# Kinematics

• Feb 16th 2009, 06:32 AM
djmccabie
Kinematics
Im sure this is very easy but just learning

V=velocity
A=acceleration
S=displacement
t=time

if V=4t+5 and S=10m when t=1s, find S when t=2s

just not sure what to do :/
• Feb 16th 2009, 07:41 AM
Quote:

Originally Posted by djmccabie
Im sure this is very easy but just learning

V=velocity
A=acceleration
S=displacement
t=time

if V=4t+5 and S=10m when t=1s, find S when t=2s

just not sure what to do :/

ds/dt=4t+5
Integrate both sides with respect to t
Get the constant of integration using the value of S= 10
Then put t=2s

• Feb 16th 2009, 08:27 AM
djmccabie
so by integrating i get 2t^2 + 5t ???

and does this = s/t?

still a bit uncertain thankyou for helping
• Feb 16th 2009, 08:40 AM
1

$
\frac{ds}{dt}= 4t+5
$

2

$
ds=(4t+5)dt

$

3

$
\int{ds} =\int{4t+5}dt
$

4

$
s=\frac{4t^2}{2} + 5t +c
$

5

Now at t=1s
s= 2 + 5 +c = 10m

hence c= 3
6

Put the value of t= 2s
s= 18+3 =21m
(Nod)
Now tell the step(s) that you dont get or have a doubt in(Smirk)
• Feb 16th 2009, 08:55 AM
djmccabie
thing is i've never really been taught what the '+C' part is just told to write it (which i forgot to do lol) but yes i understand your method thanks