Newton's 1st Law: Object at Constant Speed, No Reaction Force

[adjacent]

An object is traveling at constant speed.There is no force on it(It means it won't also exert any force on any object)What will happen if I put something in its path?If it does not exert a force there will be no reaction force(Newtons 3rd law).Then what will happen to it?

 

[WannabeNewton]

What makes you say it won't exert a force? If you put an object in the particle's path, the object exerts a force on the particle when the two come into contact, so the particle will no longer be traveling at the same velocity. This is essentially a collision problem.

 

[Doc Al]

An object is traveling at constant speed.There is no force on it(It means it won't also exert any force on any object)What will happen if I put something in its path?If it does not exert a force there will be no reaction force(Newtons 3rd law).Then what will happen to it?

If you put something in its path they will collide and exert forces on each other. Newton's first law says that an object will maintain the same speed and direction unless acted upon by some force. When the object collides with something, that something exerts a force on it.

 

[adjacent]

If there is an action there should be a reaction.If the object exert a force the particle should also exert a force on the object BUT the particle is moving without any force.That was my question

 

[WannabeNewton]

The particle was free of forces before colliding. As soon as it collides, the particle exerts a force on the object that is equal and opposite to the force the object exerts on the particle.

 

[adjacent]

The particle was free of forces before colliding. As soon as it collides, the particle exerts a force on the object that is equal and opposite to the force the object exerts on the particle.

That doesn't make a sense to me.How can some thing make a force out of nothing?

 

[WannabeNewton]

That doesn't make a sense to me.How can some thing make a force out of nothing?

It isn't coming out of nothing. It is coming due to collision with the object. If there was no force on the particle due to colliding with the object, it would move on with the same velocity it originally had which is only possible if it passes freely through the object - this does not happen classically.

 

[Dale]

adjacent,

There are only two possibilities:

1) there is no force on the object: the object will move in a straight line at a constant speed (Newton's 1st).
2) there is a force on the object: the object will accelerate (Newton's 2nd) AND will exert a force on other objects (Newton's 2nd).

You seem to be thinking that there is some third possibility, there is not.

 

[Aero_UoP]

The fact that the sum of the forces acting on the object is zero, doesn't mean that the object doesn't have momentum (unless its mass is zero or it's motionless). Now, if you put an obstacle in its path (say another object or a solid wall) there will be an impact, its speed will change and you can use the impulse-momentum theorem to find what you need.

 

[adjacent]

The fact that the sum of the forces acting on the object is zero, doesn't mean that the object doesn't have momentum (unless its mass is zero or it's motionless). Now, if you put an obstacle in its path (say another object or a solid wall) there will be an impact, its speed will change and you can use the impulse-momentum theorem to find what you need.

I have not studied momentum yet

 

[adjacent]

Another question ,If a ball gets hit on the wall,the ball as well as the wall will exert the same force.Then how come the ball bounce back.It should be in equilibrium?

 

[WannabeNewton]

You don't need to know momentum to understand this in an intuitive manner. Throw a ball at a wall - the ball will of course recoil backwards. How do you suppose its direction changed if there was no force on it? The force is coming from the contact.

The ball is not in equilibrium. There is a net force on it, the one being exerted on it by the wall. Newton's 3rd law says the wall exerts a force on the ball that is equal and opposite to the force exerted by the ball on the wall. Both will feel a force, they don't cancel out for both objects because the mutual reaction forces are acting on two different objects.

 

[Doc Al]

If there is an action there should be a reaction.

Right.

If the object exert a force the particle should also exert a force on the object

Right.

BUT the particle is moving without any force.

Until it collided with something.

 

[adjacent]

You don't need to know momentum to understand this in an intuitive manner. Throw a ball at a wall - the ball will of course recoil backwards. How do you suppose its direction changed if there was no force on it? The force is coming from the contact.

The ball is not in equilibrium. There is a net force on it, the one being exerted on it by the wall. Newton's 3rd law says the wall exerts a force on the ball that is equal and opposite to the force exerted by the ball on the wall. Both will feel a force, they don't cancel out for both objects because the mutual reaction forces are acting on two different objects.

Thanks. May be in the first question there could be inertial force or something that keeps the object moving in the same way.And exerting forces on objects?

