What are the dominant forces in different situations?

[Wes Turner]

I volunteered to help tutor students in a nearby high school in math and science. I got two seniors from an AP Physics class. I'm doing OK with most of the problems, but it's been almost 50 years since my last physics class, so I'm more than a little rusty in some details. I'm hoping I can get some help here from time to time.

In the tutoring sessions today, one of the kids brought in a test question that she got wrong. The question asked the students to describe the situations where gravity is the dominant force and explain why.

In a previous handout, the teacher had said that gravity is 30 orders of magnitude weaker than the electromagnetic force, 29 orders of magnitude weaker than the weak nuclear force, and 37 orders of magnitude weaker than the strong nuclear force.

Here's what I think the correct answer is. I'd appreciate any corrections or additions.

Gravity is dominant when the masses and/or distances are large. Gravity dominates the electromagnetic force because the EM force is usually neutral for large masses, like the earth, which have roughly equal numbers of electrons and protons. Gravity dominates the strong force because the strong force acts over very short distances (less than the radius of the nucleus). I don't really understand what the weak force is, so could not answer that part of the question.

Any help anyone?

Thanks

PS: I checked the tickbox saying that I used the template, but I'm not sure I really did. This question did not seem to fit the template well. Should I post this in some other section?

 

[mfb]

I think it would have fit into the template, but it is clear what the question and the suggested answer are.

Gravity is dominant when the masses and/or distances are large. Gravity dominates the electromagnetic force because the EM force is usually neutral for large masses, like the earth, which have roughly equal numbers of electrons and protons. Gravity dominates the strong force because the strong force acts over very short distances (less than the radius of the nucleus). I don't really understand what the weak force is, so could not answer that part of the question.

Sounds fine to me.
The weak interaction has a very short range as well. Its main relevance in today's universe is in beta decays and proton-proton fusion (without that process the Sun wouldn't be a star).

 

[haruspex]

The weak and strong forces are mediated by particles of mass, so have limited range.
As you say, at the large scale bodies tend to be electrically neutral, because if they were not they would be very powerfully attracted to oppositely charged bodies. But that leaves open two issues: why is the universe as a whole electrically neutral; why isn't there some residual charge that at least would exert forces similar in strength to gravity?
The answer to the first is probably connected with the big bang. For the second, maybe it's because any differences would long since have been neutralised by interstellar ions.

 

[FactChecker]

I would be careful about your comparison between gravity and electromagnetic force. The difference is so great that even a small scale electromagnetic force is stronger than the gravitational force from a large mass. For instance, a bowling ball dropped from 10 feet is instantly stopped by concrete. So even the gravity of the mass of the Earth does not overpower the small, local electromagnetic force of the concrete at the bowling ball contact point.

 

[Wes Turner]

The weak interaction has a very short range as well. Its main relevance in today's universe is in beta decays and proton-proton fusion (without that process the Sun wouldn't be a star).

So neither the strong or weak forces obey Newton's or Coulomb's inverse square laws, right?

 

[Wes Turner]

The weak and strong forces are mediated by particles of mass, so have limited range.

What does "mediated by particles of mass" mean?

As you say, at the large scale bodies tend to be electrically neutral, because if they were not they would be very powerfully attracted to oppositely charged bodies. But that leaves open two issues: why is the universe as a whole electrically neutral; why isn't there some residual charge that at least would exert forces similar in strength to gravity?
The answer to the first is probably connected with the big bang. For the second, maybe it's because any differences would long since have been neutralised by interstellar ions.

That is a bit beyond the scope of this high school class, for which I am grateful.

Thanks for the help

 

[Wes Turner]

I would be careful about your comparison between gravity and electromagnetic force. The difference is so great that even a small scale electromagnetic force is stronger than the gravitational force from a large mass. For instance, a bowling ball dropped from 10 feet is instantly stopped by concrete. So even the gravity of the mass of the Earth does not overpower the small, local electromagnetic force of the concrete at the bowling ball contact point.

Wow! That is an excellent point. I think the bowling ball on concrete image will really help them visualize the difference in the magnitude of the forces. Thanks.

 

[haruspex]

What does "mediated by particles of mass" mean?

It used to be mostly philosophers who bridled at the "action at a distance" concept. Turns out they were right. Modern "gauge" theories say that forces are mediated (transmitted) by virtual particles.
Electromagnetism is considered to be mediated by photons, which, after all, are just bundles of electromagnetic energy. According to quantum theory energy is uncertain over short times, so particles can pop in and out of existence (usually in opposite pairs to fit with conservation laws). Thus, "empty" space is now thought of as filled with virtual particles. A disturbance to which such a particle is sensitive can thus be transmitted to neighbours.
If the virtual particles are massless, the transmission is at light speed and knows no bounds. But the particles that mediate the weak and strong forces do have mass, which severely limits their ranges.
Is gravity also mediated by virtual particles? Is there a graviton, or is it made redundant by Einstein's curved spacetime? That's beyond my expertise.

That is a bit beyond the scope of this high school class, for which I am grateful.

Of course, but I feel the usual "large bodies are electrically neutral" explanation a bit glib. There has to be a reason that they are so nearly neutral that gravity wins on the large scale. Some bright student might point this out.

 

[mfb]

The weak and strong forces are mediated by particles of mass, so have limited range.

Gluons are massless. I know what you mean, but that is beyond the scope of this thread I think.

So neither the strong or weak forces obey Newton's or Coulomb's inverse square laws, right?

Right.

What does "mediated by particles of mass" mean?

The electromagnetic interaction can be described with the interaction of photons. Photons are massless. The weak interaction can be described with the interaction of W and Z bosons, both have mass.
In particle physics there is a relation between the mass of these particles and the range of the interaction - no mass leads to infinite range, large masses lead to short ranges.
But that is beyond the scope of this thread as well.

@haruspex: This is like a [b]-level thread. Don't start with QFT please.

 

[Wes Turner]

I want to thank everyone for their contributions. I hope I can help these kids do a little better on their finals next week. I also hope that don't confuse them with concepts that are beyond the scope of the class.

 

[Wes Turner]

I would be careful about your comparison between gravity and electromagnetic force. The difference is so great that even a small scale electromagnetic force is stronger than the gravitational force from a large mass. For instance, a bowling ball dropped from 10 feet is instantly stopped by concrete. So even the gravity of the mass of the Earth does not overpower the small, local electromagnetic force of the concrete at the bowling ball contact point.

Wouldn't your bowling ball example be more relevant if the bowling ball were just sitting on the concrete floor. When you drop it, we have momentum to deal with. If it's just sitting there, it's just gravity vs EM.

And along those lines, we could mention a fully loaded 747 sitting on the tarmac. The tires compress a little, but it doesn't sink in. Or the space shuttle sitting in the truck that carries it to the launching pad.

 

[FactChecker]

Wouldn't your bowling ball example be more relevant if the bowling ball were just sitting on the concrete floor. When you drop it, we have momentum to deal with. If it's just sitting there, it's just gravity vs EM.

And along those lines, we could mention a fully loaded 747 sitting on the tarmac. The tires compress a little, but it doesn't sink in. Or the space shuttle sitting in the truck that carries it to the launching pad.

I agree. The drop is only for dramatic effect, and is misleading.

 

[Wes Turner]

I agree. The drop is only for dramatic effect, and is misleading.

It is definitely dramatic. Thanks.