Newton's Third Law: Pushing 20kg Box w/140N Force

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Discussion Overview

The discussion revolves around the application of Newton's Third Law and the resulting acceleration of a 20kg box when a force of 140N is applied. Participants explore the implications of force, mass, and acceleration in different contexts, including floating in space and gravitational effects.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the acceleration of the box is 2m/s² in the positive direction when pushed with a force of 140N, suggesting that the box should also have some acceleration in the direction of the push due to the reaction force.
  • Another participant clarifies that the acceleration of an object is determined by the net force acting on it, not by the forces it exerts on other objects.
  • There is a discussion about the relationship between acceleration, mass, and the net force, with some participants expressing confusion about how these concepts interact.
  • A participant illustrates a scenario where a person with a mass of 70kg exerts a 140N force on the 20kg box, leading to an acceleration of 7m/s² for the box and a corresponding acceleration of 2m/s² for the person in the opposite direction.
  • Further clarification is provided that the force acting on the box is the gravitational force, which is distinct from the force exerted by the person pushing it.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding Newton's laws and the implications of force and acceleration. There is no consensus on the initial interpretation of the forces at play, indicating that multiple competing views remain.

Contextual Notes

Some participants express confusion about the distinction between net force and the forces acting on an object, highlighting a potential misunderstanding of Newton's laws. The discussion also touches on gravitational forces and their role in the context of the problem.

autodidude
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If I push a box (mass of 20kg) with a force of 140N, is my acceleration 2m/s^2 in the positive direction? I cause it to accelerate in the same direction as I'm pushing in, and that accelerates at 7m/s^2, but it reacts with 140N also, so shouldn't it have some acceleration in my direction to produce that reaction force?
 
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The acceleration of some object depends on the net force on the object, not on any forces that the object may exert on other objects.
 
If I understand you correctly, acceleration x mass of an object does not give the force exerted on others?
 
autodidude said:
If I understand you correctly, acceleration x mass of an object does not give the force exerted on others?
Acceleration x mass of an object just gives you the net force on the object.
 
I will assume that your mass is 70 kg.

If you are floating in space and you exert a 140 N force on a 20 kg object, while you are pushing, it will indeed accelerate at 7 m/s/s. And yes, likewise, while exerting the 140 N force, your center of mass will accelerate in the opposite direction by 2 m/s/s, because Newton's 2nd and 3rd laws apply to both you andthe object.
 
Doc Al said:
Acceleration x mass of an object just gives you the net force on the object.

Of the forces acting on it? Oooh...I find that a bit less intuitive.

So with weight, 686N is the force ACTING on it, not the force produced by the 70kg mass under gravity?

Bob at PC said:
I will assume that your mass is 70 kg.

If you are floating in space and you exert a 140 N force on a 20 kg object, while you are pushing, it will indeed accelerate at 7 m/s/s. And yes, likewise, while exerting the 140 N force, your center of mass will accelerate in the opposite direction by 2 m/s/s, because Newton's 2nd and 3rd laws apply to both you andthe object.

Yeah, I misunderstood Newton's laws, I thought the object would've had to accelerate in my direction to exert a force to react against my action
 
autodidude said:
Of the forces acting on it? Oooh...I find that a bit less intuitive.

So with weight, 686N is the force ACTING on it, not the force produced by the 70kg mass under gravity?
For a 70kg mass near the Earth's surface, 686 N is one of the forces acting on it. It is the gravitational force that the Earth exerts on the mass. And it's also the gravitational force that the mass exerts on the earth. (Newton's 3rd law.)

But when you apply ΣF = ma to an object, ΣF stands for the net force on the object.
 

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