Exploding Anvils With Incorrect Physics

I feel bad for pointing out errors in TV shows. I have worked with this kind of stuff before and I understand that this isn't the simplest job. Some of these ideas (like forces and momentum) aren't so straightforward. Just think, students study this for a whole semester in college and then they still don't understand it fully. Now skip to a show. You have 30 seconds to explain conservation of momentum. GO!

I've already talked about these media-based physics difficulties before: Media Guide for Physics. Look at rule number 1: Don't Be Wrong. This is followed by rule number 2: It's Better to Say Nothing Than to be Wrong. Generally, I aim my explanations for "not technically wrong". Of course, you can't get what you want. But sometimes, you get what you need. In one particular case, I said that you can't describe a force in terms of kilograms. In the end, I lost. Oh well.

Now for the example. I hate to even point out the show, but I will. This is from Outrageous Acts of Science. There, I said it. In fact, it was from the episode regarding exploding anvils.

The basic idea of anvil shooting is to first put an anvil upside down on the ground. Next, add some gunpowder into a cavity at the bottom of the anvil. Finally, put another anvil on top and light the fuse. Boom. The top anvil goes up into the air. Great fun.

The Explanation

Let's take a look at the explanation on Outrageous Acts of Science. They start with this:

"Every action has an equal and opposite reaction."

I've looked at this version of Newton's 3rd law in the past. The problem is that people use this phrases ALL the time without really understanding it. First, I think that Newton was a pretty smart human. However, this "action" and "reaction" might not mean what you think it means. The phrase works much better if you replace "action" and "reaction" with "force".

Ok, back to the show. Here is the explanation for the shooting top anvil.

"Now that gunpowder, when it is lit produces gases and they try to shoot out in every different direction. Now they can't push the Earth, it's too massive. The only thing that's going to give in the system is the anvil. So what happens - instead of the Earth moving, the anvil shoots far up into the air."

There is also a diagram that looks something like this:

Well, the actual graphic on the show looked like two anvils instead of two blocks.

What's Wrong?

Is the Earth too massive to move? Absolutely not. Is it too massive to notice its motion, yes? This is big problem. If you go into an introductory physics classroom, you might hear a student say something just like Outrageous Acts of Science. They like this idea of one thing being too big to move. In fact, any force will change the momentum of an object. Even better - the change in momentum of the Earth and the anvil have the same magnitude. However, since the Earth has a very large mass, this change in momentum means it has a very small change in velocity.

This is very similar to the popular calculation of the recoil speed of the Earth if everyone jumped at the same time. Although, I must confess that the xkcd version of this question is much better than mine.

What about the diagram that they show? Well, it's not super bad but it's just regular bad. The one good thing they show is that there are equal and opposite forces. However, if they would have just used one force on each anvil instead of these side forces it would have been better.

A Better Explanation

This is the part where I write the best possible explanation I can. I'm not going to do that. Instead, I will leave this as a homework assignment for you. See if you can create a diagram and a short explanation to go along with the shooting anvil. See if you can make it shorter than 30 seconds. Share it with your class or with your friends. Or maybe make a short video and post it online.