How to use the Laplace Transform on a DE with a time coefficent

In summary, when solving the problem with the equation ty'' + y' = 2t2 and y(0) = 0, you need to use the equation laplace(f''(t)) = s2laplace(f(t)) -sf(0) - f'(0) and (-1)(d/ds)(F(s)) and the product rule to find the solution for Y(s). This is not the same as finding the LaPlace of y'(t). Once you have solved for Y(s), you can then take the inverse to find the final solution for y(t).
  • #1
sliperyfrog
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Homework Statement


ty'' + y' = 2t2, y(0) = 0

Homework Equations


laplace(f''(t)) = s2laplace(f(t)) -sf(0) - f'(0)
(-1) (d/ds) (F(s))

The Attempt at a Solution


I know how to solve the problem except for the ty'' part. I tried using the equation and I got -d/ds(s2Y(s) - 0 - f'(0)) which becomes -2sY(s). But this is wrong because it does not lead to the correct answer y = (2/3)t3+c1t2
 
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  • #2
sliperyfrog said:

Homework Statement


ty'' + y' = 2t2, y(0) = 0

Homework Equations


laplace(f''(t)) = s2laplace(f(t)) -sf(0) - f'(0)
(-1) (d/ds) (F(s))

The Attempt at a Solution


I know how to solve the problem except for the ty'' part. I tried using the equation and I got -d/ds(s2Y(s) - 0 - f'(0)) which becomes -2sY(s). But this is wrong because it does not lead to the correct answer y = (2/3)t3+c1t2

Your equation for ##Y(s)## is a first-order differential equation, since it involves both ##Y(s)## and ##dY(s)/ds##.
 
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  • #3
So in the problem when I get the dY(s)/ds after using the product rule what am I supposed to do? Is it the same as having the laplace of y'(t)?
 
  • #4
sliperyfrog said:
I got -d/ds(s2Y(s) - 0 - f'(0)) which becomes -2sY(s).

No. ##\frac d {ds} (s^2Y(s) \ne -2sY(s))##. You need the product rule.

sliperyfrog said:
So in the problem when I get the dY(s)/ds after using the product rule what am I supposed to do? Is it the same as having the LaPlace of y'(t)?

No, it isn't the same as the LaPlace of ##y'(t)##. You already know the LaPlace of that is ##sY(s)-y(0)##. Assuming you do correctly use the product rule, as has already been pointed out to you, you will get a differential equation in ##Y(s)## which you have to solve for ##Y(s)## before you try to take the inverse.
 
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  • #5
Thanks for the help I got the answer!
 

FAQ: How to use the Laplace Transform on a DE with a time coefficent

1. What is the Laplace Transform?

The Laplace Transform is a mathematical tool used to solve differential equations (DE) with time-dependent coefficients. It transforms a function of time into a function of a complex variable, making it easier to solve the DE.

2. How do I use the Laplace Transform on a DE with a time coefficient?

To use the Laplace Transform on a DE with a time coefficient, you first need to take the Laplace Transform of both sides of the DE. This will convert the DE into an algebraic equation that can be solved for the transformed function. Once the solution is found, you can then take the inverse Laplace Transform to obtain the solution to the original DE.

3. Do all DEs with time coefficients require the Laplace Transform?

No, not all DEs with time coefficients require the Laplace Transform. However, it is a useful tool for solving DEs with non-constant coefficients, especially for higher order DEs.

4. Are there any limitations to using the Laplace Transform on DEs with time coefficients?

Yes, there are some limitations to using the Laplace Transform on DEs with time coefficients. It may not be suitable for all types of DEs, such as those with discontinuous or singular coefficients. In addition, the Laplace Transform assumes that the initial conditions are known, which may not always be the case.

5. Is the Laplace Transform the only method for solving DEs with time coefficients?

No, the Laplace Transform is not the only method for solving DEs with time coefficients. Other methods, such as the power series method or the variation of parameters method, can also be used. However, the Laplace Transform is often preferred due to its efficiency and ability to handle more complex DEs.

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