Are there two answers to this question?

[askbbj]

1. The question is found in the file attached herein. This is a Year 3 Question. You may still use higher ability concepts to help me understand your explanation as I am a University graduate and a Science educator. 2. I understand that air is the cause why the plunger is able to move downwards. 3. The Answer is No.(4). This is due to air not having a definite volume. However, I also feel it can be due to Option (3). where air does not have a definite shape. When air does not have a definite shape, it will also cause the plunger to move downwards. Isn't this true? Please let me know if there is a mistake in this question.

 

[askbbj]

Please share with me your opinion as I am not sure if my explanation is accurate.

 

[berkeman]

View attachment 219334 1. The question is found in the file attached herein. This is a Year 3 Question. You may still use higher ability concepts to help me understand your explanation as I am a University graduate and a Science educator. 2. I understand that air is the cause why the plunger is able to move downwards. 3. The Answer is No.(4). This is due to air not having a definite volume. However, I also feel it can be due to Option (3). where air does not have a definite shape. When air does not have a definite shape, it will also cause the plunger to move downwards. Isn't this true? Please let me know if there is a mistake in this question.

Welcome to the PF.

Have you studied Boyle's Law yet? If not, look that up and I think you will see why "D" is the best of the available answers...

Alternately, look up the Ideal Gas Law -- it is a more general form for how gas volume and pressure are related.

 

[NFuller]

The Answer is No.(4). This is due to air not having a definite volume. However, I also feel it can be due to Option (3). where air does not have a definite shape. When air does not have a definite shape, it will also cause the plunger to move downwards. Isn't this true? Please let me know if there is a mistake in this question.

Although technically true, I think you are reading too far into the question. Just having an indefinite shape is not sufficient to allow the plunger to be pushed down without anything escaping. For example, if the cylinder was completely filled with water then the plunger can not be pushed down. Although water has no definite shape, it is not easily compressible and has a definite volume, i.e. $1\text{g}=1\text{mL}$. Thus, option 4 is the necessary condition.

 

[askbbj]

Although technically true, I think you are reading too far into the question. Just having an indefinite shape is not sufficient to allow the plunger to be pushed down without anything escaping. For example, if the cylinder was completely filled with water then the plunger can not be pushed down. Although water has no definite shape, it is not easily compressible and has a definite volume, i.e. $1\text{g}=1\text{mL}$. Thus, option 4 is the necessary condition.

If air had a fixed shape here, you wouldn't be able to push down the plunger, would you?
We need to read deeper in order to fully understand what's happening on the surface. This is not asking for too much for a simple question like this. Anyway it's also another option presented in the array of possible solutions found herein.

 

[askbbj]

Welcome to the PF. [emoji2]

Have you studied Boyle's Law yet? If not, look that up and I think you will see why "D" is the best of the available answers...

Alternately, look up the Ideal Gas Law -- it is a more general form for how gas volume and pressure are related.

Boyle's law speaks of volume, temp n pressure-concepts which are beyond the scope of Year 3s.
Imagine for once, if air had a fixed shape here, you wouldn't be able to push down the plunger, would you?

 

[askbbj]

Welcome to the PF. [emoji2]

Have you studied Boyle's Law yet? If not, look that up and I think you will see why "D" is the best of the available answers...

Alternately, look up the Ideal Gas Law -- it is a more general form for how gas volume and pressure are related.

The concept of the interrelationship between volume, pressure n temperature as espoused in Boyle's law is beyond the scope of Year 3 s.

 

[Merlin3189]

Air not having a fixed shape (ie. not being a solid) is a necesssary, but not a sufficient condition.

Air not having a definite volume is necessary and sufficient, since no fixed volume includes no fixed shape.

1 and 2 are both true, as is 3, but none are necessary and sufficient. Only 4 meets that criterion.

As NFuller mentioned, 1,2 and 3 are also true for water which does not compress. 4 discriminates between air and water and is the critical issue here.

Edit:
Multiple choice questions have a difficult job. A discursive answer allows you to explain and show understanding. MC questions can only deduce understanding by your selection of the best and most germane answer selected from a mixture of less good answers.

 

[haruspex]

If air had a fixed shape here, you wouldn't be able to push down the plunger, would you?

You make an interesting point. If you approach the question by considering the consequence of each of the four predicates being false, it is clear that falsifying 1 or 2 would not result in the plunger being indepressable. The same cannot be said of 3. So purely as an exercise in logic, 3 is not ruled out.

