Dot Physics

People say I am picky. Ok, sometimes I am. But somebody has to stand up for what is right and just. Maybe I am that person.

Please stop using the word force if you don’t know what it is.

There. I said it. You can attack me now.

It wasn’t just one thing that got me fired up. It was two things. First, I read this article on physics and football (Physics of ‘The Hit’ from the NY Times). If it was just this article, I would have let it go and moved on. But no. One of my kids just happened to be watching MythBusters (We all love MythBusters) and there was a discussion that used the term force. I have to react.

I am going to point out a couple of the misuses of the word force in the NY Times article. One, because I don’t want to attack the MthBusters and second – the NY Times article is online. Oh, a disclaimer. Some of the quotes are quotes of other people and not necessarily the author. Still, why include wrong stuff?

The main point of the article is to examine collisions and tackling in football. In particular, this refers to amazingly hard hit near the end of the Steelers-Ravens play off game.

What is force?

I think the most important thing to remember about forces are that they are an interaction between two objects. The most important thing to NOT remember is that force is a property of an object. Let me repeat. Force is NOT a property of an object. If you want more details on forces here is a post about forces.

“Force equaled mass times acceleration. Momentum was conserved. And the bodies finally came to rest,…”

“Force equaled mass times acceleration”? That doesn’t even make sense. Then saying momentum was conserved and then they came to rest implies that momentum was NOT conserved. If you want to talk about force, the most import thing is that the NET force changes the momentum. Here is my post on collisions and conservation of momentum.

“impart the force of a deadly car crash..”

Maybe this isn’t wrong, but to me this implies that force is transfered from one player to another.

“Mass is the players’ weight, “

Me being picky again. Weight is the gravitational force on an object. Mass is essentially the number of electrons, protons and neutrons in an object.

“that has tremendous effect on the force of a football collision. Hard objects repel each other quickly; equally heavy but softer objects have “give” that allows their contact to last longer and accept the force less jarringly.”

True. If the change in momentum is the same, a longer time of interaction gives a smaller force. However, this seems to again apply force is transfered – “accept the force”?

“Which is why trying to run over the most massive running backs, from Earl Campbell to Brandon Jacobs, is asking for your action to get an equal and opposite (not to mention embarrassing) reaction,”

Maybe this is wrong on purpose. The whole action-reaction thing is just a carry over from old middle school science texts. Newton’s 3rd law should really be: “forces are an interaction between two objects”.

Ok, that is enough. I feel better now. Maybe it wasn’t as bad as I thought.

Comments

  1. #1 C. Chu
    February 4, 2009

    As a student of engineer, it makes me chuckle when people talk about the centrifugal force.

    It makes me seethe when my aerodynamics professor talks about the centrifugal force.

    :(

  2. #2 Polonius
    February 4, 2009

    “Mass is the players’ weight,”

    I have quite a lot of sympathy with this view. To a lay person, when you weigh a thing, it seems you are determining an intrinsic property of that thing. So the lay person’s concept of “weight” is quite close to the physicist’s “mass”. And when a heavy thing hits you, it hits harder than a light thing, so the lay person understands that inertial mass and gravitational mass are the same thing, but he still thinks of that thing as “weight”. It’s unfortunate, but we’re stuck with the confusion.

  3. #3 Anonymous Coward
    February 4, 2009

    1) ““Force equaled mass times acceleration”? That doesn’t even make sense.”

    I know that F=ma is usually discussed using the present tense, why wouldn’t it also be true in the past tense? Surely you’re not suggesting that Newton’s law was turned off for the game?

    2) ““impart the force of a deadly car crash..” Maybe this isn’t wrong, but to me this implies that force is transfered from one player to another.”

    Now it just seems like you’re trying to read in inaccuracies where there aren’t any. If the reader knows what force is, this sentence couldn’t possibly imply that.

    3) “However, this seems to again apply force is transfered – “accept the force”?”

    Yeah, that bugs me too. Someone needs to work on getting the word “impulse” into the vernacular.

  4. #4 Brian Anderson
    February 6, 2009

    “It makes me seethe when my aerodynamics professor talks about the centrifugal force”

    it’s a force. or at least a pseudo force resulting from being in a rotating frame of reference. same thing really. only difference is it doesn’t transform the same way, being a triple vector product. although even gravity is a pseudo force, so i guess i don’t see the problem in just calling it a force.

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