# Math Help - Rates of Decay

1. ## Rates of Decay

In the question attached what is the answer to part b) i can do part a) and c) thanks

2. Originally Posted by nath_quam
In the question attached what is the answer to part ii) i can do part 1 and 3 thanks
you could see the $\frac{dI}{dt}= -5(1-cosx)$
where x= $\frac {\pi*t}{12}$
now, 1- cosx is always greater than or equal to zero.
if 1- cosx is greater than zero., I is decreasing
I is stationary if cosx =1 i.e. $x=\frac{\pi}{2}$
or $\frac {\pi t}{12}=\frac{\pi}{2}$
hence I is stationary at $t=6$

3. What Malaygoel forgot to mention though it is implicit in what he wrote
is that a differentiable function $f(x)$ is decreasing at $x$ if and only
if $f'(x)<0$ and is stationary if and only if $f'(x)=0$

RonL

4. ## Stationary Points

Thanks guys, for this question there must be other stationary points such as 18?? and so on

5. Originally Posted by malaygoel
you could see the $\frac{dI}{dt}= -5(1-cosx)$
where x= $\frac {\pi*t}{12}$
now, 1- cosx is always greater than or equal to zero.
if 1- cosx is greater than zero., I is decreasing
I is stationary if cosx =1 i.e. $x=\frac{\pi}{2}$
or $\frac {\pi t}{12}=\frac{\pi}{2}$
hence I is stationary at $t=6$
$\cos(x)=1$ only when x is a multiple of $2\pi$,
so the stationary points are solutions of:

$
\frac{\pi t}{12}=n \pi
$
,

or: $t=12n$, $n=0,\ 1,\ ...$.

RonL

6. the question asks for when I is stationary, ain't we saying that at t = 12n the rate is stationary because for example at t = 12 -- I is decreasing still

7. Originally Posted by nath_quam
the question asks for when I is stationary, ain't we saying that at t = 12n the rate is stationary because for example at t = 12 -- I is decreasing still
I is stationary when dI/dt=0.

Stationary means the rate of change is zero.

RonL

9. Small correction, Cap'n . . .

$\cos(x)=1$ only when x is a multiple of $2\pi$,

so the stationary points are solutions of: . $\frac{\pi}{12}t \,=\,$ 2 $\pi\cdot n$

or: . $t \,= \,24n,\;\;n=0,\,1,\,2, ...$.

10. Originally Posted by Soroban
Small correction, Cap'n . . .

$\cos(x)=1$ only when x is a multiple of $2\pi$,

so the stationary points are solutions of: . $\frac{\pi}{12}t \,=\,$ 2 $\pi\cdot n$

or: . $t \,= \,24n,\;\;n=0,\,1,\,2, ...$.

Hello Soroban
Does it make any sense that I is stationary at $t=0(n=0)$.

11. Hello, malaygoel!

Does it make any sense that I is stationary at $t=0\,(n=0)$.
Yes, it does . . .

An analogous example:
A particle can be at rest at $t = 0\:(v = 0)$.
A nano-jiffy later it is moving $(v > 0)$ due to some acceleration.

12. Originally Posted by Soroban
Small correction, Cap'n . . .

$\cos(x)=1$ only when x is a multiple of $2\pi$,

so the stationary points are solutions of: . $\frac{\pi}{12}t \,=\,$ 2 $\pi\cdot n$

or: . $t \,= \,24n,\;\;n=0,\,1,\,2, ...$.

Oops

Thanks for spotting that.

Also perhaps a reminder to everyone to check what the helpers write,
after all we all make mistakes at some time

RonL

13. Originally Posted by Soroban
Hello, malaygoel!

Yes, it does . . .

An analogous example:
A particle can be at rest at $t = 0\v = 0)" alt="t = 0\v = 0)" />.
A nano-jiffy later it is moving $(v > 0)$ due to some acceleration.
Imagine the motion of the same particle.
Let $x=f(t)$ describes the motion of the particle. Consider the motion from $t=0$ to $t=10$.Can we find $\frac{dx}{dt}$at $t=0.$. In other words, if a function is defined on the closed interval [a,b], can it be differentiable on one of its end-points e.g.'a'?