Direction of an object when the net force is 0

AI Thread Summary
The discussion centers on the confusion surrounding Newton's first law of motion, particularly in relation to an airplane moving at constant speed. It is clarified that when the net force on an object is zero, it will maintain its current state of motion, whether that be at rest or moving in a straight line at a constant speed. The conversation highlights that the direction of motion is independent of the net force being zero; it only indicates that the object will not change its direction. Additionally, it is emphasized that knowing the net force alone does not provide information about the object's direction unless its initial motion is known. Overall, the principles of equilibrium and momentum conservation are reiterated in the context of Newton's laws.
albert1993
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Hello, I'm self studying physics right now and have learned about Newton's laws.
However, I am confused by the first law.
I understand that when the net force is 0, an object will continue its original motion at the same velocity. But how does this work for an airplane which is moving at a constant speed? Since it is going at a constant speed, the net force on the airplane is 0, right? Well, how do we then conclude that the airplane is moving forward, though logically that makes sense?
 
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Newton's laws tell us that as long as the net force on an object is zero, the object will keep moving at a constant speed in a straight line. This applies to an airplane, just like any other object. When the net force on the airplane is zero, it will keep moving in whatever direction it was going. If the plane was moving backward when the net force became zero, it will keep moving backward.
 
oh yea! there's still time before they all cancel each other out. thanks :D
 
time?? no. a body in equilibrium (dynamic or static) will ALWAYS remain in equilibrium PROVIDED no external net force acts on the body.

i.e. Total momentum of a system is constant, provided no external net force acts on a system.
 
unscientific said:
time?? no. a body in equilibrium (dynamic or static) will ALWAYS remain in equilibrium PROVIDED no external net force acts on the body.

i.e. Total momentum of a system is constant, provided no external net force acts on a system.

i'm sorry.. i don't understand what you mean... but i think i got my question answered..
 
Your second post implies you didn't understand. What do you think time has to do with the issue?
 
Albert1993 said:
Hello, I'm self studying physics right now and have learned about Newton's laws.
However, I am confused by the first law.
I understand that when the net force is 0, an object will continue its original motion at the same velocity.
Consider a spaceship between the Earth and the Moon. There are several points, called Langrange points, where the Earth gravity, Moon gravity, and centripital forces cancel out. The spaceship is in unstable equilibrium, because any motion away from these points would cause one force to be stronger than the other two. See
http://plus.maths.org/issue36/features/dartnell/index.html
 
russ_watters said:
Your second post implies you didn't understand. What do you think time has to do with the issue?

i meant time as, that there was a moment of time that the the net force was not 0 before they all became equal
 
Albert1993 said:
i meant time as, that there was a moment of time that the the net force was not 0 before they all became equal
That isn't necessarily true and certainly isn't relevant. Newton's 1st law doesn't care how two objects got into their current situation, all it does is say that they'll stay in that situation if there is no outside/unbalanced force.
 
  • #10
russ_watters said:
That isn't necessarily true and certainly isn't relevant. Newton's 1st law doesn't care how two objects got into their current situation, all it does is say that they'll stay in that situation if there is no outside/unbalanced force.

ahh so you're saying that there is no direction of an object when the net force is 0?
 
  • #11
Albert1993 said:
ahh so you're saying that there is no direction of an object when the net force is 0?
The fact that the net force is zero has nothing to do with the direction an object is moving in. It just means that it won't change direction.
 
  • #12
Doc Al said:
The fact that the net force is zero has nothing to do with the direction an object is moving in. It just means that it won't change direction.

kk, so is it impossible to determine the direction of an object when the net force is 0, unless we know what its original direction was (that is if it was moving in the first place), right?
 
  • #13
Albert1993 said:
kk, so is it impossible to determine the direction of an object when the net force is 0, unless we know what its original direction was (that is if it was moving in the first place), right?
Correct.

Just knowing the net force (even if it's not zero) will not tell you the direction of motion.
 
  • #14
Doc Al said:
Correct.

Just knowing the net force (even if it's not zero) will not tell you the direction of motion.

ok, thank you :biggrin:
 

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