Question about inertia and acceleration

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    Acceleration Inertia
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Discussion Overview

The discussion revolves around the concepts of inertia, force, and acceleration, particularly in the context of a 1 kg object subjected to forces of 20 N in opposing directions. Participants explore the implications of applying these forces over specific time intervals and the resulting motion of the object.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether a 1 kg object, initially at rest and acted upon by a 20 N force, would accelerate in the opposite direction if a 20 N opposing force is applied after three seconds.
  • Another participant emphasizes the importance of impulse and suggests that the initial impulse must be compared to the second impulse to determine the object's motion.
  • Some participants argue that applying a force for zero duration effectively means no force is applied, leading to confusion about the implications for acceleration.
  • There is a discussion about the definitions of inertia, mass, and momentum, with some participants suggesting that the term "inertia" is too vague for the context of the problem.
  • One participant asserts that as long as there is a net force acting on the object, it will accelerate in the direction of that force, while another clarifies that mass cannot act as a force.
  • Participants express uncertainty about the effects of inertia and how it relates to the forces applied, with some suggesting that both mass and momentum can oppose changes in velocity.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions and implications of inertia, mass, and momentum in this context. Multiple competing views remain regarding the effects of forces applied over different time intervals and the resulting motion of the object.

Contextual Notes

There are unresolved issues regarding the application of forces for zero duration and the implications for impulse and acceleration. Participants also express varying interpretations of inertia and its role in the discussion.

FieldvForce
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This question is just about a thought I was having.

If a 1 kg object which is at rest is made to accelerate with a force of 20 Newtons and then three seconds later have an opposing force of 20 N applied to it with an impulse of (1)60/(T=0), would the object begin to accelerate in the opposite direction?
 
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Tell us what you think will happen.
 
Well the inertia of the object opposes change in motion and that is what the forward force is causing, if the backward force removes the foward acceleration instantaneously more force than necessary is used to make Fnet = 0 and so Fnet is negative and large.
 
What counts here is Impulse. Work out the initial impulse. Is it greater or less than the second impulse applied.

There will be no acceleration once the forces have stopped, will there? Think Newton for every stage. The term "Inertia" is not really applied to these simple problems. If you mean Mass, say Mass and if you mean Momentum, say Momentum and follow the rules, rather than trying to 'chat' through the problem.
 
sophiecentaur said:
What counts here is Impulse. Work out the initial impulse. Is it greater or less than the second impulse applied.

There will be no acceleration once the forces have stopped, will there? Think Newton for every stage. The term "Inertia" is not really applied to these simple problems. If you mean Mass, say Mass and if you mean Momentum, say Momentum and follow the rules, rather than trying to 'chat' through the problem.

I am trying to understand the problem so that I can apply the rules, I don't think i have the brain power to 'chat' through the problems as most who are brave enough to stop clinging to the rules for one moment are usually classed as geniuses further down the line.


Maybe the way I presented the situation was inaccurate.

An object with mass 1 kg is acted upon by a force of 20 N, after three seconds an opposing force of 20 N is applied over a time T= 0.

So that the impulse is 0 and the velocity should immediately become constant. Will with object begin to accelerate backwards due to the inertia already apposing the forward acceleration?
 
If you apply a force for zero duration it has no effect.
 
FieldvForce said:
IAn object with mass 1 kg is acted upon by a force of 20 N, after three seconds an opposing force of 20 N is applied over a time T= 0.
Still unclear. The initial force of 20 N is applied for three seconds? Is it removed after three seconds?

Saying that an opposing force of 20 N is applied for 0 seconds is another way of saying that no opposing force was applied. Try again.

So that the impulse is 0 and the velocity should immediately become constant.
The velocity will be constant when the net force is zero. Did you remove the initial force?
 
CWatters said:
If you apply a force for zero duration it has no effect.

You have to constantly apply the force? I thought tF=mv ∴ if T = 0 impulse = 0 and F = ∞

Doc Al said:
Still unclear. The initial force of 20 N is applied for three seconds? Is it removed after three seconds?

Saying that an opposing force of 20 N is applied for 0 seconds is another way of saying that no opposing force was applied. Try again.


The velocity will be constant when the net force is zero. Did you remove the initial force?

ΔF/ΔT, where ΔT = 0, however.
ΔF will also = 0 since the forces cancel obviously there is an issue with dividing by 0, ΔT is just meant to be extremely small, but this is really about the inertia of the object.
 
FieldvForce said:
You have to constantly apply the force? I thought tF=mv ∴ if T = 0 impulse = 0 and F = ∞
So now the opposing force is infinite, not merely 20 N? But still applied for zero time?

Just state the real force (finite) applied for some finite time. Then you'll be able to find the impulse it delivers.
 
  • #10
Doc Al said:
So now the opposing force is infinite, not merely 20 N? But still applied for zero time?

Just state the real force (finite) applied for some finite time. Then you'll be able to find the impulse it delivers.

The less time the force is applied the larger it must be to have the desired affect.

Right, 20 N of force is applied for half a second. Then an opposing 20 N is applied for 0.5 immediately (this is why I was talking about 0 seconds) after the first force is removed; will object accelerate in the opposite direction due to the inertia of the object and the opposing 20 N.

Can the inertia act as an opposing force along with the opposing 20 N causing the 20 N to be excessive in terms of causing the object to have constant velocity.
 
  • #11
FieldvForce said:
The less time the force is applied the larger it must be to have the desired affect.
OK.

