How a net force acts on 'Earth'?

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Discussion Overview

The discussion revolves around the application of Newton's laws of motion, particularly the concepts of net force and action-reaction pairs, in the context of forces acting on objects on Earth. Participants explore how these principles apply when objects interact with air, questioning the balance of forces and the resulting motion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that when a force is applied to an object, the air exerts an equal force back, leading to confusion about net force and acceleration.
  • Others clarify that the net force is calculated by considering the applied force minus the opposing force from the air, suggesting that if there is a net force, the object will accelerate.
  • A participant questions how an object can be balanced by equal forces from both sides, using examples of forces applied to air particles.
  • Some participants highlight that the drag force from air is dependent on the speed of the object, indicating that it may not always equal the applied force initially.
  • There are discussions about the implications of the third law of motion, emphasizing that the forces between the object and air do not necessarily equate to the forces applied by the user.
  • One participant mentions that when an object is thrown, it does not continuously apply force, which affects the net force and acceleration.
  • Another participant suggests that vector diagrams could help clarify the understanding of forces involved.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of net force and action-reaction pairs, with no consensus reached on how these concepts apply in the context of objects moving through air. The discussion remains unresolved regarding the balance of forces and the conditions under which acceleration occurs.

Contextual Notes

Limitations include the dependence on specific scenarios, such as the speed of the object and the nature of the forces involved, which are not fully explored. The discussion also highlights the complexity of applying Newton's laws in real-world situations.

  • #31
CWatters said:
See the reply by Jbriggs.

You are trying to apply what I said to a totally different situation. I assumed the two blocks start out in contact and a force is applied to one of them causing the pair to accelerate. No collision involved.

The situation you describe is quite different. For example what happens after the collision depends on the what the objects are made of. eg super bouncy balls behave differently to lumps of sticky clay. Usually you can easily work out what happens if they are one or the other. If they are somewhere in between then it can be a bit harder.

There are various possible outcomes to the situation you describe..

M1 can stick to M2 and they carry on together (eg bullet into a block of wood).
M1 can bounce off M2 and then they both continue in the same direction (Bowling ball and skittles)
M1 can bounce off M2 and return the way it came, while M2 accelerates away (Ball bouncing on the ground, M2 is the earth)
M1 stops and M2 continues (Two ball Newtons cradle).

It can be tricky to work out the forces involved during a collision so we frequently use other tricks to work out what happens to each object. Sometimes you can apply conservation of energy to the problem (total energy before = total energy after) sometimes not. You can always apply conservation of momentum.
Okay...Can we just take it simple as to 'simple' objects...( no bouncy or rubbery stuff)...What i meant to say was...Like if a force of 10N is applied to an object and it continues in a straight line(of course after accelerating from rest)...And then it collides with the second object...Now if the first object applies a force of 10N to the second object then there is a reaction of 10N force on first object...Net force on first object becomes 0 and so it comes in rest or uniform velocity...But the same case is not observed with the object-air molecule relation...As you said so that the force on object is not neccesarily the force it applies to air molecule so that's how there is a net force on object...But if the reaction force on the first object is lesser than 10N than it should decelerate but still continue its motion...So it is really important for me to know that whether the reactional force in above scenario will be 10N or lesser than 10N?(Regardless of the second object's mass)
 
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  • #32
Kaneki123 said:
Okay...Can we just take it simple as to 'simple' objects...( no bouncy or rubbery stuff).
This does not bode well...
Now if the first object applies a force of 10N to the second object then there is a reaction of 10N force on first object...Net force on first object becomes 0 and so it comes in rest or uniform velocity...
How long is that 10N force applied? If we have simple objects which are perfectly rigid then the answer is... No time at all.
The resulting momentum change is... Zero.
The required mass of the object in order to change its state of motion in zero time with zero momentum change is... Zero.

Normally we do not talk about the force of impact. What we talk about instead is the quantity of momentum that is exchanged. This is the "impulse" associated with the collision. This eliminates the need to know how rigid the objects are..

So let's say that we have a head-on collision with an impulse of 10 kg m/sec. If the initially-moving object has a momentum equal to this, it stops. If it has a momentum greater than this, it keeps going. If it has a momentum less than this, it rebounds.
 
  • #33
Kaneki123 said:
...if a force of 10N is applied to an object and it continues in a straight line(of course after accelerating from rest)...And then it collides with the second object...Now if the first object applies a force of 10N to the second object then there is a reaction of 10N force on first object...Net force on first object becomes 0...
This isn't how applied forces and collisions work. The fact that a force of 10N is applied to accelerate the object at first does not imply it applies a force of 10N on the second object. Nor do forces applied at different times (10N at first to accelerate the first object, 10N later when they collide) cancel each other out.
 
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  • #34
russ_watters said:
This isn't how applied forces and collisions work. The fact that a force of 10N is applied to accelerate the object at first does not imply it applies a force of 10N on the second object. Nor do forces applied at different times (10N at first to accelerate the first object, 10N later when they collide) cancel each other out.
You said''The fact that a force of 10N is applied to accelerate the object at first does not imply it applies a force of 10N on the second object''...So is this a fact that the the force applied on first object is not neccessarily equal to the force it applies to the second object?...Because if it is not equal then ofcourse there would be a net force on first object...
 
  • #35
Kaneki123 said:
You said''The fact that a force of 10N is applied to accelerate the object at first does not imply it applies a force of 10N on the second object''...So is this a fact that the the force applied on first object is not neccessarily equal to the force it applies to the second object?...Because if it is not equal then ofcourse there would be a net force on first object...

Right, there would be a net force on the first object. That's how an airplane increases its speed - the pilot pushes the throttle forward, the jet engines spool and and apply more force to the airplane, the force of the air on the airplane doesn't change so there's a net force on the airplane and it accelerates.

In general, the force applied to the first object and the force applied by the first object to the second object are completely different forces acting on completely different bodies. There's no reason why they should be the same.
 
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  • #36
Kaneki123 said:
You said''The fact that a force of 10N is applied to accelerate the object at first does not imply it applies a force of 10N on the second object''...So is this a fact that the the force applied on first object is not necessarily equal to the force it applies to the second object?...Because if it is not equal then of course there would be a net force on first object...
Yes, these are totally unrelated issues.
 
  • #37
Kaneki123 said:
You said''The fact that a force of 10N is applied to accelerate the object at first does not imply it applies a force of 10N on the second object''...So is this a fact that the the force applied on first object is not neccessarily equal to the force it applies to the second object?...Because if it is not equal then ofcourse there would be a net force on first object...
And you can extend post #26 to calculate the net force on M1.
 
  • #38
The three laws of Newton are more clearly stated in Newton's Law: F=ma. This is most powerful statement in all of science.
 
  • #39
jim meyer said:
The three laws of Newton are more clearly stated in Newton's Law: F=ma. This is most powerful statement in all of science.
F=ma is the second law. The third law might be phrased as ##F_{ab} = -F_{ba}##
 

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