Another Mechanics Query (1 Viewer)

Hermes1

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A solid of unit mass is dropped under gravity from rest at a height of h metres. Air resistance is proportional to the speed v of the mass, acceleration under gravity is g.

okay so a = g-kv

the question is:

using a = v.dv/dx

show that:

the problem is that when v=0, x=h
this means when i solve for the constant, h gets in the way of the answer im supposed to prove.
can i use at x=0, v=0 instead. making the point of projection the origin?, this gives me the answer above, but i dont no if it is legit to do this?
 

hscishard

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I feel stupid, what is x? the only I got from the question that it's a one dimensional plane
Ok now im thinking x= height, h is a constant
 

Hermes1

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hscishard, have you done mechanics, bcuz this is a mechanics question. that culd be why u dont understand it
 

Drongoski

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Of course you may treat the drop-off point as x=0 if it makes treatment easier.
 

Hermes1

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Of course you may treat the drop-off point as x=0 if it makes treatment easier.
tnk u very much drongoski, i was worried bcuz i hadnt done anything with h in my answer so thank u very much.

drongoski remind me to buy u lunch after the hsc as a thank you for helping me so much.
 

Drongoski

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tnk u very much drongoski, i was worried bcuz i hadnt done anything with h in my answer so thank u very much.

drongoski remind me to buy u lunch after the hsc as a thank you for helping me so much.
You are always welcome = since you are polite and acknowledge help given directly to person rendering help.

Not like:" thanks in advance" or "thanks guys" or worse complete silence after helpers have taken quite a bit of trouble to type out solution.
 

hscishard

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Haven't started mechanics since most of it isn't in my trials but I still can't get the question! Isn't this just 3unit?
 

Hermes1

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Haven't started mechanics since most of it isn't in my trials but I still can't get the question! Isn't this just 3unit?
this is definitely not 3 unit. this was Q7 from Gosford high school 2010 MX2 trial

3 unit motion is very closely linked to 4 unit motion, you need to understand the basic principles of 3 unit motion to master mechanics
 

xxxzxc

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yes make x = 0 because it is where you start as you are dropping it from that height
 

hscishard

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this is definitely not 3 unit. this was Q7 from Gosford high school 2010 MX2 trial

3 unit motion is very closely linked to 4 unit motion, you need to understand the basic principles of 3 unit motion to master mechanics
It really feels like you can solve it using 3unit methods...
a=g-kv

Wouldn't it be a=kv-g? Since gravity is down and resistance is up?
 

hscishard

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One silly mistake and it stuffs you up lol

I did it with 3U methods using x=0 v=0, I think it's just a hard 3unit motion question
 

bleakarcher

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if u are considering the downward motion of the particle, when x=0,v=0.
when u are considering the upward motion of the particle, when x=h, v=0
 

hscishard

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Other way around, unless you want to take account to negative level zero values

...right?

Nvm, I was confused with displacement and x=0 being ground zero or w/e you call it
 
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hscishard

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Then if x = displacement and x=h, then the thing would either be h units up or down the drop off. Right...?
If so, v wouldn't be =0
 

bleakarcher

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that is wrong. v would be final velocity, at the instantanteous point in time it hits the ground
 

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