the soldier?

Any help appreciated
D

What makes you think that we can't say that?

I think you'd need to be more careful than that - this is an exercize given as part of your education so you have to demonstrate that you understand the physics involved.

So - there is a force of the gun on the soldier - what is it equal and opposite to?
(you have to actually say...)

The conservation of momentum answer is just quicker.
(though you didn't complete that answer either...)

Wite out both complete answers side by side and see which one you prefer.

Anyway - you may happen to know that the question is part of an exam set on "conservation of momentum" - just like you know you are expected to do the problem in Newtonian mechanics without having to be told ;)

## 1. What is momentum conservation?

Momentum conservation is a fundamental principle in physics that states that the total momentum of a closed system remains constant over time, regardless of any internal or external forces acting on the system.

## 2. How does momentum conservation relate to Newton's 3rd law?

Newton's 3rd law states that for every action, there is an equal and opposite reaction. This means that when two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. Momentum conservation is a consequence of this law, as the momentum of the two objects before and after the interaction must be equal and opposite to satisfy Newton's 3rd law.

## 3. Can momentum be created or destroyed?

No, momentum cannot be created or destroyed. This is known as the law of conservation of momentum. In any closed system, the total momentum must remain constant, even if there are internal forces acting on the system.

## 4. How is momentum calculated?

Momentum is calculated by multiplying an object's mass by its velocity. The formula for momentum is: p = mv, where p is momentum, m is mass, and v is velocity. Momentum is a vector quantity, meaning it has both magnitude and direction.

## 5. How is momentum conserved in collisions?

In collisions, the total momentum of the system before the collision must be equal to the total momentum after the collision. This means that if one object gains momentum, another object must lose an equal amount of momentum in the opposite direction to conserve the total momentum of the system.

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