Is Newton's Second Law Flawed? An Exploration of Zero Force in Space

[Roddy Hatch]

Newton's law's of motion have been used for hundred's of years, but the second law is wrong and here is why. Newton's second says that net force equals the mass of the object multiplied by the acceleration or F∑=MA. This is fairly simple and straight forward, but is easy to prove wrong. If we take two objects, both having a mass of one kilogram, and push them at each other in a place with zero friction, like space, with a constant velocity of two meters per second until they collide into each other then they should move in the opposite direction with the same force they impacted each other with. The problem with this is when they have a constant velocity they also have an acceleration of zero. The force applied to the objects is zero because the equation is zero multiplied by one. Please look at this as if you are in space and not on earth.

 

[fresh_42]

Wrong, they do not have acceleration zero during impact!

 

[Mark44]

Newton's law's of motion have been used for hundred's of years, but the second law is wrong and here is why. Newton's second says that net force equals the mass of the object multiplied by the acceleration or F∑=MA. This is fairly simple and straight forward, but is easy to prove wrong. If we take two objects, both having a mass of one kilogram, and push them at each other in a place with zero friction, like space, with a constant velocity of two meters per second until they collide into each other then they should move in the opposite direction with the same force they impacted each other with. The problem with this is when they have a constant velocity they also have an acceleration of zero. The force applied to the objects is zero because the equation is zero multiplied by one.

You are misunderstanding the differences between force, momentum, and kinetic energy. If both objects are moving at constant velocities, then there is no force acting on them (disregarding the small gravitational force attracting them). The objects have momentum, though, $mv_1$ and $mv_2$, as well as kinetic energy, ($\frac 1 2 mv_1^2$ and $\frac 1 2 mv_2^2$), with m = 1 in both sets of expressions.

Newton can rest easy -- you haven't disproved anything.

 

[kuruman]

When you say ∑F = MA, what is M? Is it one of the masses or is it the combined mass of the two objects? If it is one of the masses it accelerates during the collision and changes direction because of the external force exerted on it by the other mass. If it is the combined masses, the sum ∑F is zero because collision forces are action-reaction pairs and the center of mass of the combined system does not accelerate even though each individual mass does.

Note: NASA would not have been able to put people on the Moon without Newton's Second Law.

 

[George Jones]

with a constant velocity of two meters per second until they collide into each other then they should move in the opposite direction with the same force they impacted each other with. The problem with this is when they have a constant velocity they also have an acceleration of zero. The force applied to the objects is zero because the equation is zero multiplied by one.

Long live Dirac delta functions!

 

[Nugatory]

The collision takes a short but non-zero amount of time during which the two objects are in contact with one another, each is applying a force to the other, and each is accelerating (that is, changing its speed).

 

[Roddy Hatch]

Wrong, they do not have acceleration zero during impact!

I would like to ask a question about the first law. If an object in motion remains in motion until acted upon by an external force then why do rockets go into space even if gravity is pulling on them? The answer is simple the force of gravity can't stop the momentum that propels the rocket. What I am saying is that if any of the laws are wrong then so are the other's. The example with the rocket makes the first law wrong and thus disproves all laws.

 

[CWatters]

You haven't shown that any of them are wrong. You've only shown that you don't understand how rockets get into orbit.

I suggest you go find an explanation online and if you don't understand it come back with questions.

 

[Doc Al]

If an object in motion remains in motion until acted upon by an external force

That's not what the first law says. Try again.

 

[FactChecker]

Newton's law's of motion have been used for hundred's of years, but the second law is wrong

Don't you think that you should be cautious before you claim that all the geniuses in the last 100 years are wrong? And in such a simple way?

 

[gneill]

Don't you think that you should be cautious before you claim that all the geniuses in the last 100 400 years are wrong? And in such a simple way?

Sorry, thought you might like to include Newton, Galileo, etc., in that summary.

 

[ZapperZ]

I would like to ask a question about the first law. If an object in motion remains in motion until acted upon by an external force then why do rockets go into space even if gravity is pulling on them? The answer is simple the force of gravity can't stop the momentum that propels the rocket. What I am saying is that if any of the laws are wrong then so are the other's. The example with the rocket makes the first law wrong and thus disproves all laws.

You would have looked less like a fool if you had come here and politely asked if you had understood completely the fault in the scenario you've described, rather than proudly proclaimed that Newton's Laws are wrong. (If you truly believe that, then run out of the building you're in and do not use any bridges, because all of them have been built based on those "wrong" laws.)

In this scenario, you seem to ignore the fact that a "rocket" has thrusters, which supply enough force to overcome gravity. But what is amusing here is that the design and kinematics of rockets and space flight are based on Newton's laws! Go ask NASA if you don't believe me! I hope you are seeing the fallacy here, because you are using something that was based on Newton's laws, and then turning around saying that it works because Newton's laws are wrong!

Is this rational?

Zz.

 

[Drakkith]

I would like to ask a question about the first law. If an object in motion remains in motion until acted upon by an external force then why do rockets go into space even if gravity is pulling on them? The answer is simple the force of gravity can't stop the momentum that propels the rocket. What I am saying is that if any of the laws are wrong then so are the other's. The example with the rocket makes the first law wrong and thus disproves all laws.

There is an external force. Inside the combustion chamber of a rocket, molecules of fuel and oxidizer react with each other to produce the final propellant. This propellant is extremely hot and by virtue of its thermal motion it bounces around the chamber. Each time a molecule of propellant bounces off of the combustion chamber it exerts a small amount of force on the chamber walls. One can treat the force that each molecule generates on the combustion chamber as an external force since these molecules are not attached to the rocket and are eventually ejected out the nozzle and backwards into space. So you have 2 external forces here. The force of gravity and the combined force of trillions of trillions of propellant molecules bouncing off of the combustion chamber (the thrust).

 

[DaveC426913]

Can the OP clarify the opening argument?

The problem with this is when they have a constant velocity they also have an acceleration of zero. The force applied to the objects is zero because the equation is zero multiplied by one.

What exactly does this mean?

Once they make contact, causing their acceleration to reduce to zero, and (assuming an inelastic collision) their equal and opposite velocities have been turned into deformation and heat, their acceleration will indeed be zero. Why is that a problem?

If it's an elastic collision, on the other hand, they will indeed bounce off each other and separate with the sum of their resultant velocities equaling the sum of their initial velocities. Why is that a problem?

Is it possible that what you're struggling with is simply the difference between an elastic collision and an inelastic collision?

 

[Dale]

This thread is closed. Thanks to all those who tried to assist the OP, but he will need to go elsewhere.