# Newton's first law

Gold Member
An object is travelling at constant speed.There is no force on it(It means it wont 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?

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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
Mentor
An object is travelling at constant speed.There is no force on it(It means it wont 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.

Gold Member
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.

Gold Member
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 doesnt make a sence to me.How can some thing make a force out of nothing?

WannabeNewton
That doesnt make a sence 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
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2020 Award

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.

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.

Gold Member
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

Gold Member
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
Mentor
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. Gold Member
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
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2020 Award
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
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2020 Award
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.

Gold Member
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?

Gold Member
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

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 ;)

Gold Member
Can someone explain #18?

Doc Al
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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!

Gold Member
What about Inertia on 18#?Doc Al?

Doc Al
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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
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2020 Award
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.

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