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

[Beanyboy]

Was wondering if any Physics teachers would care to share just one common misconception that arises when students encounter Newton's Laws of Motion. I'm a learner and want to watch out for some of the pitfalls.

Appreciate your help,
Beany

 

[haruspex]

Was wondering if any Physics teachers would care to share just one common misconception that arises when students encounter Newton's Laws of Motion. I'm a learner and want to watch out for some of the pitfalls.

Appreciate your help,
Beany

Take a look at the "entire list ..." pull-down at https://www.physicsforums.com/insights/frequently-made-errors-vectors-elementary-use

 

[Beanyboy]

http://www.acarindex.com/dosyalar/makale/acarindex-1423880487.pdf

Haruspex,
Many thanks for the link. Very much appreciated. Above is a link to some research done in Thailand looking at the confusions and misconceptions that arise with vectors.

Thanks again,
Beany

 

[pixel]

Judging by the number of recent posts here, there is often confusion about the 3rd law that goes like this: A person pushes on a block, the block exerts an equal and opposite force on the person, so why does the block move? The resolution comes with the understanding that the motion of the block is determined by the net force on the block; it's reaction force on the person is not relevant to is motion.

 

[phinds]

Judging by the number of recent posts here, there is often confusion about the 3rd law that goes like this: A person pushes on a block, the block exerts an equal and opposite force on the person, so why does the block move? The resolution comes with the understanding that the motion of the block is determined by the net force on the block; it's reaction force on the person is not relevant to is motion.

If the block is heavy enough, it's the person that moves (at least to the length of their arms ... possibly farther if they are on a slippery surface)

 

[pgardn]

1. The direction of an acceleration and thus a net force.

ex. A ball is thrown up. When it is at its maximum height and v = 0 is there a net force? Yes. Then of course what is happening while the ball is in the hand while throwing it up and then catching it.

2. If an object is moving a net force must be acting on it. Not true of course. Ex. I apply a horizontal force with my finger to a block on a rough table and it moves at a constant velocity.

If one goes through the entire process of applying the finger to the block and it does not move, to applying enough force with the finger so that it BEGINS to move. To moving at a constant velocity. To removing the finger and noticing that the block slows down and stops. To asking why the block does not return to its original position like presented in #1 (i.e. friction can be confusing)

Then make the table perfectly smooth and ask the same questions as the above and realizing that the only time the block should move at a constant v is when the finger is not applying a force.

3. Then the proper application of Newtons laws to a free body diagram. Texts have different rules for making free body diagrams and it can become a mess after the student becomes use to a method meant for fairly simple situations. ( Tension and friction can be confusing)

4. Circular motion and Newton's laws. Changing direction but not speed requires a net force.

5. The whole idea that an applied force can be countered by another force that has no intentions.

Ex. A box on a table. You push down on it with your finger. How does the table know to push back up... Intention based thinking.

 

[Andy Resnick]

Was wondering if any Physics teachers would care to share just one common misconception that arises when students encounter Newton's Laws of Motion. I'm a learner and want to watch out for some of the pitfalls.

Appreciate your help,
Beany

If I can only list one, I would say "confusing the concepts of 'force' and 'velocity' (e.g. 'a force causes an object to move')". If I could list another, it would be the confusion that force resides within, or is a property of, an object ('one object imparts a force onto another')

 

[Beanyboy]

Judging by the number of recent posts here, there is often confusion about the 3rd law that goes like this: A person pushes on a block, the block exerts an equal and opposite force on the person, so why does the block move? The resolution comes with the understanding that the motion of the block is determined by the net force on the block; it's reaction force on the person is not relevant to is motion.

So, if I've understood you then, we learners struggle initially with the concept of "net force"?

 

[Beanyboy]

Thanks ever so much. I'm particularly struck by number 5. Forces being applied without obvious intention. I've encountered this confusion myself. I suppose this is more to do with the meaning attached to the word as used by scientists compared to everyday usage.

1. The direction of an acceleration and thus a net force.
ex. A ball is thrown up. When it is at its maximum height and v = 0 is there a net force? Yes. Then of course what is happening while the ball is in the hand while throwing it up and then catching it.

2. If an object is moving a net force must be acting on it. Not true of course. Ex. I apply a horizontal force with my finger to a block on a rough table and it moves at a constant velocity.

