Friction does not oppose motion!

You can’t, of course
walk or drive, with
no friction force.
Please tell your kith
and tell your kin:
You can’t begin
without friction.

So give up this
misconception
and you wont miss
its deception.
Your FBDs
you’ll find will please
your physics prof.

This was triggered by the Twisted Mouse’s post The Frictional Force of Stupidity (If you’ve never looked at his stuff – why the hell not?? It’s brilliant.).

But it’s mainly a response to years and years of trying to get students to give up the idea that “friction opposes motion”. It doesn’t!!! It opposes the sliding of one surface against another. Friction is the force that makes your car go forwards when you accelerate, so you don’t look like an idiot sitting at the lights spinning your wheels. It’s the force that allows you to walk. Ever tried walking on a really, really slippery surface? Your feet just go out from under you. Unless you’re jet-propelling yourself along (and I probably don’t want to know how you’re doing that), you need friction to move!

The internet is full of incorrect force diagrams (aka Free Body Diagrams or FBDs) showing the forces acting on a car, with friction the wrong way and a magical force due to the engine providing the force in the direction of acceleration. An engine does not exert a force on a car. Seriously, it doesn’t. It’s part of the car, so it can’t. (Ever tried pulling yourself up by your bootstraps? It only works metaphorically, not literally.)

Here is a correct Free Body Diagram (FBD) for a car accelerating forwards. Note that I’ve left off drag due to air, because at low speeds that force is (to use the technical term) buggerall. (Really, really small forces are termed buggerallsquared). This is for an all-wheel drive. Any wheels not providing a driving force have a small (buggerall) rolling friction acting on them, which would be better left off the diagram to avoid confusion.

Here endeth the lesson.  Any questions??? (Seriously, happy to take questions on this.)

23 Comments

Filed under poem, rants

23 responses to “Friction does not oppose motion!

  1. This is really well explained, I didn’t need the diagram even to be frank.🖤🖤 Lord but for a second I was so scared of what is FBD, because there are so many new acronyms coming up each day I can never keep track and I thought of a couple of hilarious ones like Friends Before Devil 🙈😅

  2. Thank you for the shout out!
    You are truly too kind.
    An excellent explanation and even better doodle.
    Next week, gravitational forces!!! 🤣

    • Oh good! I look forward to your take on gravitational forces!
      And I can easily do a rant on that too – in fact I did one yesterday to a group of students on the difference between weight and load and why even if I sit on you, my weight acts on me, not on you.

      • Oh no. I was talking about you 😉 my next science based post is on the 23rd, for mole day.
        I’m sure you could. Wouldn’t be worth your salt as a physics teacher if you couldn’t 😉

        That sounds rather threatening. Never had a teacher, even hypothetically, say they would sit on me 🤣

        • Well, I shall ponder on gravity, and look forward to your homage to mole day…
          I have never sat on a student. Not that I recall, anyway. I did have a colleague who would lie on a bed of nails and get a student to stand on him.

        • Gravity sounds good.

          I’ve seen that before. Squimish stuff to witness. On par with that, I once saw a man willing women to step on his genitals with high heels.
          Isn’t it a wonderfully fascinating world we live in.

  3. Pingback: Forces By Demons (FBDs) | anotherkatewilson

  4. What an enjoyable write. You seem to be a wonderful teacher going by the way you have explained friction here. Loved it. 😊

    • Thanks Goutam. So I shall assume that any students who fail today’s quiz do so because they have not paid attention, rather than blaming my teaching. 😀

      • Absolutely! There is a sense of philosophy in your explanation too….friction keeps things in check, else we would all be forever sliding without stability. Would have loved to be a student of a teacher who explains so beautifully. 🙏😊

  5. Certainly very disheartening! How can they not grasp this simple thing? They probably have never fallen on a slippery floor…hence unable to grasp. 😊

    • Unfortunately it’s often taught incorrectly at school. They only deal with cases like things sliding on surfaces and are told “friction opposes motion”. It gets lodged in their heads and can’t be removed. Then they extend that to all other cases. There are similar problems with Newton’s third and gravity/normal. I’ve done diagnostic tests on incoming first year uni students, and some of these misconceptions are held by 90% of them. I don’t know what it’s like elsewhere, but there seems to be a big problem with high school physics in Australia.

      • You are right….the emphasis is too much on the drag caused due to friction and probably not enough examples are put forward to explain the beneficial effects of friction.

  6. When I was in high school spinning tires at stop lights was a big thing for those with muscle cars (I walked). They thought they were “cool.” 😄 Wonderful lesson, prof! Now I’m going to pop in some “Forces by Demons” and slide into my night!

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