# Neeed help with a laplace transform

1. Oct 17, 2005

### Lorens

2. Oct 17, 2005

### Tom Mattson

Staff Emeritus
According to the section entitled "Homework Help" in the Physics Forums Global Guidelines which you agreed to:

So whatcha got?

3. Oct 17, 2005

### Lorens

It isn't so much to test, jsut to watch a tabel of formula, i know the transform for cos(x), but i can't find any rule which would let me multiplicat it with O(x).

I mean for problems like this you jsut think, and try to figure out how to do it.
I would say my problem is O(x).

Anyway i got to go to bed now 00:08.. lol...

4. Oct 17, 2005

### Tom Mattson

Staff Emeritus
Do you know what $\theta(t)$ is?

5. Oct 18, 2005

### Lorens

for $\theta(t)$ t<0 gives t=0 and t>0 gives t=1 and the transform is 1/s, but that knowledge don't help me much :( ...

6. Oct 18, 2005

### Tom Mattson

Staff Emeritus
You are right about $\theta(t)$, but that knowledge should help you a great deal.

You have a function that is defined piecewise:

$$\theta(t) = \left\{ \begin{array}{cc}0 & t<0\\1 & t \geq 0\end{array}$$

Now, if you multiply $\theta(t)$ by $\cos(t)$, then you just have to multiply both pieces by $\cos(t)$.

So...

$$\cos(t)\theta(t) = \left\{ \begin{array}{cc}0 & t<0\\\cos(t) & t \geq 0\end{array}$$

Can you take it from there?

Last edited: Oct 18, 2005
7. Oct 19, 2005

### Lorens

You missed that $\theta(o)$=1/2 my text book say so, but it don't matter.

Can I ignore $\theta(t)$ seen the laplace transform isn't defined for the second quadrant for the x-axis.

8. Oct 19, 2005

### Galileo

It's not a matter of being undefined, but you got the right idea. It's more precise to state that the laplace transform of f doesn't care what values f takes on for x<0.

What special signs?

9. Oct 19, 2005

### Lorens

Like $\theta(t)$ i just copyed him there

10. Oct 20, 2005