If there is no force being applied can there still be a net force?

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Discussion Overview

The discussion revolves around the concept of net force and acceleration in the context of Newton's laws of motion. Participants explore whether an object can continue to accelerate after an applied force has ceased, particularly in a frictionless environment. The conversation touches on theoretical implications and interpretations of classical mechanics.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants suggest that an object may continue to accelerate towards its final velocity even after the applied force is removed, raising questions about the nature of net force.
  • Others argue that according to Newton's laws, specifically the first law, if no force is applied, there can be no acceleration, leading to a contradiction in the original premise.
  • A participant mentions that it takes time for the object to finish accelerating from the original force, implying a delay in the cessation of acceleration.
  • Some participants introduce edge cases, such as the deformation of objects and the gradual reduction of force, which complicate the discussion of instantaneous changes in motion.
  • There are references to historical perspectives on motion, including Aristotle's views and the implications of dissipative forces on celestial bodies, suggesting a broader philosophical context to the discussion.
  • One participant expresses skepticism about the idea of "momentum of acceleration," questioning its validity and relevance to the discussion.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are competing views regarding the relationship between applied force, acceleration, and net force, with some asserting that acceleration ceases immediately when force is removed, while others propose that acceleration can continue for a time due to various factors.

Contextual Notes

Limitations include the ambiguity surrounding the definitions of force and acceleration, as well as the assumptions made about ideal conditions versus real-world scenarios. The discussion also highlights the complexity of applying Newton's laws in different contexts.

annamal
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If there is no force being applied can there still be a net force? For example, supposed you apply a force F forward on an object on a frictionless plane, the moment you stop applying the force, the object may still be accelerating towards its final velocity but there is no applied force. So if there is no force being applied, there can still be a net force?

If so, that would be confusing for all the physics word problems.
 
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annamal said:
the object may still be accelerating towards its final velocity but there is no applied force

Why?
 
weirdoguy said:
Why?
Can you clarify? What do you mean?
 
Why object may still be accelerating without the force?
 
annamal said:
the object may still be accelerating towards its final velocity
Mr Newton says "No force, no acceleration". Basic laws of motion.
 
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weirdoguy said:
Why object may still be accelerating without the force?
It takes time for the object to finish accelerating from the original force
 
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sophiecentaur said:
Mr Newton says "No force, no acceleration". Basic laws of motion.
Doesn't it take time for the object to finish accelerating from the original force? So if you push an object, it will stop accelerating the moment you stop pushing it?
 
annamal said:
Doesn't it take time for the object to finish accelerating from the original force? So if you push an object, it will stop accelerating the moment you stop pushing it?
Are you trying to apply your ideas to a particular situation and finding it's not making sense? Start off assuming that Newton 1 will always apply. If something actually appears to accelerate after the applied force then there could be some change of shape so that, although the centre of mass is no longer accelerating, the surface may appear to be still accelerating a bit until its equilibrium shape is reached. (e.g. a balloon with a mass suspended in its centre by rubber bands)
 
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It will stop accelerating when the force is zero.

In practice, it takes a finite time for a force to be removed during which the force decreases with corresponding lower accelerations.
 
  • #10
caz said:
It will stop accelerating when the force is zero.

In practice, it takes a finite time for a force to be removed during which the force decreases with corresponding lower accelerations.
In practice, objects deform when pushed. Nothing happens instantly - there are always masses and elasticity at work.
 
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  • #11
annamal said:
It takes time for the object to finish accelerating from the original force
It's hard to imagine where you got this idea.
 
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  • #12
annamal said:
If there is no force being applied can there still be a net force? For example, supposed you apply a force F forward on an object on a frictionless plane, the moment you stop applying the force, the object may still be accelerating towards its final velocity but there is no applied force.
"Momentum of acceleration?"
"Take your foot off the gas at fifty, 'and the momentum of acceleration will take you right on up to sixty,'" from an idiot to my dad back in the sixties.
 
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  • #13
Bystander said:
"Momentum of acceleration?"
"Take your foot off the gas at fifty, 'and the momentum of acceleration will take you right on up to sixty,'" from an idiot to my dad back in the sixties.
I know you didn't mean it this way but I feel it's important to clarify the OP is not an idiot for asking the question or for surmising his answer.

annamal said:
Doesn't it take time for the object to finish accelerating from the original force? So if you push an object, it will stop accelerating the moment you stop pushing it?
Others have pointed out edge case scenarios (forces do not reduce to zero instantly, objects deform, etc.) but the short answer is the moment no force is applied, it will stop accelerating, yes.
 
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  • #14
Bystander said:
"Momentum of acceleration?"
"Take your foot off the gas at fifty, 'and the momentum of acceleration will take you right on up to sixty,'" from an idiot to my dad back in the sixties.

Yeah, in high school I was talking with a hunting buddy who insisted that the bullet didn't get up to speed until it was way out of the barrel... I had the same thought about him that your dad did about his friend... o0)
 
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  • #15
DaveC426913 said:
I know you didn't mean it this way but I feel it's important to clarify the OP is not an idiot for asking the question or for surmising his answer

Indeed Aristotle was not stupid and his intuition was really (and yet understandibly) incorrect.
 
  • #16
Newton's first law is actually quite obscure, as there are almost no clear examples of it. Things do naturally tend to lose speed. It was a great insight that this is entirely due to dissipative forces; and, that in the absence of such forces uniform motion would continue indefinitely.

I've never heard of the idea that things keep speeding up even after the impulse has ceased. "The momentum of acceleration" seems a somewhat bizarre concept.
 
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  • #17
The heavenly bodies are hardly subjected to dissipative forces. I'm sure that's where Newton got the idea.
 
  • #18
Hornbein said:
The heavenly bodies are hardly subjected to dissipative forces. I'm sure that's where Newton got the idea.
Those were subject to the laws of heaven and kept moving by the hand of god. Newton's Principia also established the idea of universal laws of physics: not one law for Earth and another for the heavens. This idea may not have originated with Newton, but the first law required that insight as well.

It is genuinely hard to construct your own experiment to demonstrate the first law.
 
  • #19
PeroK said:
Newton's first law is actually quite obscure, as there are almost no clear examples of it. Things do naturally tend to lose speed. It was a great insight that this is entirely due to dissipative forces; and, that in the absence of such forces uniform motion would continue indefinitely.

Hornbein said:
The heavenly bodies are hardly subjected to dissipative forces. I'm sure that's where Newton got the idea.
Intuition can lead to all sorts of 'wrong' ideas. There was a very reasonable theory, based on observation, that "Everything" slows down (+"eventually", not actually stated). It's common (was universal) terrestrial experience, even with low friction clock mechanisms.

It was much easier to deny that Physics would apply everywhere and that God was turning the handle up there.

To be fair, it took a lot of arguing the subject out, at a very high level, to arrive at a consistent model that includes Force, Mass, Momentum and Kinetic Energy. It can take a lot of self discipline sometimes to approach a new situation, using the 'right' terms to describe and predict what happens.
 
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