# Physics qns relating stokes lawWatch

#1
Hi ! May I know when do we use stokes law in Edexcel alevels ? In projectile motion I never came across a question where it was being used . Do we only use it when they want us to find the upthrust in a given question or when it requires us to use it ?
1
6 months ago
#2
(Original post by KrippyKush)
Hi ! May I know when do we use stokes law in Edexcel alevels ? …
It seems that you don’t really understand Stokes Law at A level within Edexcel syllabus. It depends on the context of the problem.

(Original post by KrippyKush)
…In projectile motion I never came across a question where it was being used.
It can be used in projectile motion. Again it depends on the context. In most of the projectile motion, the speed is relatively high, so Stoke’s law would not be valid to describe the air resistance.

(Original post by KrippyKush)
…Do we only use it when they want us to find the upthrust in a given question or when it requires us to use it ?
Again it really depends on the context of the question. Upthrust and "Stoke’s law" are two different forces acting on an object moving in a fluid.
1
6 months ago
#3
As the person above said, it depends on the context of the q. Stokes law gives you the drag force when something spherical moves through a fluid provided the flow is laminar (turbulent flow would mean air resistance, which would make stoke’s law inapplicable).
If you have motion within the fluid you can use the given information to find viscous drag. Then find the weight of the object. Many questions in past papers have previously asked to find upthrust. Imagine you had a ball going up with terminal v, that would mean all forces (weight, upthrust and drag) balance, you would construct an equation (such as weight +drag =upthrust if ball was going up) which you would then use the weight and drag to find the upthrust. Ive never seen a projectile motion q with it but they could always throw it in but no idea how.
Hope this helped
1
6 months ago
#4
(Original post by Htn_02)
As the person above said, it depends on the context of the q. Stokes law gives you the drag force when something spherical moves through a fluid provided the flow is laminar (turbulent flow would mean air resistance, which would make stoke’s law inapplicable).
Adding to what Htn_02 had described on laminar and turbulent flow: in A-level, students are required know in laminar flow, the “fluid resistance” is directly proportional to the velocity of the object and in turbulent flow, the “fluid resistance” is directly proportional to the square of the speed of the object.

(Original post by Htn_02)
…If you have motion within the fluid you can use the given information to find viscous drag. Then find the weight of the object. Many questions in past papers have previously asked to find upthrust. Imagine you had a ball going up with terminal v, that would mean all forces (weight, upthrust and drag) balance, you would construct an equation (such as weight +drag =upthrust if ball was going up) which you would then use the weight and drag to find the upthrust. Ive never seen a projectile motion q with it but they could always throw it in but no idea how.
Hope this helped
It is possible to set a projectile motion with Stoke’s law depending on whether the question is asking for conceptual understand or quantitative understanding of the forces involves. The latter type of question is more difficult to set if we want it to describe the real world situation.
0
#5
Eimmanuel Sup man I got i what you mean .

One question I have is that for upthrust to be included in our calculations in projectile motion , the questionare will be directly asking us to find it right ? Otherwise we don't ever use it when a ball is being thrown in air , only came across it being used when smtg is displaced in liquid .

And secondly how will the question require us to use stokes law ?
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#6
Htn_02
Hi man thanks for the reply
So If the object is not spherical then do we neglect the Stoke law equation ? & What do u mean by turbulent aka air resistance ? Isn't drag force due to a fluid which can be air too a resistance provided by the particles of the medium the object is moving ?

Secondly how do we know smtg is in laminar flow ? ( My school skip the topic lol pls can you explain me the difference between both laminar and turbulent flow)
0
6 months ago
#7
(Original post by KrippyKush)
Eimmanuel Sup man I got i what you mean .

One question I have is that for upthrust to be included in our calculations in projectile motion , the questionare will be directly asking us to find it right ? Otherwise we don't ever use it when a ball is being thrown in air , only came across it being used when smtg is displaced in liquid .

And secondly how will the question require us to use stokes law ?
Again, i cannot say definitely a yes for your first question. Sometimes, the question expects the students to make the judgement themselves instead of telling them what to do.