If one goes through the entire process of applying the finger to the block and it does not move, to applying enough force with the finger so that it BEGINS to move. To moving at a constant velocity. To removing the finger and noticing that the block slows down and stops. To asking why the block does not return to its original position like presented in #1 (i.e. friction can be confusing)

Then make the table perfectly smooth and ask the same questions as the above and realizing that the only time the block should move at a constant v is when the finger is not applying a force.

3. Then the proper application of Newtons laws to a free body diagram. Texts have different rules for making free body diagrams and it can become a mess after the student becomes use to a method meant for fairly simple situations. ( Tension and friction can be confusing)

4. Circular motion and Newton's laws. Changing direction but not speed requires a net force.

5. The whole idea that an applied force can be countered by another force that has no intentions.

Ex. A box on a table. You push down on it with your finger. How does the table know to push back up... Intention based thinking.

If I can only list one, I would say "confusing the concepts of 'force' and 'velocity' (e.g. 'a force causes an object to move')". If I could list another, it would be the confusion that force resides within, or is a property of, an object ('one object imparts a force onto another')

1. The direction of an acceleration and thus a net force.

ex. A ball is thrown up. When it is at its maximum height and v = 0 is there a net force? Yes. Then of course what is happening while the ball is in the hand while throwing it up and then catching it.

2. If an object is moving a net force must be acting on it. Not true of course. Ex. I apply a horizontal force with my finger to a block on a rough table and it moves at a constant velocity.

If one goes through the entire process of applying the finger to the block and it does not move, to applying enough force with the finger so that it BEGINS to move. To moving at a constant velocity. To removing the finger and noticing that the block slows down and stops. To asking why the block does not return to its original position like presented in #1 (i.e. friction can be confusing)

Then make the table perfectly smooth and ask the same questions as the above and realizing that the only time the block should move at a constant v is when the finger is not applying a force.

3. Then the proper application of Newtons laws to a free body diagram. Texts have different rules for making free body diagrams and it can become a mess after the student becomes use to a method meant for fairly simple situations. ( Tension and friction can be confusing)

4. Circular motion and Newton's laws. Changing direction but not speed requires a net force.

5. The whole idea that an applied force can be countered by another force that has no intentions.

Ex. A box on a table. You push down on it with your finger. How does the table know to push back up... Intention based thinking.

 

[lychette]

If the block is heavy enough, it's the person that moves (at least to the length of their arms ... possibly farther if they are on a slippery surface)

I think you mean 'massive' enough.
I don't think weight is part of this expanation?

 

[phinds]

I think you mean 'massive' enough.
I don't think weight is part of this expanation?

I'm thinking of it sitting on a rough surface, so that its friction with the surface is what causes the human to move instead of the block, in my example. In this consideration, weight clearly does matter. With no weight, both objects always move, proportional to their weight and the force.

 

[lychette]

I'm thinking of it sitting on a rough surface, so that its friction with the surface is what causes the human to move instead of the block, in my example. In this consideration, weight clearly does matter. With no weight, both objects always move, proportional to their weight and the force.

I was thinking in a more general physics sense, such as in space outside the space station with a massive object or on ice with something massive...mass is the important property in general...not weight.
misconceptions in this area arise by confusing 'weight' with mass

 

[Isaac0427]

I was thinking in a more general physics sense, such as in space outside the space station with a massive object or on ice with something massive...mass is the important property in general...not weight.
misconceptions in this area arise by confusing 'weight' with mass

The OP should, IMHO, not go overboard with this. My former science teacher, in an attempt to get people using mass instead of weight, actually used mass all the time, even when weight was the correct term. For example, she said something like "objects with more mass are heavier," when how heavy an object is has to do with weight. Whether this is to technical or not, instead of saying "mass is generally the correct term" as I viewed the post implied, just know the difference between mass and weight.

Also, learn the "second part" of Newton's third law-- third law pairs must be the same type of force (i.e. gravitational, tension, contact, electric, etc.), which will show you that the normal force (a contact force) is not the third law pair of mg*cosθ (a gravitational force), though they are equal and opposite. There are many misconceptions about the third law, many of which have been mentioned. I'd pay extra close attention to all of them.

 

[lychette]

The OP should, IMHO, not go overboard with this. My former science teacher, in an attempt to get people using mass instead of weight, actually used mass all the time, even when weight was the correct term. For example, she said something like "objects with more mass are heavier," when how heavy an object is has to do with weight. Whether this is to technical or not, instead of saying "mass is generally the correct term" as I viewed the post implied, just know the difference between mass and weight.

