B What Are Common Misconceptions About Newton's Laws of Motion?

[sophiecentaur]

A rigid has no deformation.

The problem with the term 'rigid' is that it is not an intuitive term. It doesn't occur in real life and most misconceptions are based on intuitions about experiences of real life. The conservation laws that Science has invented just sidestep the problem of what goes on during ideal processes: situation before...event ... situation after. Until people are happy with ignoring the event itself, they get too involved with the nuts and bolts. The word 'intention' gets introduced and that, at least in my opinion, suggests consciousness in objects and that is madness and definitely not Scientific.
The confusion between Mass and Weight goes back to before Newton's time because experience was (and still is) dominated by the consequences of Weight as a force which will account for friction and not by Mass. The confusion is still with us. "Power Weight Ratio" is still used much more commonly than Power Mass Ratio.

 

[haruspex]

The problem with the term 'rigid' is that it is not an intuitive term. It doesn't occur in real life

True, but in post #19 I could have written "unpenetrated surface" instead of "rigid body", and gone on to say "penetration or further deformation". Just trying to avoid further rabbit holes in the thread.

 

[haruspex]

Post #5 refers to 'weight' being important...it is not...friction has to be introduced to cover up this misconception.

You seem to have lost your way in this particular rabbit hole. You were challenging my interpretation of post #5, not whether post #5 was crystal clear to all potential readers.

 

[gannordean]

Hi everyone,
When teaching for middle schoolers, I dealt with many sort of misunderstanding they "swirl in", like finding right from left sides in caclulating cos or sin! Or the famous "rounded number" dilemma when I ask for a fraction, when inserting the Pi number in a volume equation..OMG why does pi freak them out so muchOO! For some, an acid is the same as a base but with less "water" in it..??! I remember finding some doing multiplication's table when on exams, hilarious!
At junior, my friends didn't know what is the difference between the dot(scalar) "." and the start(vector) " x" product. Believe me, some even didn't learn it until doing their bachelor degree in mechanics! Also the "Braket" complex in QM. The weight vector pointing upwards for a departing airplane and downards when landing, exotic -400 C° in thermodynamic, equalling force and acceleration, equalling linear with angular frequency(forget the radian!),...etc
My favorite:
-Me: what is an electron?
-x : It's electricity not visible!
-Me: euh..ok..so then can you explain what's visible?
-y: electricity..?..
-...
That's it.
Samir.

 

[pgardn]

Intentionality is a legal and psychological concept, not a physical one.
If you want to think of objects having it, you could say a rigid body has the intention of not being penetrated by another object. This is actually useful. It tells you that the magnitude of the force is just sufficient to prevent penetration. In particular, it will be normal to the contact plane.

Yes.

It's prevalent when one first examines the subject which is why I mentioned it as being a problem.

It is not intuitive for students to think if I push you, you push me. It's a problem mentioned by Knight who has closely examined beginning physics and the problems associated with the ideas.

 

[pgardn]

This misconception arises, I think, because it is tempting to ask the 'how does it know?' question. When you push an object, there is always a transitional time, during which the two surfaces (your finger and the ball) deform until the (N3 pair of) forces reach a final value. The same sort of 'how does it know?" question is often asked about the currents and voltages around an electrical circuit. They are only there after a certain transitional delay, during which the various circuit elements respond to their individual situation

Yep.

 

[berkeman]

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