As for the second question, i again cannot say for sure. A simple application is finding the resultant force acting on the projectile object with a pacticular velocity at a particular time.
0
#8
(Original post by Eimmanuel)
Again, i cannot say definitely a yes for your first question. Sometimes, the question expects the students to make the judgement themselves instead of telling them what to do.
Eimmanuel
As for the second question, i again cannot say for sure. A simple application is finding the resultant force acting on the projectile object with a pacticular velocity at a particular time.
Eimmanuel

Sorry man I have one more qns

Regarding flies why is it that laminar flow comes about with lower velocity compared to turbulent velocity ? And why is it we can't apply stokes low on spherical objects which are big ?
( My school skipped on the flows is it possible for you to do a quick summary on it plsssssssssss I would appreciate it man I went through YouTube and all of the videos are focusing on university level flow motion.)
Last edited by KrippyKush; 6 months ago
0
6 months ago
#9
(Original post by KrippyKush)
Htn_02
Hi man thanks for the reply
So If the object is not spherical then do we neglect the Stoke law equation ? & What do u mean by turbulent aka air resistance ? Isn't drag force due to a fluid which can be air too a resistance provided by the particles of the medium the object is moving ?

Secondly how do we know smtg is in laminar flow ? ( My school skip the topic lol pls can you explain me the difference between both laminar and turbulent flow)

The flow in a fluid is either laminar or turbulent. Laminar is when the fluid lines around the object in a fluid are parallel and dont cross, turbulent is the exact opposite. (See below). If viscous drag exists then you assume the flow is turbulent (not laminar). Stokes law is only for spherical objects undergoing laminar flow. Whether or not flow is laminar can be determined from the context of the q: if speed of lets say a ball is fast in a fluid then you could say it’s turbulent but if it’s slow it can be said its laminar.
If a ball is dropped into a tube containing some fluid, and it drops near the edges of the cylindrical tube, then because flow near edges is turbulent you say stokes law isn’t applicable etc. You should find more stuff on this in your revision guide or physics and maths tutor if you need more help on this.
0
#10
Htn_02

(Original post by Htn_02)
The flow in a fluid is either laminar or turbulent. Laminar is when the fluid lines around the object in a fluid are parallel and dont cross, turbulent is the exact opposite. (See below). If viscous drag exists then you assume the flow is turbulent (not laminar). Stokes law is only for spherical objects undergoing laminar flow. Whether or not flow is laminar can be determined from the context of the q: if speed of lets say a ball is fast in a fluid then you could say it’s turbulent but if it’s slow it can be said its laminar.
If a ball is dropped into a tube containing some fluid, and it drops near the edges of the cylindrical tube, then because flow near edges is turbulent you say stokes law isn’t applicable etc. You should find more stuff on this in your revision guide or physics and maths tutor if you need more help on this.
We use stokes law to find viscous force on spherical object with low speed which will have laminar flow
Than why did you relate turbulent flow to the motion of an object such that if object experiencing a viscous force than it's a turbulent flow ?
0
6 months ago
#11
(Original post by KrippyKush)
@Eimmanuel
Sorry man I have one more qns
Regarding flies why is it that laminar flow comes about with lower velocity compared to turbulent velocity? And why is it we can't apply stokes low on spherical objects which are big?
Sorry for the late reply. What is the flies that you are referring to? What do you mean by turbulent velocity?
I don’t really understand your two why’s questions. There are not many why’s questions that we can really answer in science. Science tends to explain the what and the how rather than the why’s unless we can phrase the why question to how.

(Original post by KrippyKush)
….( My school skipped on the flows is it possible for you to do a quick summary on it plsssssssssss I would appreciate it man I went through YouTube and all of the videos are focusing on university level flow motion.)

I would suggest that you go through the set of material first. A summary should be helpful for people who have some fundamental understanding of the topic.
https://opentextbc.ca/physicstestboo...iseuilles-law/
https://opentextbc.ca/physicstestboo...of-turbulence/
https://opentextbc.ca/physicstestboo...viscous-fluid/

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