Help with easy mechanics question!!! (1 Viewer)

[hk31]

i need help with a mechanics question...
i'm so clueless about this topic...that i have no idea what to do..

anyway....the question is from ARNOLD and ARNOLD'S CAMBRIDGE 4U MATHS book...

exercise 7.2
question 1

thank you....

 

[McLake]

Can you post the Q?

 

[hk31]

am i allowed to? as in....for some legal reason?...

 

[McLake]

Originally posted by hk31
am i allowed to? as in....for some legal reason?...

Yes, you can post (there's no legality issues that I know of)

a particle is moving vertically downward in a medium which exerts a resistance to the motion which is proportional to the speed of the particle. The particle is released from rest at O, and at time t its position is at a distance x below O and its speed is v. If the terminal velocity is V, show that gx +Vv = Vgt

ma = kmv (km is a convient constant)
a = kv
dv/dt = kv
dt/dv = 1/kv
dt = 1/kv dv
t = I (1/kv) dv
t = [1/k*ln(kv)] between V and v
t = 1/k[ln(v/V)]
v/V = e^kt

OK, try this
v*dv/dx = kv
dv/dx = k
dx/dv = 1/k
x = [v/k] between V and v
x = (V-v)/k

Hmm ...

 

[hk31]

umm....what?

i thought since its downward motion....does
a= g - kv?
and then i'm suppose to do something with that....

 

[Grizzly]

Oh, ive done this exact question...im at school at the moment
when i get back home i get my book and try to type it all up

Basically, what Mclake posted
You gotta find v in terms of x, and t.
Then...bah, i forget, ill get bak to u when i look in my book.