# just a quick m3 question...Watch

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#1
...which i thnk im making it out harder than it is lol (usually the case)

If a=4s+2 and initially s=0 and v=1ms^-1, find

a) the value of v when s=3m
b) the value of t when s=3m

Iv worked out a), which is 7m and which is correct lol

but i can't work out part b)?

plz help!
0
14 years ago
#2
(Original post by posh_git)
...which i thnk im making it out harder than it is lol (usually the case)

If a=4s+2 and initially s=0 and v=1ms^-1, find

a) the value of v when s=3m
b) the value of t when s=3m

Iv worked out a), which is 7m and which is correct lol

but i can't work out part b)?

plz help!
a = 4x + 2
vdv/dx = 4x + 2
0.5v^2 = 2x^2 + 2x + C
x=0, v=1:
0.5 = C

v^2 = 4x^2 + 4x + 1
0.25v^2 = x^2 + x + 0.25
0.25v^2 = (x+0.5)^2
Taking square roots of both sides gives:
0.5v = (x+0.5)
v = 2x + 1

Now v=dx/dt
dx/dt = 2x + 1
dt/dx = 1/(2x+1)
Int 1 dt = INT 1/(2x+1) dx
t = 0.5ln[2x+1] + C
t=0, x=0
0 = 0.5ln1 + c
Hence c=0 as ln1=0
t = 0.5ln[2x+1]
At x=3, t=0.5ln7
0
#3
thank you v much!!!!! lol i wud never had got tht...always leads me to think why in hell am i doing further maths!!!!!!!!!

thanks once again!
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