# Most misunderstood physics concept

1. Mar 7, 2017

### Greg Bernhardt

What do you think is the most misunderstood concept in physics and why? I'm guessing it's something in QM or relativity, but maybe somewhere else?

2. Mar 7, 2017

### George Jones

Staff Emeritus
Misunderstood by whom? By the general public? By physics students? By physicists? Examples from any of the previous categories?

3. Mar 7, 2017

### Greg Bernhardt

I was thinking joe public but both might be interesting!

4. Mar 7, 2017

### Staff: Mentor

Well, those certainly are target-rich domains. Relativity has "Time slows down as you move faster" and "infaller never crosses the event horizon". Quantum mechanics has the endless misunderstandings generated by what I sometimes call the Copenpop Interpretation: "particle went through both slits", dead and alive cats, conscious observers collapsing wave functions, and the like.

Classical physics has its own conceptual hard spots: Newton's third law and the horse-cart problem; fictional forces and rotating motion; come to mind. However most people get through these fairly quickly once they see a competent explanation.

5. Mar 7, 2017

### OmCheeto

O.M.G.

Now I have to buy 3 more t-shirts......

ps. I don't even try to understand QM anymore, so my personal latest "most misunderstood" concept is entropy: "It's like a jigglyness per cubic meter, or something like that...."

6. Mar 7, 2017

### Staff: Mentor

(Att., opinion) I had to laugh, too, but please don't. It's a bit unfair with respect to the time it was created. Everything was in development and people tried to grasp the new ideas. To make fun of it almost a century later is a bit mean.
Yeah. Something with $- n \log n$, wasn't it.

7. Mar 7, 2017

### phyzguy

I'm not sure that you have to go to QM or special relativity to find widely misunderstood concepts. Two simple concepts that I find are widely misunderstood by the general public are:

(1) Newton's first law. Many people think that I have to keep pushing on an object to keep it moving.
(2) The difference between energy and power. I find even people working in the power industry don't get this right. Look at this link, for example. It says,

"On Thursday, the California Public Utilities Commission unanimously approved its proposed mandate (PDF) that will require the state’s big three investor-owned utilities to add 1.3 gigawatts of energy storage to their grids by decade’s end."

Say, what? Gigawatts is a measure of power, not energy. Storing 1.3 gigawatts is easy - I can probably hold a capacitor in my hand that will do that. Of course, it won't do it for very long....

8. Mar 7, 2017

### Rive

The most popular misunderstanging might be something else, but I think the most common will be something around mass <> weight.

9. Mar 7, 2017

### rootone

That 'a singularity', such as the one referred to in the case a black hole, is a physical entity with defined properties.

10. Mar 7, 2017

### Andy Resnick

Force. The overwhelming majority of people have a pre-Newtonian view of inertia and regularly confuse force with velocity.

11. Mar 7, 2017

### TurtleMeister

12. Mar 7, 2017

### Staff: Mentor

I'm not making fun of the Copenhagen interpretation, I'm making fun of the popularizers who to this day continue to misrepresent quantum mechanics.

13. Mar 8, 2017

### Devils

Something simple that students have trouble with is how rockets can move in space ie speed up or change direction, when there is nothing to "push" against. At this learning stage they probably know about Newton' laws, but need to understand conservation of momentum.

14. Mar 8, 2017

### RogueOne

I saw this "brain teaser" on facebook and was appalled by the amount of people who got the wrong answer. I am not sure where to begin when describing the misconceptions that I heard, but it was saddening. This is very obvious to anybody who understands any physics

Here is the riddle.

15. Mar 8, 2017

### Bandersnatch

I don't think it's that trivial.

16. Mar 9, 2017

### OmCheeto

As a subscriber to the Copenpop interpretation, I can assure you, I was not making fun of Quantum Mechanics.
Good lord. I just checked wiki, and it says that "Entropy" has the units of "energy/temperature".
So, this implies, that as I slow down in the winter, and my body temperature stays the same, my entropy goes up?*
I will never understand QM, nor entropy, as both seem to be described by incomprehensible maths.

(I spent the last two days trying to relearn how to derive an equation from a curve fit of a set of numbers. My spreadsheet gave me the answer; "It's a parabola, Om!", but I decided that I can no longer comprehend even the simplest of maths, to do it on my own. Fortunately, I can still add and subtract.)

*I promise not to start one of those whackadoodle threads: "Entropy is wrong!"

17. Mar 10, 2017

### TurtleMeister

I think what makes it non-trivial is that it is badly worded. I think what is actually meant is that the conveyor will match the ground speed of the aircraft in the opposite direction. Of course the aircraft can take off regardless of how fast the conveyor is moving, provided the ratings of the tires are not exceeded, because an aircraft does not depend on it's wheels for propulsion.

18. Mar 10, 2017

### Bandersnatch

Yes. Specifically, the way it's worded, any non-zero ground speed would make the conveyor try and accelerate to infinity. Then the question becomes: 'for realistic belt accelerations, can the plane take off before its wheels explode/break off/create more friction than the engines produce thrust?'

And to stay on topic, for the most misunderstood concept, I nominate Big Bang. It seems especially prone to breeding misplaced sense of understanding both in laymen and in people who should know better. I blame the name.

19. Mar 10, 2017

### Devils

I was reading about answers to the aircraft problem and the moving belt will interact with the air, and the air will move due to a surface effect. So the air will move a well as the belt. That's not simple.

20. Mar 10, 2017

### rootone

It might give the plane a tiny amount of lift, but no more than an average gentel wind, the plane stays on the ground.