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"