 

[Dale]

Another question ,If a ball gets hit on the wall,the ball as well as the wall will exert the same force.Then how come the ball bounce back.It should be in equilibrium?

The best approach for understanding this is using free-body diagrams (FBD). Draw one FBD for the ball and one for the wall. On the FBD for the ball there are two forces, the weight of the ball pointing down, and the normal force from the wall pointing backwards. On the FBD for the wall there are also two forces, the weight of the wall pointing down, and the normal force from the ball pointing forwards.

According to Newton's third law the normal force from the wall on the ball in the first FBD is equal and opposite to the normal force from the ball on the wall in the second FBD. Note that those two forces are on different FBD's because they act on different objects.

Now consider Newton's second law. The mass of the ball times its acceleration is equal to the sum of the forces acting on the ball, i.e. the sum of the forces on the first FBD. The mass of the wall times its acceleration is equal to the sum of the forces acting on the wall, i.e. the sum of the forces on the second FBD.

The two forces in a Newton's third law pair act on DIFFERENT bodies so they NEVER get summed together.

 

[Dale]

Thanks. May be in the first question there could be inertial force or something that keeps the object moving in the same way.And exerting forces on objects?

Inertial forces (aka fictitious forces) show up in non-inertial reference frames. You don't want to deal with those at this point. Stick to inertial reference frames where Newton's laws hold until you have fully mastered them. Only then look at inertial forces, etc.

 

[adjacent]

Right.


Right.


Until it collided with something.

I have confusion on this Until it collided with something From where is the energy provided for it to produce a force on the object?

 

[adjacent]

Inertia should be involved.Object wants to be in the same way.When another object collides on it,Inertia exerts a force on the other object.That is my thinking

 

[Aero_UoP]

Another way to think of this is to think about billiard. When you hit the ball with the cue, the ball starts moving. You can suppose that the friction between the ball and the table as well as the aerodynamic drag are neligible. Thus, sum of the forces acting on the ball is zero and the ball will keep on moving at constant speed until it hits another ball. Newton's first law applies up to here. From the moment of impact on, the ball's velocity and direction will change and the other ball will start moving as well. This phenomenon is now described my the impulse-momentum theorem. Google it ;)

 

[adjacent]

Can someone explain #18?

 

[Doc Al]

I have confusion on this Until it collided with something From where is the energy provided for it to produce a force on the object?

Two points:
(1) Force doesn't require energy; they are different things.
(2) The object was moving!

 

[adjacent]

What about Inertia on 18#?Doc Al?

 

[Doc Al]

Inertia should be involved.Object wants to be in the same way.When another object collides on it,Inertia exerts a force on the other object.That is my thinking

That's an OK way to think of it. (Almost.)

Objects have inertia (mass). When moving, they "want" to keep going at the same speed in the same direction. But when they collide with something, they are prevented from doing that. The colliding objects exert forces on each other.

 

[Dale]

Inertia should be involved.Object wants to be in the same way.When another object collides on it,Inertia exerts a force on the other object.That is my thinking

Your thinking is slightly off. When an object collides with another the force is the normal or contact force. Inertia may be the reason that the objects are in contact, but it is the contact which exerts the force. Inertia itself does NOT produce a force in inertial reference frames.

Inertial forces are an advanced topic that you need to wait for until you have mastered Newtons laws with real forces.

 

[adjacent]

Is Doc Als Explanation correct,DaleSpam? Is there any mathematical explanation for it?

 

[Doc Al]

Is Doc Als Explanation correct,DaleSpam? Is there any mathematical explanation for it?

I think we're basically saying the same thing. (I don't think you had "inertial forces" in mind when you mentioned inertia. As DaleSpam says, that's a more advanced subject.)

 

[Dale]

I think we're basically saying the same thing. (I don't think you had "inertial forces" in mind when you mentioned inertia. As DaleSpam says, that's a more advanced subject.)

He specifically mentioned inertial forces in post 14:

May be in the first question there could be inertial force or something that keeps the object moving in the same way.

I am trying to steer him away from that concept until he masters standard Newtonian mechanics concepts.

It could be that he used the words without realizing that they have a specific meaning, which he did not intend.

