Laplace transform by replacing s with jω provided
Mathematics · FE Reference Handbook section
Core formulas for this FE topic
Definitions, applicability, units, assumptions and worked examples for each relation.
Worked exam-style examples
The four ways this section is written on the real exam — thoughts first, then equations, then substitution.
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 5.7000 1/s; complex frequency (s) = 5.8000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.0870 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.1739 — kept a factor of two that cancels in the correct rearrangement.
- 0.0435 — dropped that same factor in the other direction.
- 0.0957 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
A student looks up a Laplace transform pair for a first-order system. Given decay constant (a) = 6.9000 1/s; transform value (F) = 0.1050 s, determine the complex frequency (s) in 1/s.
Given
Find
complex frequency (s), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except s is given, so isolate s symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for s:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning s = 2.6238 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 5.2476 — kept a factor of two that cancels in the correct rearrangement.
- 1.3119 — dropped that same factor in the other direction.
- 2.8862 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
The Laplace transform of a circuit's impulse response is evaluated at s. Given complex frequency (s) = 0.7000 1/s; transform value (F) = 0.5800 s, determine the decay constant (a) in 1/s.
Given
Find
decay constant (a), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for a:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning a = 1.0241 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 2.0483 — kept a factor of two that cancels in the correct rearrangement.
- 0.5121 — dropped that same factor in the other direction.
- 1.1266 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 5.6000 1/s; complex frequency (s) = 8.9000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.0690 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.1379 — kept a factor of two that cancels in the correct rearrangement.
- 0.0345 — dropped that same factor in the other direction.
- 0.0759 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
A student looks up a Laplace transform pair for a first-order system. Given decay constant (a) = 5.6000 1/s; transform value (F) = 1.2750 s, determine the complex frequency (s) in 1/s.
Given
Find
complex frequency (s), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except s is given, so isolate s symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for s:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning s = -4.8157 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -9.6314 — kept a factor of two that cancels in the correct rearrangement.
- -2.4078 — dropped that same factor in the other direction.
- -5.2973 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
The Laplace transform of a circuit's impulse response is evaluated at s. Given complex frequency (s) = 2.8000 1/s; transform value (F) = 1.1150 s, determine the decay constant (a) in 1/s.
Given
Find
decay constant (a), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for a:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning a = -1.9031 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -3.8063 — kept a factor of two that cancels in the correct rearrangement.
- -0.9516 — dropped that same factor in the other direction.
- -2.0935 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 2.7000 1/s; complex frequency (s) = 2.0000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.2128 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.4255 — kept a factor of two that cancels in the correct rearrangement.
- 0.1064 — dropped that same factor in the other direction.
- 0.2340 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
A student looks up a Laplace transform pair for a first-order system. Given decay constant (a) = 2.6000 1/s; transform value (F) = 0.4310 s, determine the complex frequency (s) in 1/s.
Given
Find
complex frequency (s), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except s is given, so isolate s symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for s:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning s = -0.2798 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -0.5596 — kept a factor of two that cancels in the correct rearrangement.
- -0.1399 — dropped that same factor in the other direction.
- -0.3078 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
The Laplace transform of a circuit's impulse response is evaluated at s. Given complex frequency (s) = 10.0000 1/s; transform value (F) = 0.6200 s, determine the decay constant (a) in 1/s.
Given
Find
decay constant (a), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for a:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning a = -8.3871 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -16.7742 — kept a factor of two that cancels in the correct rearrangement.
- -4.1935 — dropped that same factor in the other direction.
- -9.2258 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 6.7000 1/s; complex frequency (s) = 9.8000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.0606 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.1212 — kept a factor of two that cancels in the correct rearrangement.
- 0.0303 — dropped that same factor in the other direction.
- 0.0667 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms