# Homework Help: Conservation of momentum and apple falling

1. Jan 18, 2008

### evansbluefan

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.

2. Jan 18, 2008

### jdg812

The larger system = ( An apple + the Earth )

3. Jan 19, 2008

### esalihm

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. Jan 19, 2008

### basePARTICLE

As an aside all momentum within the apple is conserved! This would be the momentum the apple held before it fell...

5. Jan 19, 2008

### Staff: Mentor

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. Jan 19, 2008

### belliott4488

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. Jan 19, 2008

### awvvu

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. Jan 20, 2008

### basePARTICLE

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.

9. Jan 20, 2008

### Staff: Mentor

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. Jan 20, 2008

### belliott4488

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.