Conservation of momentum and apple falling

In summary: The momentum of the apple falling to the ground is not conserved because the external force of gravity acts on it. But the momentum is conserved in a larger system. The larger system is composed of the apple and the Earth, and there is no external force applied to this system. The momentum of the apple in this larger system is not conserved because the Earth also moves in response to the gravitational force, but the ratio of their movements is proportional to their masses. This means that while the momentum of the apple may change, the total momentum of the system remains constant. This is known as the conservation of momentum.
  • #1
evansbluefan
1
0
The momentum of an apple falling to the ground is not conserved because the external force of gravity acts on it. But the momentum is conserved in a larger system. Explain?

I am confused because gravity is the external force acting upward, so what's the force that is acting up on the apple? Is it air resistence? Or does it have to do with some kind of energy. I am really confused as to what the larger system is. Can someone help me figure this out? Thanks.
 
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  • #2
evansbluefan said:
what the larger system is.
The larger system = ( An apple + the Earth )
 
  • #3
there is no external force applied to "the larger system"
when u think of the apple alone, Earth applies the gravitational force, that's why momentum of the apple is not conserved.
 
  • #4
evansbluefan said:
The momentum of an apple falling to the ground is not conserved because the external force of gravity acts on it. But the momentum is conserved in a larger system. Explain?

I am confused because gravity is the external force acting upward, so what's the force that is acting up on the apple? Is it air resistence? Or does it have to do with some kind of energy. I am really confused as to what the larger system is. Can someone help me figure this out? Thanks.
As an aside all momentum within the apple is conserved! This would be the momentum the apple held before it fell...
 
  • #5
basePARTICLE said:
As an aside all momentum within the apple is conserved! This would be the momentum the apple held before it fell...
What's that supposed to mean? What's the momentum "within the apple"? If you mean the momentum of the apple in a frame in which it is not moving... well, that momentum is zero, of course.
 
  • #6
evansbluefan said:
The momentum of an apple falling to the ground is not conserved because the external force of gravity acts on it. But the momentum is conserved in a larger system. Explain?

I am confused because gravity is the external force acting upward, so what's the force that is acting up on the apple? Is it air resistence? Or does it have to do with some kind of energy. I am really confused as to what the larger system is. Can someone help me figure this out? Thanks.
If you want to be more accurate, both the Earth and the apple move as a result of the equal and opposite gravitational forces acting on them. The Earth moves up and the apple moves down, it's just that the ratio of the distance the Earth moves to the distance the apple moves is the same as the ratio of the apple's mass to the Earth's mass (this is all in the center-of-mass reference frame for the Earth + apple system). In other words, you'd be hard-pressed to observe the motion of the Earth as it flies up towards the apple, but that's how the total momentum is conserved.
 
  • #7
evansbluefan said:
I am confused because gravity is the external force acting upward, so what's the force that is acting up on the apple?

Think about it from a Newton's 3rd Law point of view. If the apple has a force downwards by gravity, then the equal and opposite force is the Earth going upwards.
 
  • #8
Doc Al said:
What's that supposed to mean? What's the momentum "within the apple"? If you mean the momentum of the apple in a frame in which it is not moving... well, that momentum is zero, of course.
Simply the summation of all momenta of the apple's atomic structure. I have realized that green apples are not the same as ripening apples simply because of the changed atomic structure. It would be this changed atomic structure especially of the stem connecting the apple to the branch of an apple tree that has undergone enough change to allow its strain and stress vectors to become equivalent to mg (mass of apple * acceleration due to gravity) as the apple starts to break away. Smart people prefer planting apple trees on slopes. :wink:
 
  • #9
basePARTICLE said:
Simply the summation of all momenta of the apple's atomic structure.
I assume you mean: The summation of the momenta of each particle of the apple. Again, you must specify the frame in which you are evaluating the momentum. If the center of mass of the apple is at rest, then--trivially--the sum of the momenta of apple "particles" is zero. But if the center of mass is moving at speed v--as the apple falls--then the sum of the momenta is mv, where m is the total mass of the the apple.
 
  • #10
basePARTICLE said:
Simply the summation of all momenta of the apple's atomic structure. I have realized that green apples are not the same as ripening apples simply because of the changed atomic structure. It would be this changed atomic structure especially of the stem connecting the apple to the branch of an apple tree that has undergone enough change to allow its strain and stress vectors to become equivalent to mg (mass of apple * acceleration due to gravity) as the apple starts to break away. Smart people prefer planting apple trees on slopes. :wink:
It's good that the OP seems to have gone away, because I don't see how this post helps to answer his question at all.
 

Related to Conservation of momentum and apple falling

1. What is conservation of momentum?

Conservation of momentum is a fundamental principle in physics that states that the total momentum of a closed system remains constant, regardless of any external forces acting on the system. This means that in a collision or interaction between objects, the total momentum before the interaction is equal to the total momentum after the interaction.

2. How does conservation of momentum apply to an apple falling from a tree?

When an apple falls from a tree, it is subject to the force of gravity pulling it towards the ground. As it falls, its momentum increases due to its acceleration. However, according to the principle of conservation of momentum, the total momentum of the apple and the Earth must remain constant. This means that as the apple's momentum increases, the Earth's momentum must decrease by an equal amount.

3. Why does an apple falling from a tree not violate conservation of momentum?

Although it may seem like the apple's momentum is changing while it falls, the total momentum of the apple and Earth is still constant. This is because the Earth's mass is so much greater than the apple's, that the change in the Earth's momentum is too small to be noticeable. Additionally, the force of gravity on the apple is an external force, not an internal force, so it does not affect the total momentum of the system.

4. Does conservation of momentum only apply to objects falling on Earth?

No, conservation of momentum applies to all closed systems, regardless of location or objects involved. This principle is a fundamental law of physics and is applicable in all situations where there are no external forces acting on the system.

5. How does conservation of momentum relate to Newton's third law of motion?

Conservation of momentum and Newton's third law of motion are closely related. Newton's third law states that for every action, there is an equal and opposite reaction. This means that when two objects interact, their forces on each other are equal and in opposite directions. Conservation of momentum is a result of this law, as the equal and opposite forces between the two objects result in a conservation of the total momentum of the system.

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