# PhysicsCalculating Terminal Velocity

#### jc91

##### New member
Hi All,

I am struggling with the question (attached) and was hoping to get some guidance and explanation on how to solve it.

Im thinking that it is force down is equal to force up? But I could be wrong.

Any help greatly appreciated.

Regards,

James

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#### Ackbach

##### Indicium Physicus
Staff member
I would definitely use a tilted coordinate system. Terminal velocity will occur when the forces down the slope are equal and opposite to the forces up the slope. What is your model for air resistance?

#### jc91

##### New member
I would definitely use a tilted coordinate system. Terminal velocity will occur when the forces down the slope are equal and opposite to the forces up the slope. What is your model for air resistance?
So would I be using Force = mass x velocity
Therefore F down = F up
Therefore mv = mv
??

I have no model for air resistance.

#### Country Boy

##### Well-known member
MHB Math Helper
You are told that "the frontal area is 0.45 m^2 and the drag coefficient is 1.1". If that doesn't mean anything to you- in particular if you don't know what "drag coefficient" means- then you need to discuss this with your teacher.

#### Ackbach

##### Indicium Physicus
Staff member
So would I be using Force = mass x velocity
Therefore F down = F up
Therefore mv = mv
??

I have no model for air resistance.
Force definitely does not equal mass times velocity. You have $F=ma,$ and this holds up and down the slope, as well as normal to the slope (that is, in each direction of the tilted coordinate system). I would recommend you use the following attached Problem-Solving Strategy for this problem.

View attachment Mechanics Problem-Solving Strategy.pdf

From a physics perspective, the key here is to recognize that at terminal velocity, which is constant, there can be no acceleration. That is, the acceleration must be zero.

As far as air resistance, you can model it as proportional to the speed, or proportional to the square of the speed. Look it up in your book to see which one your book is assuming.

#### jc91

##### New member
Force definitely does not equal mass times velocity. You have $F=ma,$ and this holds up and down the slope, as well as normal to the slope (that is, in each direction of the tilted coordinate system). I would recommend you use the following attached Problem-Solving Strategy for this problem.

From a physics perspective, the key here is to recognize that at terminal velocity, which is constant, there can be no acceleration. That is, the acceleration must be zero.

As far as air resistance, you can model it as proportional to the speed, or proportional to the square of the speed. Look it up in your book to see which one your book is assuming.
Thanks you very much for the assistance

- - - Updated - - -

You are told that "the frontal area is 0.45 m^2 and the drag coefficient is 1.1". If that doesn't mean anything to you- in particular if you don't know what "drag coefficient" means- then you need to discuss this with your teacher.
Im sorry, I didnt realise that this was a judgement forum?

Im after returning to education after 8 years as a construction worker and am now currently studying to be a mechanical engineer on night courses. My ability in fluid dynamics is limited to date and that is why I posted here for help and guidance, not just the answer.

If youve nothing useful to add to this forum then just ignore it, thanks

#### Ackbach

##### Indicium Physicus
Staff member
Im sorry, I didnt realise that this was a judgement forum?

Im after returning to education after 8 years as a construction worker and am now currently studying to be a mechanical engineer on night courses. My ability in fluid dynamics is limited to date and that is why I posted here for help and guidance, not just the answer.

If you`ve nothing useful to add to this forum then just ignore it, thanks
This is a volunteer forum. Country Boy is a volunteer, like all the helpers on this forum. There is no obligation to help anyone. Therefore, it is appropriate to keep your tone polite. These comments are borderline. Country Boy 's comments are not out of line: he's just saying you need to make sure you understand the concepts required to solve the problem, and if you need to go to your teacher, then go to your teacher. Math Help Boards is not a substitute for your teacher. We're here to help you get unstuck, not to do the heavy lifting for you.

#### jc91

##### New member
This is a volunteer forum. Country Boy is a volunteer, like all the helpers on this forum. There is no obligation to help anyone. Therefore, it is appropriate to keep your tone polite. These comments are borderline. Country Boy 's comments are not out of line: he's just saying you need to make sure you understand the concepts required to solve the problem, and if you need to go to your teacher, then go to your teacher. Math Help Boards is not a substitute for your teacher. We're here to help you get unstuck, not to do the heavy lifting for you.
Ok, thank you

#### Country Boy

##### Well-known member
MHB Math Helper
Where was there any "judgement" involved? If you have trouble understanding something in a class then you should let your teacher know that so that he/she can clarify.