The question would have been better if it had contrasted the situation with the case of no air being present. Since water has no definite shape either, option 3 could have been eliminated.

By the way, Ahmed did not fill the cylinder with water; he partially filled it.

Edit: as Merlin points out, falsifying 3 (i.e. saying it does have fixed shape) falsifies 4 as well, so the possible answers are "4" and "3 and 4".

 

[PeroK]

If air had a fixed shape here, you wouldn't be able to push down the plunger, would you?

This is false logic. The three cases under question are:

Solid: fixed shape and fixed volume (incompressible)

Liquid: no fixed shape and fixed volume (incompressible)

Gas: no fixed shape and no fixed volume (compressible)

The "no fixed volume" is the critical property for compressibility. Logically, having "no fixed shape" is necessary but not sufficient. If the question was to be logically phrased you could replace "Why ..." by "What is the necessary and sufficient condition ...".

 

[jbriggs444]

Imagine for once, if air had a fixed shape here, you wouldn't be able to push down the plunger, would you?

Sure you would. The air in the container could then shrink away from the walls and allow the piston to be depressed. Having no fixed shape is neither a necessary nor a sufficient condition for an object or substance to allow the plunger to be depressed.

There are real world objects with a fixed shape but not a fixed volume. Air happens not to be one of them.

 

[askbbj]

Air not having a fixed shape (ie. not being a solid) is a necesssary, but not a sufficient condition.

Air not having a definite volume is necessary and sufficient, since no fixed volume includes no fixed shape.

1 and 2 are both true, as is 3, but none are necessary and sufficient. Only 4 meets that criterion.

As NFuller mentioned, 1,2 and 3 are also true for water which does not compress. 4 discriminates between air and water and is the critical issue here.

Edit:
Multiple choice questions have a difficult job. A discursive answer allows you to explain and show understanding. MC questions can only deduce understanding by your selection of the best and most germane answer selected from a mixture of less good answers.

Answers and their interpretation are always debateble as in this case. Perhaps I was thinking too deeply but then again that's how a great mind works. Correct me if I sound wrong...

 

[askbbj]

Sure you would. The air in the container could then shrink away from the walls and allow the piston to be depressed. Having no fixed shape is neither a necessary nor a sufficient condition for an object or substance to allow the plunger to be depressed.

There are real world objects with a fixed shape but not a fixed volume. Air happens not to be one of them.

By "shrinking away from the walls" wouldn't you in effect have changed the shape of air also?

Also here we are just concerned with air per se and not about other matters. And we all know that air having no fixed shape allows for the plunger to move down (option 3). Similarly, air having no fixed volume also allows for the plunger to move down (option 4).

 

[jbriggs444]

By "shrinking away from the walls" wouldn't you in effect have changed the shape of air also?

No. You can retain shape while shrinking. A smaller cylinder (for instance) can have the same shape as a larger cylinder.

 

[Ray Vickson]

By "shrinking away from the walls" wouldn't you in effect have changed the shape of air also?

Also here we are just concerned with air per se and not about other matters. And we all know that air having no fixed shape allows for the plunger to move down (option 3). Similarly, air having no fixed volume also allows for the plunger to move down (option 4).

No, we do not know that air having no fixed shape allows the plunger to move down (or up, if we pull on it). It has already been explained that if you filled the chamber with water---which also has not shape---instead of an air-water mixture, you could not push the plunger down (at least, not by hand and not easily).

 

[jbriggs444]

No, we do not know that air having no fixed shape allows the plunger to move down (or up, if we pull on it). It has already been explained that if you filled the chamber with water---which also has not shape---instead of an air-water mixture, you could not push the plunger down (at least, not by hand and not easily).

The point is that whether air has a fixed shape or not has no bearing whatsoever on the question of whether the plunger can be plunged.

A claim had circulated in this thread that having no fixed shape was a necessary condition to having the plunger plunge. It is not.

1. Something with no fixed shape can resist the plunge (e.g. water)
2. Something with fixed shape can resist the plunge (e.g. steel)
3. Something with no fixed shape can fail to resist the plunge (e.g. air)
4. Something with fixed shape can fail to resist the plunge. (e.g. a hoberman sphere)

 

[askbbj]

No. You can retain shape while shrinking. A smaller cylinder (for instance) can have the same shape as a larger cylinder.

