A new season of MythBusters has started. Unfortunately, I already have a complaint.
In this week's episode, the MythBusters were seeing if they could make a merry go round spin by hitting it with a bullet. By the way, this is almost identical to a problem from one of my favorite textbooks - Matter and Interactions.
Let me just get to my complaint even though I have said this before. MythBusters: Please just leave out the physics explanations. Your show is fine without them and you are making a mess of the physics. Just stop.
If you watch their explanations, the following ideas are presented (and these are ideas that do not agree with evidence or Newtonian physics). Here is a clip, in case you missed it.
Mistake 1: Kinetic energy is transferred in a collision
This is a tough one. Suppose I shoot a pool ball such that it collides with another ball. In this collision, the original ball stops and the target ball recoils with the same speed. Doesn't this transfer kinetic energy? I guess it depends on how you look at it. If you use the work-energy principle:
Then the target ball increase in kinetic energy because the launched ball exerts a force on it over some distance. It may look like I am splitting hairs, but I think it is an important difference. Suppose the you shoot bullet into some clay and it stops. The clay recoils a bit, but it doesn't have as much kinetic energy as the bullet.
Mistake 2: In a collision, kinetic energy is more important than momentum.
Suppose two objects collide. One object is a ball of clay and the other is a steel ball. Here is a diagram before and during the collision.
During this collision the clay (labeled A) and the steel ball (labeled B) exert forces on each other. Since a force is an interaction between two objects, the magnitudes of these forces are the same. The time these forces interact is also the same. Using this and the momentum principle (which says forces change the momentum of an object) I can say:
This is essentially where conservation of momentum comes from.
What about kinetic energy? Since both balls exert a force over a distance, they both do work on the other ball. However, the distances do not have to be the same. The steel ball can move farther (or less) than the clay ball. So the work done on B by A does not have to the same as the work done on A by B.
My point: when you are dealing with collisions, momentum is conserved if there are no external forces (or if the time of the collision is so short you can ignore the external forces). Kinetic energy is only conserved in special cases.
Mistake 3: Inertia is what keeps an object from moving
Consider a force (and only one force) exerted on an object. What will happen? If the object is at rest, it will start to move and speed up. If the object is already moving, the force will either change the speed, change the direction of motion, or both.
Now consider a single force with a magnitude of 1 Newton. Suppose this exact same force is on two different objects - a 1 kg block and 2 kg block.
Both objects will increase in speed. However, the 1 kg block will have a greater increase in speed. Inertia is the property of matter for which the change in speed is inversely proportional. Greater the inertia, smaller the change. So, inertia doesn't keep it from moving, it makes the change in speed smaller.
Mistake 4: You need a certain amount of energy to overcome inertia
In this case, the MythBusters are thinking about energy like money. Suppose you want to buy some huge boxes of cereal that they have at these discount stores. To buy this, you have to be a member and pay a fee. For the MythBusters, inertia is like the fee to get in the store. Once you pay it (overcome it), it doesn't matter.
Really, the first problem is that they are comparing energy and inertia. You can't make a comparison between two things that are different. I guess that is really the biggest problem with this idea.
Mistake 5: Force is a property of an object and can be transferred
They actually say this a lot. It will be something like this:
Or
Actually, force is an interaction between two objects. It is similar to distance. What is the distance of New York City? Wait, that doesn't make sense, right? But that is essentially what the MythBusters are saying about force. Instead, distance is not an attribute of one location. You could say "what is the distance between New York and Chicago". But it isn't a property of one thing. Force is the same way.
Oh, and since force isn't a property of an object, it wouldn't make sense to say it is transferred. Right?
How to fix this?
Like I said before, this is an easy thing to fix. Just don't formally try to explain things. In the early episodes of MythBusters, they just did cool stuff. There were no official "science" explanations. Oh sure, Adam and Jamie would say stuff and say it wrong. But in these cases, they were just being normal people.
Why did they start adding in science explanations? Who knows. If they really want to keep these in the show, maybe they should just send me a note. I would be happy to look over any of their stuff before hand. Honestly, I would be happy. Oh, and I am still a big MythBusters fan. Just to be clear.
