How Does Acceleration Occur Despite Equal and Opposite Forces?

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Homework Help Overview

The discussion revolves around the concepts of Newton's third law and the conditions for acceleration in physics. Participants are exploring how acceleration can occur in the presence of equal and opposite forces, particularly in the context of action-reaction pairs and the implications of mass and net force.

Discussion Character

  • Conceptual clarification, Assumption checking, Exploratory

Approaches and Questions Raised

  • Participants are questioning how acceleration can happen when forces are equal and opposite, particularly focusing on the distinction between action-reaction forces acting on different objects. They are also discussing the implications of mass and net force in relation to acceleration.

Discussion Status

Some participants have provided insights into the nature of forces acting on different objects and the role of net force in determining acceleration. There is an ongoing exploration of how these principles apply to specific examples, such as a block being pushed and the interaction with a fly and a train.

Contextual Notes

There are references to Newton's laws, particularly the first and second laws of motion, and the discussion includes considerations of mass and friction, which are relevant to the problem context.

Reugar
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I am doing a bit of review and have discovered something I haven't thought about before. We all know Newton's third law (Fab)=-(Fba). So, how can there be acceleration? Say that Fab is 10N, then -Fba is -10N...10N - 10N = 0N. In an action reaction pair, if this net force is 0N...how can there be acceleration?
 
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Reugar said:
I am doing a bit of review and have discovered something I haven't thought about before. We all know Newton's third law (Fab)=-(Fba). So, how can there be acceleration? Say that Fab is 10N, then -Fba is -10N...10N - 10N = 0N. In an action reaction pair, if this net force is 0N...how can there be acceleration?

Action and reaction forces NEVER act on the same object. They can never cancel.
 
Yes...they must act on different objects, same magnitude opposite direction. Say for example I push a block with 50N of force...that block pushes back on me with 50N as well...why does the block accelerate?
 
Reugar said:
Yes...they must act on different objects, same magnitude opposite direction. Say for example I push a block with 50N of force...that block pushes back on me with 50N as well...why does the block accelerate?
Why doesn't a fly make a train crash if they hit and the fly is accelerating and the train is not, after all F=ma.

:rolleyes:
 
Okay, so go back to the first law of inertia then. The fly doesn't make the train crash since it is of neglegable mass. Bit it DID still exert a force on the train. Now...im sorry if this seems foolish but there's just something that isn't clicking here. I push a block and exert 50N on that block, it in turn exerts 50N on me. Why does the block accelerate?
 
Reugar said:
Yes...they must act on different objects, same magnitude opposite direction. Say for example I push a block with 50N of force...that block pushes back on me with 50N as well...why does the block accelerate?

Because by Newton's second law, the acceleration of the block is the result of forces acting on the block, not forces the block is exerting on something else. If the 50N is the only force acting on the block, it must accelerate.

What happens to you? If the 50N the block is exerting on you is the only force acting on you, then you must accelerate in the opposite direction to the block. If the block is really massive (like the whole Earth for example), it will hardly move, but you will. If the block has little mass it will accelerate so quickly that the force will only last for a small fraction of a second, and you will not change your velocity very much, while the block experiences a much larger velocity change.

This is the origin of the conservation of momentum principle. If you and the block start at rest, and exert force upon one another, each of you will accelerate for the same time interval and acquire velocities in opposite directions. Your speed times your mass will equal the block's speed times its mass at all times during the acceleration and thereafter.
 
That's much better...thank you. I was having a blonde moment there, but thanks for explaining it :smile:
 
Reugar said:
Yes...they must act on different objects, same magnitude opposite direction. Say for example I push a block with 50N of force...that block pushes back on me with 50N as well...why does the block accelerate?
You're missing the point. The fact that the block pushes back on your does not change the fact that it's experiencing a 50N force itself.

Draw a free-body diagram showing all the forces on the block: your pushing, and friction. If your pushing exceeds the friction force, the net force is non-zero, and the block accelerates.

- Warren
 

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