How Does Acceleration Occur Despite Equal and Opposite Forces?

In summary, 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?
  • #1
Reugar
7
0
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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  • #2
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.
 
  • #3
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?
 
  • #4
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:
 
  • #5
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?
 
  • #6
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.
 
  • #7
That's much better...thank you. I was having a blonde moment there, but thanks for explaining it :smile:
 
  • #8
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
 

FAQ: How Does Acceleration Occur Despite Equal and Opposite Forces?

What are Newton's Laws of Motion?

Newton's Laws of Motion are three fundamental principles that describe the behavior of objects in motion. The first law states that an object will remain at rest or in constant motion unless acted upon by an external force. The second law states that the force applied to an object is equal to its mass multiplied by its acceleration. The third law states that for every action, there is an equal and opposite reaction.

How do Newton's Laws apply to everyday life?

Newton's Laws can be observed in many aspects of everyday life. For example, the first law can be seen when a soccer ball comes to a stop after being kicked, or when a car comes to a stop after the brakes are applied. The second law can be seen when a person pushes a shopping cart, with the force applied resulting in a change in the cart's acceleration. The third law can be seen when a person jumps off a diving board, with the action of the person pushing down on the board resulting in an equal and opposite reaction that propels them upwards.

Why are Newton's Laws important?

Newton's Laws are important because they form the basis of classical mechanics and help us understand the behavior of objects in motion. They have been extensively tested and are still used today to predict and explain the motion of objects in various scenarios, from simple movements to complex systems. They also laid the foundation for many other scientific discoveries and advancements.

Who was Sir Isaac Newton?

Sir Isaac Newton was an English physicist and mathematician who is widely recognized as one of the most influential scientists in history. He is best known for his work on gravity and the three laws of motion, which were published in his book "Philosophiæ Naturalis Principia Mathematica" in 1687. He also made significant contributions to the fields of optics, calculus, and astronomy.

How did Newton develop his Laws of Motion?

Newton developed his Laws of Motion through his observations and experiments. He was inspired by the work of previous scientists such as Galileo and Kepler, and also conducted his own experiments with pendulums and falling objects. He used mathematics to express his laws and prove their validity. These laws were a groundbreaking contribution to the field of physics and continue to be a fundamental concept in the study of motion.

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