What is the laplace transform of u(-t)?

In summary, the Laplace transform of u(-t) is equal to 0 for t > 0 and undefined for t < 0. This is because the unit step function approaches from the negative side and the value at t = 0 is finite, resulting in a transform of 0. However, for functions that have an infinite value at t = 0, the Laplace transform may still be non-zero. This is only applicable to one-sided Laplace transforms, as two-sided transforms have a different approach.
  • #1
asd1249jf

Homework Statement


Find the laplace transform of u(-t)


Homework Equations





The Attempt at a Solution


For u(t), the laplace transform of it is 1/s, basically taking the integral of e^-st from 0 to infinity.

In this case, since the unit step function approaches from the negative side, do I just take the integral of e^(-st), but switch the limit (from infinity to 0) for the purpose of transformation, leaving with -1/s?
 
Physics news on Phys.org
  • #2
What is the value of u(-t) for t>0... what does that say about the integral of u(-t)*[e^(-st)]?
 
  • #3
learningphysics said:
What is the value of u(-t) for t>0... what does that say about the integral of u(-t)*[e^(-st)]?

u(-t) for t > 0 is equal to 0, does that immediately lead to the conclusion that the transformation of u(-t) = 0? What if t < 0?
 
  • #4
l46kok said:
u(-t) for t > 0 is equal to 0, does that immediately lead to the conclusion that the transformation of u(-t) = 0? What if t < 0?

Yes, though a function can be 0 for t>0, but still have non-zero laplace transform... like the unit impulse function... but that's because the value at 0 is infinite... the laplace transform is defined from 0- (a value slightly less than 0) to infinity to include what happens at t=0... what happens for t<0 doesn't matter... but what happens at zero does matter.

But for u(-t) the value at t=0 is finite... so the integral is 0.

Hope I'm not missing anything... I think this is all true...

This is all for one-sided laplace transforms... two-sided is different...
 
Last edited:

Related to What is the laplace transform of u(-t)?

1. What is a unit step function transform?

A unit step function transform, also known as the Heaviside step function, is a mathematical function that is defined as 0 for negative input values and 1 for positive input values. It is commonly used in signal processing and control systems to model sudden changes or jumps in a system.

2. How is a unit step function transform represented mathematically?

The unit step function transform is typically represented by the symbol u(t), with t representing the input variable. It can also be represented as H(t) or θ(t). The function can be written as u(t) = 0 for t < 0 and u(t) = 1 for t ≥ 0.

3. What is the Laplace transform of a unit step function?

The Laplace transform of a unit step function is 1/s, where s is the complex variable used in the Laplace transform. This can be derived by using the definition of the Laplace transform and integrating the unit step function from 0 to infinity.

4. How is the unit step function transform used in real-world applications?

The unit step function transform is commonly used in engineering and physics to model systems with sudden changes or jumps. It can also be used in economics and finance to represent events such as changes in interest rates or stock prices.

5. What is the inverse transform of a unit step function?

The inverse transform of a unit step function is 1, as the function is discontinuous and has no inverse. However, the inverse Laplace transform of 1/s is the unit step function, as it is the function that, when transformed, results in 1/s.

Similar threads

  • Calculus and Beyond Homework Help
Replies
3
Views
2K
  • Calculus and Beyond Homework Help
Replies
2
Views
227
  • Calculus and Beyond Homework Help
Replies
10
Views
1K
  • Calculus and Beyond Homework Help
Replies
4
Views
2K
  • Calculus and Beyond Homework Help
Replies
1
Views
646
  • Calculus and Beyond Homework Help
Replies
3
Views
815
  • Calculus and Beyond Homework Help
Replies
4
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
1
Views
151
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
Back
Top