Also, learn the "second part" of Newton's third law-- third law pairs must be the same type of force (i.e. gravitational, tension, contact, electric, etc.), which will show you that the normal force (a contact force) is not the third law pair of mg*cosθ (a gravitational force), though they are equal and opposite. There are many misconceptions about the third law, many of which have been mentioned. I'd pay extra close attention to all of them.

I wonder why your former teacher would try to get you to use 'mass' instead of 'weight' ? they (as you know) are different things. Your teacher's wording is OK as part of her attempt to make physics interesting and understandable.
I think you misunderstood my point. Newtons laws refer to masses not weights.
I know about the third law.
just know the difference between mass and weight

 

[Isaac0427]

I wonder why your former teacher would try to get you to use 'mass' instead of 'weight' ? they (as you know) are different things. Your teacher's wording is OK as part of her attempt to make physics interesting and understandable.
I think you misunderstood my point. Newtons laws refer to masses not weights.
I know about the third law.
just know the difference between mass and weight

I posted that because of this line

mass is the important property in general

Which is probably what my teacher was trying to get at.

Also, the second paragraph was for the OP, it wasn't in response to you.

 

[sophiecentaur]

5. The whole idea that an applied force can be countered by another force that has no intentions.

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

 

[haruspex]

I was thinking in a more general physics sense, such as in space outside the space station with a massive object or on ice with something massive...mass is the important property in general...not weight.
misconceptions in this area arise by confusing 'weight' with mass

No, that would only result in the block accelerating more slowly than the person. phinds was describing the case where the person moves andthe block does not, which would be a consequence of weight and friction.

 

[lychette]

No, that would only result in the block accelerating more slowly than the person. phinds was describing the case where the person moves andthe block does not, which would be a consequence of weight and friction.

have you checked with phinds that this is indeed what he meant. Too many misconceptions here
presumably we agree that there is no misunderstanding about the meaning of weight and mass. Life would become nore complicated if friction had to be included in sorting out misconceptions..

 

[haruspex]

Ex. A box on a table. You push down on it with your finger. How does the table know to push back up... Intention based thinking.

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.

 

[haruspex]

have you checked with phinds that this is indeed what he meant

Did you read post #11?

 

[lychette]

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.

but not to prevent deformation?

 

[haruspex]

but not to prevent deformation?

I stated 'rigid'

 

[lychette]

Did you read post #11?

yes!

 

[lychette]

I stated 'rigid'

OK...but deformation does not imply penetration (your word)

 

[lychette]

yes!

but did you confirm with phinds this is what was meant or are you simply re-quoting

 

[haruspex]

but did you confirm with phinds this is what was meant or are you simply re-quoting

I based my post on phinds' post #5, where it was already clear. I later realized that phinds post #11 said pretty much the same as I did. I have no idea why you continue to challenge this.

 

[lychette]

this is a post about misconceptions !
Post #5 refers to 'weight' being important...it is not...friction has to be introduced to cover up this misconception.
It is easier to admit to mistakes than to try to cover them up.
Your use of the word rigid...what do you mean by 'rigid'...is there any misconception.
This forum is about physics...the terminology is impotant. You should be concerned if posts here are not challenged.

 

[nasu]

It is easier to admit to mistakes than to try to cover them up.

You should follow your own advice more often.

A rigid has no deformation. It's OK if you did not realize he mentioned rigid. Or that you were not sure what does it mean. Once he pointed this out, what is the point to follow it and trying to cover under the "misconception" umbrella?

 

[lychette]

You should follow your own advice more often.

A rigid has no deformation. It's OK if you did not realize he mentioned rigid. Or that you were not sure what does it mean. Once he pointed this out, what is the point to follow it and trying to cover under the "misconception" umbrella?

none whatsoever...thankyou for your contributions

 

[Fervent Freyja]

this is a post about misconceptions !
Post #5 refers to 'weight' being important...it is not...friction has to be introduced to cover up this misconception.
It is easier to admit to mistakes than to try to cover them up.
Your use of the word rigid...what do you mean by 'rigid'...is there any misconception.
This forum is about physics...the terminology is impotant. You should be concerned if posts here are not challenged.

You are being ridiculous. You can't just go around picking on terminology when you don't even really know what you are writing about...

You need to chill out. Are you more interested in arguing with people on here or helping people?

 

[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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