# Direction of an object when the net force is 0

1. Jun 24, 2009

### albert1993

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?

2. Jun 24, 2009

### Staff: Mentor

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.

3. Jun 24, 2009

### albert1993

oh yea!! there's still time before they all cancel each other out. thanks :D

4. Jun 24, 2009

### unscientific

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.

5. Jun 24, 2009

### albert1993

i'm sorry.. i don't understand what you mean... but i think i got my question answered..

6. Jun 24, 2009

### Staff: Mentor

Your second post implies you didn't understand. What do you think time has to do with the issue?

7. Jun 24, 2009

### Bob S

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

8. Jun 24, 2009

### albert1993

i meant time as, that there was a moment of time that the the net force was not 0 before they all became equal

9. Jun 24, 2009

### Staff: Mentor

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. Jun 24, 2009

### albert1993

ahh so you're saying that there is no direction of an object when the net force is 0?

11. Jun 24, 2009

### Staff: Mentor

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. Jun 24, 2009

### albert1993

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. Jun 24, 2009

### Staff: Mentor

Correct.

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

14. Jun 24, 2009

### albert1993

ok, thank you