0.2 Thinking Like A Physicist

Critical Questions:

  • What does it take to really understand physics?

The crucial problem in teaching physics is that most people have a certain understanding of physical principles before they ever approach the subject in a classroom or book. The reason this is a problem rather than a benefit is that the average person’s understanding of physics is wrong.

To be more specific, there are two kinds of “understanding” one can have about physics. The first is the one that babies slowly gain as they teeter precariously on their pudgy little legs and try to manipulate solid objects with their hands and, occasionally, mouths. This is baby science in action: after a few hundreds trials, even an infant’s brain knows that if you push an object to the right, it will generally move in that direction.

"Baby Mum-Mum" by Joe Shlabotnik on Flickr
Experiment #745: Put Thing In Mouth. Results: Inconclusive.

By the time we’ve grown up, this understanding has solidified into that intuitive, unconscious awareness of the relationship between cause and effect which allows us to catch baseballs, flip pancakes, or juggle chainsaws.

The second kind of understanding is the one that physicists have been broadening and improving on for hundreds of years. And the funny thing about it is that it seems to contradict all of our everyday experiences about the world.

This is not as much of a problem in the other sciences, even ones that impact our daily lives. You can use toothpaste (created by a chemist) without having to first experiment with smoking chemicals in glass beakers. You can squish a mosquito against your arm without first memorizing its precise biological taxonomy. But you do need to spend every second of your life trying to survive in a world governed by physical laws, and that can create a deceptive familiarity.

Take, for example, a simple problem relating to motion. Imagine that you tape a golf ball to a ceiling fan, because that’s just the kind of person you are. You stand under this fan and watch the golf ball spinning around at high speeds. Suddenly, the tape comes off and the golf ball goes flying. Aside from the fact that the golf ball will fall downwards, how would you describe the ball’s motion as it flies through the air?

Fan with a golf ball taped to it
(Do not try this at home.) (Or… do?)

A. It curves towards the fan.
B. It travels in a straight line.
C. It curves away from the fan.

Most people would guess A.1 The correct answer, though, is B. The reason this seems contradictory to everyday experience is that when something is spinning, it usually keeps spinning.

Physics does more than offer a mere explanation for the golf ball’s behaviour; it offers a whole new perspective on the situation. When you think like a physicist, you no longer see a golf ball and some bits of tape. You imagine forces pushing the ball in different directions, energy shifting form and transferring from one object to another, and, if you’re really lucky, mathematical variables changing within the constraints of elegant formulae.

The first step towards this new awareness is to accept that everything you think you know about reality is probably wrong.

However, this should not be a terrifying prospect: instead, I suggest that it will liberate you from the dead weight your mind has accumulated. To truly learn physics is to approach the universe once again as a child — but this time, armed with the power of science.

Before we get into the really meaty stuff, you’ll need to to familiarize yourself with the language of physics. As you read this website, you will come across words which mean one thing in everyday language, but which in physics have either very different or much more specific definitions. Our first examples come from the world of motion, which is the subject of the first chapter.

Big Ideas:

  • When learning about physics, it is important to recognize and challenge your preconceptions about physical phenomena.
  • On this site, you will learn not just facts but also the technique of Thinking Like A Physicist.

Next: 1.1 – Introduction to Motion

Previous: 0.1 – Welcome

  1. If you know a little bit of physics, you probably guessed B. If you know a lot of physics and can accurately describe fluid dynamics and the Magnus effect, you might guess A, but in that case you may be reading the wrong website. If you guessed C, you probably had your reasons.

8 thoughts to “0.2 Thinking Like A Physicist”

    1. Well I really shouldn’t be discussing fluid dynamics this far in advance of Chapter 4, but:

      The golf ball will be spinning as it flies off from the fan. In the image above, the ball will spin counter-clockwise (when seen from below) as it moves forwards. That spin will deflect more air to the left side of the ball than to the right.

      This is much like what happens with an airplane’s wing, which deflects air up over the top edge. The deflected air moves faster and (by something called Bernoulli’s principle, which will come up in Chapter 4), has a lower pressure.

      So the ball has low-pressure air on its left and high-pressure air on its right. This results in it getting pushed to the left, which means it follows the path labelled ‘A’ in the image.

      This is the same effect that causes a curveball to drop quickly as it approaches the plate.

      1. C Liam Brown, as I look at the ceiling fan picture above I’m confused…if the fan is spinning in a “counter-clockwise” direction, then wouldn’t the golf ball spin off the fan in a “clock-wise” direction relative to the person standing beneath it? What force produces the spin?

        Thank you.

        1. I guess I was imagining a pretty idealized situation, where the golf ball just came free instantly. In that case, it would retain the same direction of spin as the fan. In the real world, there’d probably be a piece of tape or something that would catch on it and pull back one side, which might cause it to spin the other direction.

          Good point though.

  1. Thank you, C. Liam Brown, for creating this website. It makes physics so much more interesting than it already is.
    I just have one doubt in the ball-fan system. What about the centripetal force on the ball?

    1. Thanks, glad you like the site!

      “Centripetal” just means “pointing towards the center” – something moving in a circle requires a force (or a combination of forces) to constantly turn it in towards the center of that circle. In this case, the tape would provide a centeripetal force, but once the tape comes off, there is no more centripetal force, so the ball has nothing to do but move in a straight line.

    1. The tape might change the angle of the ball’s motion, but only the ball’s spin could cause a curved trajectory. The key point is that aside from some very minor fluid dynamics effects, once the ball leaves the fan, there’s nothing keeping it moving in a circular path. In the absence of forces, objects will always tend to move in a straight line.

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