Momentum conservation - falling object

In summary, when considering momentum conservation, it is important to clearly define the system being considered. If the system includes only the falling object and the Earth, then momentum is conserved. However, if the system also includes the air, then external forces such as air resistance may affect the overall momentum. It is important to specify the system in any question regarding momentum conservation.
  • #1
jsmith613
614
0
If an object falls there are two ways to consider momentum conservation

Way 1:
The system involves just the object therefore the gravitational force is an EXTERNAL force so momentum is NOT conserved

Way 2:
The system involves object and earth. The increase in momentum of the object down = increase in momentum of Earth up (therefore momentum is conserved)

Is this correct?
 
Physics news on Phys.org
  • #2
Perfectly correct.
 
  • #3
Doc Al said:
Perfectly correct.

ok so when we have a falling ball that reaches terminal velocity would I be correct in thinking momentum is NOT conserved because of the EXTERNAL force of air resistance?
 
  • #4
jsmith613 said:
ok so when we have a falling ball that reaches terminal velocity would I be correct in thinking momentum is NOT conserved because of the EXTERNAL force of air resistance?
Not sure what you mean. If something falling has reached terminal velocity, that means the net force on it is zero. It's moving at constant velocity. The momentum's not changing.

There are two external forces acting, which cancel each other: gravity and air resistance.
 
  • #5
Doc Al said:
Not sure what you mean. If something falling has reached terminal velocity, that means the net force on it is zero. It's moving at constant velocity. The momentum's not changing.

There are two external forces acting, which cancel each other: gravity and air resistance.

ok then before terminal velocity the momentum is falling because of air resistance...in this case as it APPROACHES terminal velocity, momentum is NOT conserved because of the external force AIR resistance..is that more correct?
 
  • #6
jsmith613 said:
ok then before terminal velocity the momentum is falling because of air resistance...in this case as it APPROACHES terminal velocity, momentum is NOT conserved because of the external force AIR resistance..is that more correct?
Momentum is not conserved because there's a net force on the body, even without air resistance. Gravity is an external force also.
 
  • #7
Doc Al said:
Momentum is not conserved because there's a net force on the body, even without air resistance. Gravity is an external force also.

but i though we said that from "way 2" gravity is not an external force because we can consider the Earth and ball as one system...that;s why momentum is conserved in way 2??
 
  • #8
jsmith613 said:
but i though we said that from "way 2" gravity is not an external force because we can consider the Earth and ball as one system...that;s why momentum is conserved in way 2??

In your case of terminal velocity, there is an external force of the air on the falling object. In the 2nd scenario, there was no air resistance therefore no external force. IF you consider the object, Earth (gravity), and air (resistance) as a single system, momentum is conserved as the object is impacting the molecules in the air which push back on the object.
 
  • #9
jsmith613 said:
but i though we said that from "way 2" gravity is not an external force because we can consider the Earth and ball as one system...that;s why momentum is conserved in way 2??
I didn't realize you were talking about 'way 2'. In any case, why is the air not considered as part of the earth?
 
  • #10
Doc Al said:
I didn't realize you were talking about 'way 2'. In any case, why is the air not considered as part of the earth?

I believe he meant the Earth as something that produces gravity. The atmosphere is usually not considered when thinking of gravity.
 
  • #11
Doc Al said:
I didn't realize you were talking about 'way 2'. In any case, why is the air not considered as part of the earth?

ok so your saying that using "Way 2" momentum is ALWAYS conserved (regardless of air-resistance / gravity) for falling objects
 
  • #12
jsmith613 said:
ok so your saying that using "Way 2" momentum is ALWAYS conserved (regardless of air-resistance / gravity) for falling objects
It depends on what you are consider as your system.

If your system is ball + Earth (including air), then there are no external forces and total momentum is conserved.

But if your system is ball + Earth (excluding air), then there are external forces and total momentum of that system is not necessarily conserved.
 
  • #13
Doc Al said:
It depends on what you are consider as your system.

If your system is ball + Earth (including air), then there are no external forces and total momentum is conserved.

But if your system is ball + Earth (excluding air), then there are external forces and total momentum of that system is not necessarily conserved.

so I doubt I would be asked a question like this in an exam UNLESS it is clearly stated what is and what is not part of the system...right?

e.g: system is Earth and ball ONLY means air is an external force...
 
  • #14
jsmith613 said:
so I doubt I would be asked a question like this in an exam UNLESS it is clearly stated what is and what is not part of the system...right?
Right. If they ask whether momentum is conserved they must specify the system they are considering, if there is any possibility of confusion.
 
  • #15
Doc Al said:
Right. If they ask whether momentum is conserved they must specify the system they are considering, if there is any possibility of confusion.

thanks a lot for you help :)
 
  • #16
If you consider the momentum of the Earth and air and ball together, it will be conserved. If something if falling in air, then it's pushing some parcels of air down with it.
 
  • #17
A concise way to say this is that momentum is always conserved in a closed system, which just means including everything that is exerting forces on each other as part of the same system. It's all how you regard the system, and it should be clear in any question, but often there is value in breaking up the system into parts. When you do that, the concept of conservation of momentum means you are only "moving momentum around" from one part to another, just like with energy conservation (except bear in mind that momentum has a direction and energy doesn't).
 

1. What is momentum conservation in relation to a falling object?

Momentum conservation is a fundamental principle in physics that states that the total momentum of a system remains constant, as long as there are no external forces acting on the system. In the case of a falling object, the momentum of the object is conserved as it falls due to the absence of external forces like air resistance.

2. How is momentum conserved in a falling object?

Momentum is conserved in a falling object because the force of gravity is the only force acting on the object, causing it to accelerate. As the object falls, its velocity and momentum increase, but the total momentum of the object and Earth remains constant.

3. What happens to the momentum of a falling object when it reaches terminal velocity?

When a falling object reaches terminal velocity, the force of gravity is balanced by the force of air resistance. This means that the object's acceleration and velocity become constant, and its momentum also remains constant. The object's momentum is conserved even though its speed is no longer increasing.

4. How does momentum conservation impact the motion of a falling object?

Momentum conservation impacts the motion of a falling object by ensuring that the object's momentum remains constant as it falls. This means that the object will continue to accelerate until it reaches terminal velocity, at which point its momentum will remain constant as it falls at a constant speed.

5. Is momentum conservation always applicable to falling objects?

Yes, momentum conservation is always applicable to falling objects as long as there are no external forces acting on the object. In real-world scenarios, there may be other factors at play such as air resistance, but as long as these external forces are negligible, momentum conservation will hold true for a falling object.

Similar threads

Replies
4
Views
818
Replies
6
Views
699
Replies
3
Views
1K
Replies
52
Views
2K
Replies
16
Views
1K
Replies
5
Views
934
Replies
30
Views
1K
  • Mechanics
2
Replies
53
Views
2K
Replies
11
Views
1K
Replies
12
Views
783
Back
Top