What is with the law of conservation of momentum?

In summary, in a scenario where someone stands on the surface of the Earth and throws punches into the air, all the forces acting on the system of man and Earth are internal, but the momentum of the person's arm is constantly changing. This may seem like a violation of the law of conservation of momentum, but it is not because the momentum of the rest of the person's body and the Earth are also taken into account. The total momentum of the person-Earth system is conserved, not just the momentum of the arm alone. This is due to Newton's 3rd law, where for every action there is an equal and opposite reaction. The Earth may seem at rest, but it actually moves slightly in response to the force of
  • #1
siddharth5129
94
3
Consider a scenario where someone stands on the surface of the Earth and throws punches into the air. Now all the forces(which i am taking to be the tension in the muscle fibres of the arm ) acting on the system of man and Earth are internal ( internal because they are exerted by one component within the system on another component within the system.) , but the momentum of this dude's arm is constantly changing. Anyone else see a violation of the law of conservation of momentum. What am i missing here?
 
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  • #2
siddharth5129 said:
but the momentum of this dude's arm is constantly changing. Anyone else see a violation of the law of conservation of momentum. What am i missing here?
You're forgetting about the momentum of the rest of his body and the Earth itself. Total momentum of the person-earth system is conserved, not the momentum of his arm alone.
 
  • #3
Yes but the rest of his body and the Earth are at rest , aren't they?
 
  • #4
siddharth5129 said:
Yes but the rest of his body and the Earth are at rest , aren't they?
No. The Earth moves, ever so slightly.
 
  • #5
How exactly , there aren't any forces on the earth.
 
  • #6
siddharth5129 said:
How exactly , there aren't any forces on the earth.
Sure there are. If you push on the arm to give it momentum, the arm pushes back on you (and the earth) giving you and the Earth equal and opposite momentum. That's Newton's 3rd law.
 
  • #7
Try the same thing while standing on a skateboard and you'll see the motion.
 

1. What is the law of conservation of momentum?

The law of conservation of momentum is a fundamental principle in physics that states that the total momentum of a closed system remains constant. This means that in the absence of external forces, the total momentum of the system before an event is equal to the total momentum after the event.

2. Why is the law of conservation of momentum important?

The law of conservation of momentum is important because it helps us understand and predict the behavior of objects in motion. It is used in many areas of physics, such as mechanics, thermodynamics, and fluid dynamics, and is a crucial concept in understanding the laws of motion.

3. How is the law of conservation of momentum related to Newton's third law of motion?

The law of conservation of momentum is closely related to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. This means that when two objects interact, the total momentum of the system remains constant, as the forces between the objects are equal and opposite.

4. Can the law of conservation of momentum be violated?

No, the law of conservation of momentum is a fundamental principle in physics and has been proven to hold true in all physical interactions. It is a law of nature and cannot be violated.

5. How is the law of conservation of momentum used in real-life applications?

The law of conservation of momentum has many practical applications, such as in rocket propulsion, car collisions, and sports. It is also used in the design of machines and structures, such as cranes and bridges, to ensure their stability and safety.

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