physics question

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.

  1. 👍 1
  2. 👎 0
  3. 👁 703
asked by Kim

Respond to this Question

First Name

Your Response

Similar Questions

  1. technology

    Need help on the question below. I have read all about all 3 laws but still don't understand if this would be 1st or 2nd law. "A ball is rolling in a straight line on a hardwood floor. A cat hits it from the right, so the ball

    asked by Brandi on March 14, 2014

  2. Relationship between Rotational & Linear Momentum

    Please imagine you have a chassis (mass M) that is placed on rails (it can move only to one direction i.e axis y). On the chassis centre we have placed a rod that can rotate for half cycle (180 degs) with a high angular velocity

    asked by Nikos on February 24, 2007

  3. Science

    10. __________ is a law which states that for every action, an equal and opposite reaction results. Newton’s first law of motion Newton’s third law of motion Bernoulli’s principle Newton’s second law of motion It is the

    asked by Ryder C. on June 13, 2019

  4. statistics

    Here are 6 measurements of the electrical conductivity of a iron rod: 10.08, 9.89,10.05,10.16,10.21,1011 The iron rod is supposed to have conductivity 10.1. Do the measurements give good evidence that the true conductivity is not

    asked by Anonymous on November 5, 2012

  5. Physics

    I have a few physics questions that I don't understand. Two are thermal expansion problems and the other is an ideal gas law problem. 1. Two rods, one made of iron and the other of aluminum, have the same initial length at 64.00

    asked by Alyssa on February 8, 2016

  6. physics

    An iron rod has a length of 1000 meters (or 1 km) at 0 degrees Celsius. If the rod must expand 10 cm to bridge a gap, to what temperature must the rod be heated?

    asked by chris on November 1, 2013

  7. physics

    An iron rod(a=11.8x10-6 C-1) has a length of 1000 meters (or 1 km) at 0 degrees Celsius. If the rod must expand 10 cm to bridge a gap, to what temperature must the rod be heated?

    asked by chris on November 1, 2013

  8. SCIENCE

    1. How does mass and acceleration affect force? (What happens to force if mass increases? What happens to force if acceleration increases? ) 2. What is the equation for Newton’s 2nd Law of motion? 3. How do you calculate mass

    asked by Khaliyl on February 3, 2019

  9. physics

    1.What if Newton’s Third Law did not exist?. In your response to this inquiry, think of some of the consequences if Newton’s Third Law were not true. Describe in detail at least two consequences that would result if the Third

    asked by maria on April 26, 2011

  10. physics

    Proponents of astrology claim that the positions of the planets at the time of a baby's birth will affect the life of that person in important ways. Some assert that this effect is due to gravity. Examine this claim with Newton's

    asked by cb on March 14, 2009

More Similar Questions