Right, 20 N of force is applied for half a second. Then an opposing 20 N is applied for 0.5 immediately (this is why I was talking about 0 seconds) after the first force is removed; will object accelerate in the opposite direction due to the inertia of the object and the opposing 20 N.
As long as there is a net force on the object, it will accelerate. While the initial force acts, the object accelerates in the direction of that force. When that force is removed and the opposing force acts, the object will accelerate in the direction of that new force.

In this case, assuming the object started from rest it will end up at rest, since the two impulses are equal and opposite. The first force speeds it up; the second force slows it back down.

Can the inertia act as an opposing force along with the opposing 20 N causing the 20 N to be excessive in terms of causing the object to have constant velocity.
I don't know what you mean.
 
  • #12
FieldvForce said:
will object accelerate in the opposite direction due to the inertia of the object and the opposing 20 N.

Can the inertia act as an opposing force along with the opposing 20 N causing the 20 N to be excessive in terms of causing the object to have constant velocity.
As a wise person once recommended to you a long time ago, the term "inertia" is too vague to be of use in this discussion. It can either mean mass (in which case use the term "mass") or it can mean momentum (in which case use the term "momentum").

Assuming that you mean mass then the answer to your first question is: yes, according to f=ma the object will accelerate in the opposite direction in response to a net force in the opposite direction. Under the same assumption the answer to your second question is: no, mass cannot act as force, they have different units.
 
  • #13
DaleSpam said:
As a wise person once recommended to you a long time ago, the term "inertia" is too vague to be of use in this discussion. It can either mean mass (in which case use the term "mass") or it can mean momentum (in which case use the term "momentum").

Assuming that you mean mass then the answer to your first question is: yes, according to f=ma the object will accelerate in the opposite direction in response to a net force in the opposite direction. Under the same assumption the answer to your second question is: no, mass cannot act as force, they have different units.

Ah I see what he meant now, both the mas and the momentum can act to appose a change velocity.
 
  • #14
Consider two objects with the same momentum but different mass. It will take the same impulse to stop them.
 
  • #15
FieldvForce said:
Ah I see what he meant now, both the mas and the momentum can act to appose a change velocity.

No. I can see that you are still wanting to describe the effects of force on motion in the pre-Newtonian way. This approach was put aside long ago because of its limits. The whole business of Dynamics (as we used to call it) is essentially a Quantitative thing and the Maths is needed, even at an initial level. Mass needs Force to accelerate it. Putting N2 this way round: a = F÷m, shows that the force is 'divided up' between the elements of mass in an object. N3 says that the reaction is in a direction acting against the force.
Momentum doesn't "oppose" the force, it is just changed 'by' the force being applied for a finite time. (That's Impulse). However fast an object is moving (whatever its momentum), in a given direction, a Force on it will produce exactly the same acceleration on it (Force = rate of change of Momentum) and remember all this involves Vectors and the directions are important.
It may be tiresome but using the well known equations always gives the right answer in this sort of situation (non-relativistic etc. etc.).
 
  • #16
sophiecentaur said:
No. I can see that you are still wanting to describe the effects of force on motion in the pre-Newtonian way. This approach was put aside long ago because of its limits. The whole business of Dynamics (as we used to call it) is essentially a Quantitative thing and the Maths is needed, even at an initial level. Mass needs Force to accelerate it. Putting N2 this way round: a = F÷m, shows that the force is 'divided up' between the elements of mass in an object. N3 says that the reaction is in a direction acting against the force.
Momentum doesn't "oppose" the force, it is just changed 'by' the force being applied for a finite time. (That's Impulse). However fast an object is moving (whatever its momentum), in a given direction, a Force on it will produce exactly the same acceleration on it (Force = rate of change of Momentum) and remember all this involves Vectors and the directions are important.
It may be tiresome but using the well known equations always gives the right answer in this sort of situation (non-relativistic etc. etc.).

After reading these posts and thinking for a while I have found that I made quite a mistake and misunderstood the natures of force and momentum.

I aslo mistook this equation F = mΔv/Δt as meaning an change in momentum in close 0 seconds results in a massive as apposed to a change in momentum in close to 0 seconds requires a massive force.

So considering the situation now.

20 N has to be applied over some defined time, as apposed to applied for an infinitesimal period as this would mean no change in momentum and thus no force applied at all.If the initial force is removed forward acceleration no longer continues and the objects mass (inertia) opposes the change in motion caused by the second force, if the initial force is not removed and the second force is applied the sum of forces acting on the object is 0 and so the acceleration is 0.

One question, is the mass (inertia) of the object only observable when the object is not in equilibrium? I think that was the question beneath my thought processes before I posted this thread.
 
Last edited:
  • #17
It can be in equilibrium under gravitational forces of course.
 
  • #18
sophiecentaur said:
It can be in equilibrium under gravitational forces of course.

Orbiting objects?
 
  • #19
I was thinking of a mass resting on a table. Gravity and equilibrium.
 
  • #20
FieldvForce said:
Orbiting objects?
Orbiting objects are not in equilibrium.
 
  • #21
sophiecentaur said:
I was thinking of a mass resting on a table. Gravity and equilibrium.
Ah ok
Doc Al said:
Orbiting objects are not in equilibrium.

Lol yeah, I couldn't think of anything else.

I just suggested them because there average displacement is 0 and their mechanical energy is constant.
 

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