Solving an Unexpected Problem: Newton's Third Law

In summary, the conversation revolves around a problem given by the professor regarding the violation of Newton's third law in relation to two current wires. The question is raised and multiple attempts are made to solve it, with the conclusion being that the third law is not actually violated. However, the concept of considering the magnetic field from an infinitesimal current element in isolation is also brought up. The conversation ends with confusion about the representation of point P in the diagram.
  • #1
gazepdapi1
54
0
my professor kind of sprung this problem out of nowhere for the class, and I'm having just a little trouble getting started? The problem goes like this:
A current wire pointing in the +j direction has an Idl. A second current wire in the +i direction has a different Idl.
The question he proposed was, is Newtons third law violated?

http://img241.imageshack.us/img241/7699/untitledxy4.jpg
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
for the bottom wire the 2's should be one's, my mistake.
Can anyone give me a hint?
 
  • #3
Can you restate the problem?
 
  • #4
Here's a new picture of the problem, the first one was not drawn correctly.http://img110.imageshack.us/img110/9952/untitledey8.jpg
The question is, given the two current elements as shown above, with current in the direction of the arrow, is Newtons third law violated. This is what I got. B on 2 due to 1 is in the (+k) direction given by the right hand rule. F on 2 due to B on 1 = I2dl2 X B1 = I2dl2B1 (+j x +k) = (+i) (the numbers are subscripts and the x's represent cross products. B on 1 dues to 2, is 0 because the current is in the same direction as p, the point where we are trying to measure the B. (dl X r) = 0. Therefore there is no force on 1 due to 2. This means that Newton's third law is violated. But the professor told us that it only appears that it's violated and now we have to prove why its not violated.

Can anyone tell me why? I can't seem to figure it out.
thanks a lot, nertil
 
Last edited by a moderator:
  • #5
nobody knows
?
 
  • #6
This question is ill-posed as written because you can't consider the magnetic field from an infinitesimal current element in isolation. This point is made in every E&M book I'm aware of (see the discussion on the Law of Biot Savart).

If you're being asked to consider two infinitely long wires along i and j directions, of which just a little is pictured, then it's easy to show that the net force on each wire produced by the other is zero, and the third law holds as expected.

As for point P, I don't know what it's supposed to represent in your diagram.
 

1. What is Newton's Third Law of Motion?

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will exert a force back on the first object that is equal in magnitude but opposite in direction.

2. How does Newton's Third Law apply to solving unexpected problems?

In solving unexpected problems, Newton's Third Law reminds us to consider all forces involved and to anticipate the reactions to our actions. By understanding and applying this law, we can better predict and manage any unexpected consequences that may arise.

3. Can you give an example of Newton's Third Law in action?

A classic example of Newton's Third Law is a rocket launching into space. The rocket exerts a force downwards with its powerful engines, and this causes an equal and opposite reaction in the form of an upward force, propelling the rocket upwards.

4. How does Newton's Third Law impact our daily lives?

Newton's Third Law can be seen in many everyday situations, such as walking, driving a car, or throwing a ball. In each of these actions, we are exerting a force on the ground, the car, or the ball, and the ground, car, or ball are exerting an equal and opposite force back on us.

5. Are there any exceptions to Newton's Third Law?

While Newton's Third Law holds true in most situations, there are a few exceptions. For example, in some cases, the forces involved may not be equal or opposite due to factors such as friction or air resistance. However, these exceptions do not invalidate the overall principle of the law.

Similar threads

Newton's Third Law for book on a table
    • Introductory Physics Homework Help
Replies
8
Views
2K
Newton's third law problem/kinematics (Airboat problem/no friction)
    • Introductory Physics Homework Help
Replies
3
Views
3K
Newton's Third Law Problem: Two masses, a rope and a pulley
    • Introductory Physics Homework Help
Replies
21
Views
9K
Is Newton's third law valid for rotation / Torque?
    • Mechanics
Replies
23
Views
3K
The violation of Newton's third Law
    • Thermodynamics
Replies
28
Views
3K
Newton's Laws Homework question: Double incline mass problem
    • Introductory Physics Homework Help
Replies
4
Views
1K
Why can't I show Newton's Third Law by different means?
    • Introductory Physics Homework Help
Replies
5
Views
1K
I Acceleration in Newton's second law
    • Mechanics
Replies
4
Views
640
Two Questions on Newton Law Problems
    • Introductory Physics Homework Help
Replies
5
Views
2K
Assumption on central forces between two particles
    • Introductory Physics Homework Help
Replies
1
Views
1K

Hot Threads

Recent Insights

Back
Top