 

[Dale]

adjacent, at this point in your education, it is best to think simply that inertia is another word for mass. The forces are exerted on an object through various interactions (contact, electromagnetism, gravity, etc.) and it responds to a force by accelerating by an amount related to its inertia by f=ma. So inertia is a description of the response of the object to an external force, not the source of that force. The source of the force is the appropriate interaction.

 

[Doc Al]

He specifically mentioned inertial forces in post 14:I am trying to steer him away from that concept until he masters standard Newtonian mechanics concepts.

I missed that explicit reference. Oops!

It could be that he used the words without realizing that they have a specific meaning, which he did not intend.

That's what I assumed.

 

[adjacent]

Thanks

 

[technician]

adjacent,

There are only two possibilities:

1) there is no force on the object: the object will move in a straight line at a constant speed (Newton's 1st).
2) there is a force on the object: the object will accelerate (Newton's 2nd) AND will exert a force on other objects (Newton's 2nd).

You seem to be thinking that there is some third possibility, there is not.

Newton's laws do net relate to there being NO FORCE...they relate to NO RESULTANT FORCE.
The word RESULTANT is important and is omitted too often, causing confusion...as here.

 

[technician]

An object is traveling at constant speed.There is no force on it(It means it won't also exert any force on any object)What will happen if I put something in its path?If it does not exert a force there will be no reaction force(Newtons 3rd law).Then what will happen to it?

Newton's laws refer to NO RESULTANT FORCE or NO UNBALANCED FORCE ...not NO FORCE.
There is a subtle but important difference

 

[Dale]

Newton's laws do net relate to there being NO FORCE...they relate to NO RESULTANT FORCE.
The word RESULTANT is important and is omitted too often, causing confusion...as here.

I think you are overstating it a bit. Newtons 2nd certainly does relate to resultant forces, and Newtons 3rd law certainly does not. I have seen Newtons 1st law expressed both ways:

A body moves in a straight line at a constant speed unless acted on by an unbalanced force
Or
A body moves in a straight line at a constant speed unless acted on by a force

I prefer the second, because experimentally it is generally easier to set up a no-force situation than a balanced-force situation. If there are no forces then there are automatically no unbalanced forces. Then the special case of balanced forces is taken care of in Newtons 2nd law.

So I usually think of only the 2nd as referring to resultant forces, and the others as referring to individual forces.

 

[technician]

Newtons laws relate to resultant forces...simple fact.

 

[Dale]

Newtons laws relate to resultant forces...simple fact.

No, they don't. I have never seen a single statement of Newton's laws where all three refer to resultant forces. None of the first 5 hits on Google relate all three laws to resultant forces, nor does my Serway textbook.

Can you provide even one example where all three laws relate to resultant forces? If not then your statement is not simple fact, it is simply false.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
"3.Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body."

http://teachertech.rice.edu/Participants/louviere/Newton/law3.html
"for every force there is a reaction force that is equal in size, but opposite in direction"

http://csep10.phys.utk.edu/astr161/lect/history/Newton3laws.html
"I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. "

http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#nt1
"an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force"

http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
"in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object"

 

[SammyS]

In my experience, textbooks generally refer to the NET force acting on a body: the net force being the resultant of ALL of the forces acting on a body.

Newton's 1st Law is sometimes written with reference to net force. Sometimes it's written without reference to net force, only referring to what happens to the body unless some force acts on the body.

Newton's 2nd Law, unless it's written in terms of impulse and momentum, is always written (in my experience) with reference to the net force acting on a body.

Net force has no place in Newton's 3rd Law.

 

[WannabeNewton]

There is a much more fundamental way to look at Newton's 1st law that has nothing to do with resultant forces: the assertion that inertial systems exist i.e. that it is always possible to find a frame / coordinate system in which isolated bodies move uniformly.

 

[ehild]

I have confusion on this Until it collided with something From where is the energy provided for it to produce a force on the object?

No need of energy to produce force. Just the opposite: The work of force adds energy to a body.

