## baldymcgee6 2 years ago A certain object of mass 3kg is dropped into a liquid and, as it falls through it, it is subject to the downward force of gravity and the upward force of drag, which is given by one fifth of its speed. Express this statement as a differential equation and solve such equation to identify the velocity function for the object.

1. baldymcgee6

@zepdrix :)

2. baldymcgee6

I should be fine to solve the linear ODE, I just don't know where to start with contructing one.

3. zepdrix

hmm :(

4. zepdrix

hmm i dunno, this one is too "physics"y for me lol

5. baldymcgee6

haha, who else on here does calculus?

6. zepdrix

Hmm I dunno. There are a bunch of smarty pants online right now though :O Zarkon, Satellite, King George ...

7. baldymcgee6

@satellite73, @UnkleRhaukus ?

8. baldymcgee6

these guys probably get so many tags that they don't even bother.. lol... the joys of a 99 smartscore.

9. baldymcgee6

oh goody @hartnn came online :)

10. hartnn

seen such question for the first time, but still i'll give it a try... downward force of gravity =ma = mg (acceleration is due to gravity, so a=g) upward force of drag = ma = m dv/dt (v=velocity) given that mdv/dt = 1/5th of its speed = v/5 mdv/dt = v/5 in itself is a DE, donno how to incorporate mg ...

11. hartnn

v=velocity function ..

12. baldymcgee6

haha. Well that is a good start, I don't have any idea either of how to incorporate mg

13. baldymcgee6

Fnet = mg - v/5

14. hartnn

if the question meant that 'both' downward and upward forces together are 1/5th of speed, then m dv/dt -mg = v/5 ...

15. baldymcgee6

m*dv/dt = mg-v/5 ???

16. hartnn

or that^ i am still unsure..

17. baldymcgee6

no I think just the drag is v/5

18. baldymcgee6

hmm, I guess I will just go with one and see what I get.. :) thanks guys!