F=ma, no acceleration with constant velocity?

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Discussion Overview

The discussion centers around the concept of force as described by Newton's second law (F=ma) in the context of an object moving at a constant velocity in space. Participants explore the implications of force during motion without acceleration and the nature of force during collisions.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that a 30kg rock traveling at a constant speed of 100m/s in space should experience no force according to F=ma, as there is no acceleration.
  • Another participant agrees that there is no force when the rock is not accelerating, but acknowledges that force exists during the impact with another object.
  • Some participants suggest that the understanding of force may be confused with momentum, indicating that forces describe interactions rather than being inherent properties of objects.
  • It is noted that the rock is force-free until it collides with another object, at which point forces are imposed, resulting in non-zero acceleration during the collision.
  • There is a distinction made between force-free motion and motion involving forces, emphasizing that once contact is broken, the rock returns to a force-free state.

Areas of Agreement / Disagreement

Participants generally agree that there is no force acting on the rock while it is moving at a constant velocity in space. However, there is disagreement regarding the interpretation of force during collisions and the relationship between force and momentum.

Contextual Notes

The discussion reflects varying interpretations of force and momentum, and the assumptions underlying the definitions of force in different contexts, particularly during collisions versus constant motion.

PhotonW/mass
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I got a 30kg rock in space, no gravity, no friction. It is traveling at a CONSTANT speed of 100m/s. According Newton's second law, There should be NO force because there is NO acceleration. But I know if it hits another object it will either move the object or put a hole in it so I know there is force there but according to F=ma there is no force. How do I calculate the force?

Please, I am not taking physics, so I am learning this on my own. So please try to put this in words a layman can understand.
 
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Sure, there is force during the impact, but when it isn't accelerating, no force.
 
You may be having a common misunderstanding about forces. Forces describe interactions between objects, it is not a property of the objects themselves. If an object is not interacting with another object or field, then there are no forces at work.
 
PhotonW/mass said:
... But I know if it hits another object it will either move the object or put a hole in it so ...

Sounds like your intuitive idea of 'force' is closer to 'momentum.'
 
Your rock, moving freely in space was force free until the instant that it collided with the second object. When the collision occurred, there were forces imposed on your rock, and it was no longer a force free motion. It still obeys F = ma, but at that instant, F becomes nonzero, and therefore a becomes nonzero as well. As long as your rock is in contact with the second body (or any subsequent body), it will not be a force free motion. Whenever contact is broken, force free motion will again obtain, but it will be a new force free motion.
 

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