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

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Discussion Overview

The discussion centers around common misconceptions that students encounter when learning about Newton's Laws of Motion. Participants share various misunderstandings related to the concepts of force, motion, and the application of these laws in different scenarios.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants highlight confusion regarding the third law of motion, particularly the misconception that the reaction force on a person pushing a block is relevant to the block's motion.
  • Others discuss the misunderstanding that a net force must always be acting on an object that is moving, using examples of constant velocity motion on a rough surface.
  • There is mention of the confusion surrounding the direction of acceleration and net force, particularly in scenarios involving objects at rest or in motion.
  • Some participants propose that students often confuse the concepts of 'force' and 'velocity', suggesting that learners may think a force is necessary for an object to move.
  • A later reply questions the idea of intention behind forces, noting that students may struggle with the concept of forces acting without an obvious cause.
  • Discussion includes the challenges of applying Newton's laws to free body diagrams, with varying methods leading to confusion.
  • Participants also mention the complexities of circular motion and the requirement of a net force when changing direction.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the misconceptions about Newton's Laws of Motion, and the discussion remains unresolved with no consensus on a single common misconception.

Contextual Notes

Some limitations include varying interpretations of force and motion, dependence on specific examples, and the complexity of free body diagrams that may differ across educational texts.

Beanyboy
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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
 
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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.
 
pixel said:
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)
 
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.
 
Beanyboy said:
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')
 
pixel said:
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"?
 
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.
pgardn said:
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 said:
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')
pgardn said:
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.
 
  • #10
phinds said:
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?
 
  • #11
lychette said:
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.
 
  • #12
phinds said:
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
 
  • #13
lychette said:
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.
 
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  • #14
Isaac0427 said:
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
 
  • #15
lychette said:
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
lychette said:
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.
 
  • #16
pgardn said:
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
 
  • #17
lychette said:
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.
 
  • #18
haruspex said:
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..
 
  • #19
pgardn said:
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.
 
  • #20
lychette said:
have you checked with phinds that this is indeed what he meant
Did you read post #11?
 
  • #21
haruspex said:
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?
 
  • #22
lychette said:
but not to prevent deformation?
I stated 'rigid'
 
  • #23
haruspex said:
Did you read post #11?
yes!
 
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  • #24
haruspex said:
I stated 'rigid'
OK...but deformation does not imply penetration (your word)
 
  • #25
lychette said:
yes!
but did you confirm with phinds this is what was meant or are you simply re-quoting
 
  • #26
lychette said:
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.
 
  • #27
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.
 
  • #28
lychette said:
It is easier to admit to mistakes than to try to cover them up.
You should follow your own advice more often. :smile:

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?
 
  • #29
nasu said:
You should follow your own advice more often. :smile:

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
 
  • #30
lychette said:
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?
 
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