The force between the wall and ball does not come from nothing. You know that all materials are made of atoms. The atoms have the positive nucleus and the electron cloud around it, relative far from the nucleus. Imagine the ball gets very near to the wall. The electron clouds of the surface atoms of the wall and of the ball gets so close that the Coulomb force becomes appreciable between them. It is the force between like charges, repulsive. That force presses the surface atoms inward, but then the bonds between the atoms get shortened. It is like compressing springs, and a force opposite to the initial velocity of the ball is exerted to it.

ehild

 

[technician]

No, they don't. I have never seen a single statement of Newton's laws where all three refer to resultant forces. None of the first 5 hits on Google relate all three laws to resultant forces, nor does my Serway textbook.

Can you provide even one example where all three laws relate to resultant forces? If not then your statement is not simple fact, it is simply false.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
"3.Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body."

http://teachertech.rice.edu/Participants/louviere/Newton/law3.html
"for every force there is a reaction force that is equal in size, but opposite in direction"

http://csep10.phys.utk.edu/astr161/lect/history/Newton3laws.html
"I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. "

http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#nt1
"an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force"

http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
"in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object"

I am surprised by the hyperphysics wording. It is bad and I usually find this to be a great site.
It confirms to me that standard textbooks are the only reliable source of information.
Out of interest which of the following would you judge to be best:
1st law: An object will remain at rest or moving with constant velocity unless it is acted on by a resultant force
1st law ; An object will remain at rest or moving with constant velocity unless it is acted on by an external force.

 

[WannabeNewton]

I am surprised by the hyperphysics wording. It is bad and I usually find this to be a great site.
It confirms to me that standard textbooks are the only reliable source of information.
Out of interest which of the following would you judge to be best:
1st law: An object will remain at rest or moving with constant velocity unless it is acted on by a resultant force
1st law ; An object will remain at rest or moving with constant velocity unless it is acted on by an external force.

Neither of your statements capture the true implication of Newton's first law. What you have said is simply a result of Newton's second law as applied to vanishing net forces. Newton's first law has to do with inertial reference frames, as I have already said: There is a much more fundamental way to look at Newton's 1st law that has nothing to do with resultant forces: the assertion that inertial systems exist i.e. that it is always possible to find a frame / coordinate system in which isolated bodies move uniformly.

See Kleppner and Kolenkow "An Introduction to Mechanics", pages 55-56.

 

[technician]

Neither of your statements capture the true implication of Newton's first law. What you have said is simply a result of Newton's second law as applied to vanishing net forces. Newton's first law has to do with inertial reference frames, as I have already said: There is a much more fundamental way to look at Newton's 1st law that has nothing to do with resultant forces: the assertion that inertial systems exist i.e. that it is always possible to find a frame / coordinate system in which isolated bodies move uniformly.

See Kleppner and Kolenkow "An Introduction to Mechanics", pages 55-56.

Can we have a statement regarding Newton's first law that is appropriate for students in the first stages of studying physics...something they can cope with and something that will encourage them to want to find out more?
something along the lines of ...Nelkon&Parker, Duncan, Young&Freedman etc etc

 

[WannabeNewton]

Well 1. Kleppner is a first year text and 2. we can't downplay a law of physics and misrepresent its true meaning just for the sake of getting a newcomer interested. This is what leads to misconceptions about Newton's laws, misconceptions that fail to get removed unless he/she happens upon a good mechanics book like Kleppner (well Kleppner is IMO the best mechanics book out there, not merely good). Inertial reference frames are a pivotal part of Newtonian mechanics and the first law sets the stage for that.

 

[D H]

Out of interest which of the following would you judge to be best:
1st law: An object will remain at rest or moving with constant velocity unless it is acted on by a resultant force
1st law ; An object will remain at rest or moving with constant velocity unless it is acted on by an external force.

Some texts say the first, others the latter. The distinction is irrelevant. By focusing on this minutia you are missing the point of Newton's first law.

Can we have a statement regarding Newton's first law that is appropriate for students in the first stages of studying physics...something they can cope with and something that will encourage them to want to find out more?
something along the lines of ...Nelkon&Parker, Duncan, Young&Freedman etc etc

Rhetorical question: Since Newton's second law implies the first, why do we need a first law at all?

The answer depends upon historical perspective. Newton was vying against the age-old tradition of Aristotelian physics. Aristotelian physics held that the natural state of an object was at rest and that some force was needed to keep an object in motion. Newton's first law was a flat-out rejection of that age-old tradition.