The issue here is 'shape'...By definition "shape is defined by a set of parameters, one of them being the length of the dimensions in space..." Hence, by using the term 'shrinking' , you would have effectively altered the shape of air as its dimensions would have been altered.

 

[askbbj]

The issue here is 'shape'...By definition "shape is defined by a set of parameters, one of them being the length of the dimensions in space..." Hence, by using the term 'shrinking' , you would have effectively altered the shape of air as its dimensions would have been altered.

A cylinder will transform into a disc if it's vertical dimension is reduced sufficiently. 'cylinder' and 'disc' are two different shapes altogether. Likewise the above example when the air is compressed...

 

[jbriggs444]

The issue here is 'shape'...By definition "shape is defined by a set of parameters, one of them being the length of the dimensions in space..." Hence, by using the term 'shrinking' , you would have effectively altered the shape of air as its dimensions would have been altered.

Then we disagree about the meaning of the word "shape". I do not understand the scale of an object to be relevant. An object which has its size scaled up or down equally in all three directions retains, per my understanding of the word, its "shape".

 

[jbriggs444]

A cylinder will transform into a disc if it's vertical dimension is reduced sufficiently. 'cylinder' and 'disc' are two different shapes altogether. Likewise the above example when the air is compressed...

A cylinder which is scaled down in all three directions will not transform into a disc.

 

[askbbj]

A cylinder which is scaled down in all three directions will not transform into a disc.

When the plunger is depressed, only the vertical dimension is reduced (correct me if I am wrong here) while the other 2 dimensions remain unchanged. As such, going by the definition of a shape , one of which is "a 3D space that is defined by the length of its dimensions', we can conclude that there is indeed a change in shape. This refers to a change in the ratio of the 3 dimensions. In your explanation, you are telling me all three dimensions are changing at one go when the plunger is depressed. How is this possible? Pray tell.

 

[jbriggs444]

When the plunger is depressed, only the vertical dimension is reduced (correct me if I am wrong here) while the other 2 dimensions remain unchanged. As such, going by the definition of a shape , one of which is "a 3D space that is defined by the length of its dimensions', we can conclude that there is indeed a change in shape. This refers to a change in the ratio of the 3 dimensions. In your explanation, you are telling me all three dimensions are changing at one go when the plunger is depressed. How is this possible? Pray tell.

The shape of a container need not match the shape of the contents.

 

[askbbj]

The shape of a container need not match the shape of the contents.

Apologies but you don't seem to make much sense at all.

 

[jbriggs444]

Apologies but you don't seem to make much sense at all.

Do you dispute the truth of that statement?

 

[askbbj]

Do you dispute the truth of that statement?

How can I when I don't even understand what you are talking about here...

 

[jbriggs444]

How can I when I don't even understand what you are talking about here...

Can you fit a square block in a round tin?

 

[askbbj]

Can you fit a square block in a round tin?

What happens to the dimensions of air when the plunger is depressed as per question.?

 

[jbriggs444]

What happens to the dimensions of air when the plunger is depressed as per question.?

If air were a substance with a fixed shape but no fixed volume, it would shrink away from the sides, reducing in all three dimensions at the same time. This involves no logical contradiction.

Edit: I notice that you have refused to answer the question about square objects in round canisters.

 

[askbbj]

If air were a substance with a fixed shape but no fixed volume, it would shrink away from the sides, reducing in all three dimensions at the same time. This involves no logical contradiction.

Edit: I notice that you have refused to answer the question about square objects in round canisters.

The thing is air isn't a substance with a fixed shape to begin with. And I deliberately didn't answer your question as it's irrelevant to what I am trying to get at.

 

[jbriggs444]

The thing is air isn't a substance with a fixed shape to begin with. And I deliberately didn't answer your question as it's irrelevant to what I am trying to get at.

We are asked about which attributes of air allow it to fail to resist the plunger. A lack of fixed shape is not such an attribute because an object with a fixed shape can still fail to resist the plunger. QED.

Edit: Let us take the other side of the argument for a moment. The challenge then is to present an object or substance which has a fixed volume but which nonetheless fails to resist the plunger.

 

[NFuller]

The correct answer has been explained in detail by several members. @jbriggs444 has given examples of objects with fixed shape but not fixed volume and explained why this is a poor answer. If the OP is unsatisfied with these explanations, then I would recommend that he or she ask their instructor or search google for another explanation.

In any case, the original question has been answered and I think this thread has run its course.