Graph of viscous force and velocity

[hello478]

Homework Statement

image below

Relevant Equations

weight = viscous force

my answer was A
but i dont understand
because when the viscous force equals the weight speed becomes constant
but why does the viscous force needs to be equal of weight?
is it weight = viscous force?
or speed = viscous force?

 

[PeroK]

or speed = viscous force?

That's an interesting equation!

 

[hello478]

That's an interesting equation!

is it wrong?

 

[PeroK]

is it wrong?

Yes!

 

[hello478]

Yes!

so then why does viscous force increases as speed increases?

 

[PeroK]

so then why does viscous force increases as speed increases?

I don't see how that relates to your equation. Speed can't equal force. That's dimensionally invalid.

 

[hello478]

I don't see how that relates to your equation. Speed can't equal force. That's dimensionally invalid.

ok, yeah i get it
but then can you pls explain why they both are directly proportional?

 

[jbriggs444]

so then why does viscous force increases as speed increases?

I am having trouble even understanding the problem.

If the ball is unchanging from one trial to the next then $F=mg$. So $F$ is unchanging from one trial to the next.

If the fluid is also unchanging from one trial to the next then $v$ is a fixed monotone increasing function of $F$. But since $F$ is unchanging, so is $v$.

So the graph should consist of just a single dot.

What is being changed from one trial to the next so that we have a meaningful graph instead of a single dot?

Perhaps this is similar to a Millikan oil drop experiment so that $F$ is being allowed to change from one trial to the next (variation in the charge on the ball in a fixed electrostatic field, maybe).

 

[hello478]

I am having trouble even understanding the problem.

If the ball is unchanging from one trial to the next then $F=-mg$. So $F$ is unchanging from one trial to the next.

If the fluid is also unchanging from one trial to the next then $v$ is a fixed monotone increasing function of $F$. But since $F$ is unchanging, so is $v$.

So the graph should consist of just a single dot.

What is being changed from one trial to the next so that we have a meaningful graph instead of a single dot?

Perhaps this is similar to a Millikan oil drop experiment so that $F$ is being allowed to change from one trial to the next (variation in the charge on the ball in a fixed electrostatic field).

im sorry but i dont understand this...

 

[PeroK]

ok, yeah i get it
but then can you pls explain why they both are directly proportional?

What's directly proportional to what?

A is the only graph where $F$ increases with $v$. Which I guess you are supposed to recognise as a characteristic of drag.

 

[hello478]

What's directly proportional to what?

viscous force and velocity

 

[hello478]

A is the only graph where $F$ increases with $v$. Which I guess you are supposed to recognise as a characteristic of drag.

yes but can you please explain it...

 

[PeroK]

yes but can you please explain it...

1) It shouldn't need an explanation to answer this question. It's something you ought to know.

2) An Internet search for "viscous drag" will give you plenty of information and explanations.

 

[jbriggs444]

I located a verbatim copy of the question on page 6 at https://dynamicpapers.com/wp-content/uploads/2015/09/9702_s23_qp_11.pdf

No additional context is provided to clarify the intent of the question. So it seems that @PeroK has the most plausible interpretation. We are asked for a plausible force versus velocity graph for a fixed ball in a fixed fluid forced to move at a chosen velocity.

 

[PeroK]

I located a verbatim copy of the question on page 6 at https://dynamicpapers.com/wp-content/uploads/2015/09/9702_s23_qp_11.pdf

No additional context is provided to clarify the intent of the question. So it seems that @PeroK has the most plausible interpretation. We are asked for a plausible force versus velocity graph for a fixed ball in a fixed fluid forced to move at a chosen velocity.

The point to note is that the time dependence of $v$ is not shown. Instead, we could add a constant gravitational force and map the net force against $v$ and that would make sense for graph $A$..

 

[haruspex]

because when the viscous force equals the weight speed becomes constant

"when" is a question about time, but as @PeroK points out there is no time axis in the diagrams, so they would not show whether the speed becomes constant.