Dot Physics

Tom and Jerry, or Fetch! with Ruff Ruffman?

Answer: Tom and Jerry.

What? Yes. The reason: Tom and Jerry has bad physics but does not pretend like it has good physics. I know this is probably going to “type-cast” me as “that guy that keeps attacking Ruff Ruffman.” Soon to be followed by “Leave Ruff ALONE!” I am sorry, I can’t help it. Here is my problem. If you are going be a show, do whatever you like (I might still make some comments). If you are going to be a show that attempts to teach some stuff, don’t you think you should get it correct or at least not reinforce bad ideas?

The bad idea: There is a force that keeps things moving.

You see this idea all over the place. Aristotle even had this idea. It is an idea that seems to make sense, however it is really wrong. I really don’t mind if people don’t have this idea right, just don’t teach this idea. Or…don’t say stuff that can reinforce these ideas if you are in fact trying to teach the correct thing. Here are some made up samples of things people say that are wrong. I hear these things a lot on Mythbusters and even sometimes of Ruff Ruffman.

  • When the force from the ball hits the wall….
  • This object moves with a lot of force.
  • The ball reaches a constant speed when the force of it moving balances the force of air resistance.

It’s not that all these are exactly wrong (well, mostly they are) it is that they imply the idea that a moving object “carries” a force with it.

Now, what about Ruff? What did he do this time to get me fired up? The episode was about something to do with hockey. Some of the kids were trying to make goalie padding – maybe? It started off fine. They had someone talking about what the gear does. 1) allows the interaction from the hockey puck to act over a larger area and 2) increase the time of the interaction. The time is increased by putting some squishy stuff in there. Good.

Next, it got a little odd. Some guy pulled out a Newton’s cradle (you know the clicky swinging balls thing). Not sure how this relates to the main point that I think they are trying to make. Typically Newton’s cradle is used to show that in an elastic collision, both momentum and kinetic energy are conserved. When a hockey puck collides with a person, yes momentum is conserved – depending on the system you choose. Really, this should be all about the following idea:

i-a1a574adec599d7c13f80191114c8153-momentuprinc.jpg

If the puck starts with the same speed and stops after the collision, then the change in momentum will be the same. Thus, if the time of the collision is increased, Fnet will be smaller.

So, bringing out the Newton’s cradle was bad, but in their pseudo explanation, they added this graphic:

i-54474ffda16b109a096f3fb8baef090a-newtoncradleforce.jpg

They were talking about momentum and then they show this double arrow for force? I really don’t get this. If I don’t get it, I doubt the kids get it. Maybe it would be better to just not show it? Right? Here is my interpretation of that double arrow. To me, it is saying “hey look, the force goes this way and then that way”. Or force in the direction of motion. This is a really bad diagram.

If the objective was to help kids understand the momentum principle – how about dropping eggs. Drop an egg on a cement floor – BOOM. Drop an egg on a big cushy pillow, no BOOM. Right? Same idea?

Comments

  1. #1 Rob
    March 25, 2009

    It would have to be early Tom and Jerry….

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