Expressing y(t) including the effects of air resistance & gravity

[Cooojan]

Homework Statement

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Hi everyone! So I've got this similar problem as I posted yesterday, but this one
is slightly different due to the presence of gravity:

A particle in gravitational field ##~~g##
starts traveling upward (positive direction) along the y-axis from ##~~y=0##
with the initial speed ##~~v_0≠0~~##,
where it faces air resistance ##~~F_R##

##F_R = -mbv~~~~## (where ##~v~## is the speed of the particle)

I have to show that the position of particle at any time can be expressed as following:

##y(t)= \frac 1b (v_0+ \frac gb)(1-e^{-bt})- \frac gb t##

Homework Equations

## \frac{dv}{dt}+bv = -g ##

The Attempt at a Solution

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So I found out that the expression for speed of the particle in this case is:

##v(y) = \frac 1b(b(v_0)e^{-bt}+ge^{-bt}-g))~~~~## (I believe this should be correct)

Further:

## v=\frac{dy}{dt} ~~~~⇒~~~~dy=v~dt~~~~⇒~~~~ \int \,dy= \int_0^t v\,dt ~~~~## (unsure if I've taken limits correctly)

When I do this integration, what I get - is something that somehow reminds of solution,
which I was suppose to come to:

Iget:

##y(t)= \frac1b(v_0)(1-e^{-bt}) + g- e^{-bt} - \frac gbt ##

when I should get:

##y(t)= \frac1b(v_0+ \frac gb )(1-e^{-bt}) - \frac gbt ##

If someone could point out what exactly am I doing wrong, would be awesome!
Also if you can comment on if I'm taking limits correctly.
In this case it doesn't really metter, I guess, but generally speaking - should I take same integration limits on both sides or not?
(as with: ##~~ \int \,dy= \int_0^t v\,dt ~~##)##~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Thanx~a~lot !
~~##

 

[berkeman]

FR=−mbv F_R = -mbv~~~~ (where v ~v~ is the speed of the particle)

Just to clarify, you are given that the retarding force due to air resistance is proportional to the object's mass? That doesn't seem correct...

 

[Cooojan]

No no! Its proportional to particles speed. Acceleration would then be equal to "-bv"
As you can see further below - mass is not mentioned anywhere else in equations

 

[willem2]

You made a mistake when integrating. The integral of e^at is very easy to get wrong by mistake. I also think you dropped a g somewhere.
You can also look at the units too see which terms are wrong. y(t) = g + ... can't be right.

 

[Cooojan]

 

[Cooojan]

! Oh !
I forgot to mention that originaly, this was a 2D problem, so that is why I'm using ##v_0~sinθ## in my calculations on paper,
and not just ##v_0##, as I did in my post.
But I already solved for x-axis, so I didnt want u guys to think of any unnecessities, while helping me out.
So I just turned the whole thing into a 1d problem. I hope u don't mind. :)
And also I used ##b## instead of ##α##, just so it would be faster for me to write it down in ##LaTeX##.

So that's what I did to get there...
I still can't get this right. If u have any suggestions, u r more then welcome to share))
Many thanx!

 

[willem2]

So that's what I did to get there...
I still can't get this right. If u have any suggestions, u r more then welcome to share))
Many thanx!

You're nearly there. The 1/a factors are ok now, but you forgot to multiply both terms with g here.

 

[Cooojan]

You're nearly there. The 1/a factors are ok now, but you forgot to multiply both terms with g here.

View attachment 213529

True story, my fault! Thank u)) How didnt I see this??
Was pretty close and even checked thrue integration several times... Now I got it right! Cant thank you enough))))))))))