I have a question about a problem regarding Newton's third law.

a. An iron rod is held up by a magnet. The magnet is held up by a string (from a ceiling, let's say). Draw a free-body diagram for the magnet and a separate free-body diagram for the iron rod.

For the rod I have a gravitational force going downward, and magnetic force pointing upward and a contact force pointing downward.

For the magnet I have a gravitational force pointing downward and a tension force and contact for pointing upward.

Is this correct?

b. For each of the forces drawn, identify the corresponding force that completes the Newton's third law pair.

I know the two contact forces are pairs, but am unclear about the other pairs.

No. THe rod has gravity down, magnetic force up equal to the rods'weight. For the magnet, gravity down equal to rods weight, magnetic force up. THen for the string, magnetic and rod weight down, and tension up equal to that weight.

I don't agree with Bob's answer here. The iron rod is not levitating here (it's theoretically possible, but I don't think that is meant here), so we must assume that it is in contact with the magnet.

"For the rod I have a gravitational force going downward, and magnetic force pointing upward and a contact force pointing downward."

This is correct.

"For the magnet I have a gravitational force pointing downward and a tension force and contact for pointing upward."

Here you forgot the magnetic force exerted by the rod on the magnet. This force is pointing downward.

Besides the tension forces you have the magnetic forces as third law pairs.

If you consider the other forces (the tension force and the gravitational force), you must include the objects in which that force is exerted.

So, the tension force exerted by the sring is on the magnet is opposite to the force exerted by the magnet on the string.

The gravitational force exerted by the Earth on the rod is opposite to the gravitational force exerted by the rod on the entire Earth. Sounds strange but it is true! And the same is true for gravitational force exerted on the magnet.

Now for the complete pairings:

- Gravitational force on the rod (downward) is paired with the gravitational force on the Earth (upward).
- Magnetic force on the rod (upward) is paired with the magnetic force on the magnet (downward).
- Contact force on the rod (downward) is paired with the contact force on the magnet (upward).
- Tension force on the string (upward) is paired with the tension force on the magnet (downward).

So, we have four pairs of forces in accordance with Newton's third law. Keep in mind that this is a simplified explanation and assumes no other external forces acting on the system.

To summarize the forces and their corresponding pairs:

For the iron rod:
- Gravitational force points downward.
- Magnetic force points upward.
- Contact force points downward.
- The gravitational force exerted by the Earth on the rod is the corresponding force to the gravitational force exerted by the rod on the Earth.

For the magnet:
- Gravitational force points downward.
- Magnetic force points upward.
- Contact force points upward.
- The magnetic force exerted by the rod on the magnet is the corresponding force to the magnetic force exerted by the magnet on the rod.
- The tension force exerted by the string on the magnet is the corresponding force to the tension force exerted by the magnet on the string.

Remember, according to Newton's third law, every force has an equal and opposite force acting on a different object.

b. Now, let's identify the corresponding force pairs for each of the forces drawn:

- For the contact force on the rod (pointing downward), the corresponding force is the contact force on the magnet (pointing upward). These two forces are action-reaction pairs according to Newton's third law.

- For the contact force on the magnet (pointing upward), the corresponding force is the contact force on the rod (pointing downward). As mentioned earlier, these two forces are action-reaction pairs.

- For the gravitational force on the rod (pointing downward), the corresponding force is the gravitational force on the Earth (pointing upward). These forces form an action-reaction pair based on Newton's third law.

- For the gravitational force on the magnet (pointing downward), the corresponding force is the gravitational force on the Earth (pointing upward). Again, these forces form an action-reaction pair.

- For the magnetic force on the rod (pointing upward), the corresponding force is the magnetic force on the magnet (pointing downward). These two forces are action-reaction pairs as well.

So, in summary, the contact forces and the magnetic forces in the system have corresponding forces that complete the Newton's third law pairs.