Confusion about Newton's laws, sum of forces equals zero

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Discussion Overview

The discussion revolves around understanding Newton's laws, particularly the conditions under which the sum of forces acting on an object equals zero. Participants explore the implications of Newton's third law and the distinction between action and reaction forces in the context of motion and equilibrium.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about when the sum of forces equals zero, linking it to Newton's third law and the idea of equal and opposite forces.
  • Another participant clarifies that the forces being summed are only those acting on the object, not the forces the object exerts back.
  • A participant provides an example of pushing a car, noting that the net force on the car is determined by the applied force, friction, gravity, and the normal force, and that it is not automatically zero unless there is no acceleration or the car moves at a constant speed.
  • There is agreement on the clarification that the net force is not always zero, depending on the conditions of motion.

Areas of Agreement / Disagreement

Participants generally agree on the clarification that the forces summed are those acting on the object, not the reaction forces. However, there remains some confusion about the conditions under which the sum of forces equals zero, indicating unresolved aspects of the discussion.

Contextual Notes

The discussion highlights the need for a clear understanding of the distinction between action and reaction forces, as well as the conditions for equilibrium, which may not have been fully articulated by all participants.

LogarithmLuke
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I've lately began working with Newtons laws problems at school again, and I've already ran into a few problems.

When making calculations and solving problems, it is often nessecary to understand when forces are equal to zero, and when they are not. Since every force has an equal and opposite counter force, according to Newtons third law, it seems as though the sum of forces are always equal to zero, in both y and x directions.

The reason objects are still able to move though, the way i understand it, is because if let's say i push a book across the table, my mass is far greater than the books and therefore my acceleration will be negligible. So it makes sense to me that objects can move, but always using that the sum of forces equal zero when solving problems causes a lot of issues.

How do i know when the sum of forces in a direction equal zero, and when they do not?
 
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LogarithmLuke said:
How do i know when the sum of forces in a direction equal zero, and when they do not?
You are mixing together two separate issues. The forces you are summing are not the equal and opposite forces at every point, they are only one of each pair: only the forces applied to the object (not the forces the object applies back).
 
russ_watters said:
You are mixing together two separate issues. The forces you are summing are not the equal and opposite forces at every point, they are only one of each pair: only the forces applied to the object (not the forces the object applies back).

So let's say I am pushing a car, using a force. The car pushes back at me with a force that has the same magnitude, but the force is directed at me. Therefore the sum of forces on the car are the force i use to push, friction and gravity as well as the normal force. The net force on the car will be zero if it has no acceleration or travels at a constant pace, but it is not automatically zero. Is my thinking right?
 
LogarithmLuke said:
So let's say I am pushing a car, using a force. The car pushes back at me with a force that has the same magnitude, but the force is directed at me. Therefore the sum of forces on the car are the force i use to push, friction and gravity as well as the normal force. The net force on the car will be zero if it has no acceleration or travels at a constant pace, but it is not automatically zero. Is my thinking right?
Yep!
 

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