We aren't fighting Aristotelian physics anymore, but we still teach Newton's first law. Why? Any good introductory physics text will state that Newton's first law in one of the two forms stated in your first quote, but it will also state that Newton's first law ultimately is a statement about inertial frames of reference. This latter statement is key. Newton's first law provides a framework for testing whether a frame of reference is inertial, and hence whether Newton's second and third laws do apply.

Just as Newton rejected Aristotelian physics, modern classical physics reject some minor points in Newton's physics. One of those minor points is Newton's concept of an inertial frame. Newton had a concept of an absolute frame of reference, God's frame if you will. (Newton was a deeply religious man, even by the standards of his own time.) That concept of absolute motion is not needed and is not used in modern treatments of Newtonian mechanics. What is needed is some way to determine if a frame of reference is inertial. That is what Newton's first law is all about from a modern perspective.

 

[technician]

Rhetorical question: Since Newton's second law implies the first, why do we need a first law at all?

Probably the most sensible, meaningful quote in this whole thread...could not agree more.
first and second laws could be stated together...first statement describes what happens when there is no RESULTANT force...second statement therefore must describe what happens when there is a RESULTANT force...doesn't get much simpler.

The statements could be reversed and it makes no difference. (unless somebody knows otherwise ;))

 

[WannabeNewton]

No it doesn't! Obviously if you plug in a zero net force in the 2nd law, you will get what happens when there is no net force so why in the world would you think it only applies when there is a non-vanishing net force? Maybe you need to go back and read a proper mechanics text as well. You are missing the point of the first law. I love how you ignored the bulk of D H's post, which clearly elucidates the true point of the first law.

 

[sophiecentaur]

That doesn't make a sense to me.How can some thing make a force out of nothing?

Reading all your questions and comments makes me think of the early (pre-Newton) ideas about forces and motion. He had a hard time convincing people that "things don't naturally slow down", for instance. It was assumed that things slow down unless 'helped' to keep going and this lead to the false conclusion that the planets had to be driven round in their orbits. He reconciled the two phenomena of unchanging planetary motion and the way wheeled carts always slow down by introducing the idea of friction as a force.

Newton 1 and 2 imply that there is no force on an object if it is not changing velocity in some way which really says that forces are only there when a mass changes its motion. The force only 'appears' during a collision (or when some field starts to act) so, to put it your way, a change of velocity 'makes a force out of nothing'. (I know this seems to be stated the wrong way round but I think it's the best you can hope for .)

 

[D H]

Thanks. May be in the first question there could be inertial force or something that keeps the object moving in the same way.And exerting forces on objects?

Others have interpreted that "inertial force" as talking about non-inertial frames of reference.

I'm going to assume you might be thinking along rather different lines, that some force is needed to keep an object in motion. This is after all how the world appears to work. Stop stepping on the accelerator and a car will coast to a stop. Slide a hockey puck on a long, smooth sheet of ice and it will eventually come to rest. At rest with respect to the Earth is apparently the natural state of motion for terrestrial objects. Per this viewpoint, some force is needed to alter that natural state.

That Aristotelian point of view was precisely what Newton was arguing against with his first law of motion. Newton's first law says that the natural state of motion of some object is to continue moving however it was moving in the past. A force is not needed to keep some object in motion. Contrary to what Aristotelian physics says, a force is needed to bring an object in motion to at rest with respect to the Earth. That car coasts to a stop because of aerodynamic drag and rolling friction. The hockey puck slows down and comes to a stop because of dynamic friction. Whenever some object undergoes a change in velocity there is necessarily a non-zero net force acting on that object.

 

[Bhumble]

ehild, I don't agree with your assertion that no energy is needed to produce a force. There is some potential or kinetic energy required for work to be performed and while it seems that there could be an ideal problem where there is a force with absolutely no motion that does not seem to be practical.

Can you think of any case where there is no energy but a force is exerted?

Thanks to D H and DaleSpam as I am one of those that missed the point of the first law and always saw it as a wordier way of expressing Newton's second law.

 

[Doc Al]

Can you think of any case where there is no energy but a force is exerted?

Rest a book on a table. The book exerts a force on the table without expending any energy.

 

[Bhumble]

Rest a book on a table. The book exerts a force on the table without expending any energy.

I'm not saying work must be performed just that there must be potential or kinetic energy for a force to be present.