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Laplace transform by replacing s with jω provided

Mathematics · FE Reference Handbook section

Mathematics
2 formulas
10 exam-style examples
~49 min
All Mathematics lectures

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.

Example 1
Laplace transform of an exponential — solve for transform value — Laplace transform by replacing s with jω provided

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

  • decayconstant(a)=5.70001/sdecay constant (a) = 5.7000 1/s
  • complexfrequency(s)=5.80001/scomplex frequency (s) = 5.8000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=5.70001/s,complexfrequency(s)=5.80001/sList the givens: decay constant (a) = 5.7000 1/s, complex frequency (s) = 5.8000 1/s
  4. Step 4 — Substitute the given values:

    F=15.8000+5.7000F = \dfrac{1}{5.8000+5.7000}
  5. Step 5 — Evaluate:

    F=0.0870 sF = 0.0870\ \text{s}
  6. Step 6 — Check: returning F = 0.0870 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.0870 sF = 0.0870\ \text{s}

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

Example 2
Laplace transform of an exponential — solve for complex frequency — Laplace transform by replacing s with jω provided (2)

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

  • decayconstant(a)=6.90001/sdecay constant (a) = 6.9000 1/s
  • transformvalue(F)=0.1050stransform value (F) = 0.1050 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for s:

    s=1F−as = \dfrac{1}{F} - a
  3. Step 3

    Listthegivens:decayconstant(a)=6.90001/s,transformvalue(F)=0.1050sList the givens: decay constant (a) = 6.9000 1/s, transform value (F) = 0.1050 s
  4. Step 4 — Substitute the given values:

    s=10.1050−6.9000s = \dfrac{1}{0.1050} - 6.9000
  5. Step 5 — Evaluate:

    s=2.6238 1/ss = 2.6238\ \text{1/s}
  6. Step 6 — Check: returning s = 2.6238 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
s=2.6238 1/ss = 2.6238\ \text{1/s}

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

Example 3
Laplace transform of an exponential — solve for decay constant — Laplace transform by replacing s with jω provided (3)

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

  • complexfrequency(s)=0.70001/scomplex frequency (s) = 0.7000 1/s
  • transformvalue(F)=0.5800stransform value (F) = 0.5800 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for a:

    a=1F−sa = \dfrac{1}{F} - s
  3. Step 3

    Listthegivens:complexfrequency(s)=0.70001/s,transformvalue(F)=0.5800sList the givens: complex frequency (s) = 0.7000 1/s, transform value (F) = 0.5800 s
  4. Step 4 — Substitute the given values:

    a=10.5800−0.7000a = \dfrac{1}{0.5800} - 0.7000
  5. Step 5 — Evaluate:

    a=1.0241 1/sa = 1.0241\ \text{1/s}
  6. Step 6 — Check: returning a = 1.0241 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=1.0241 1/sa = 1.0241\ \text{1/s}

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

Example 4
Laplace transform of an exponential — solve for transform value (case 2) — Laplace transform by replacing s with jω provided (4)

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

  • decayconstant(a)=5.60001/sdecay constant (a) = 5.6000 1/s
  • complexfrequency(s)=8.90001/scomplex frequency (s) = 8.9000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=5.60001/s,complexfrequency(s)=8.90001/sList the givens: decay constant (a) = 5.6000 1/s, complex frequency (s) = 8.9000 1/s
  4. Step 4 — Substitute the given values:

    F=18.9000+5.6000F = \dfrac{1}{8.9000+5.6000}
  5. Step 5 — Evaluate:

    F=0.0690 sF = 0.0690\ \text{s}
  6. Step 6 — Check: returning F = 0.0690 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.0690 sF = 0.0690\ \text{s}

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

Example 5
Laplace transform of an exponential — solve for complex frequency (case 2) — Laplace transform by replacing s with jω provided (5)

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

  • decayconstant(a)=5.60001/sdecay constant (a) = 5.6000 1/s
  • transformvalue(F)=1.2750stransform value (F) = 1.2750 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for s:

    s=1F−as = \dfrac{1}{F} - a
  3. Step 3

    Listthegivens:decayconstant(a)=5.60001/s,transformvalue(F)=1.2750sList the givens: decay constant (a) = 5.6000 1/s, transform value (F) = 1.2750 s
  4. Step 4 — Substitute the given values:

    s=11.2750−5.6000s = \dfrac{1}{1.2750} - 5.6000
  5. Step 5 — Evaluate:

    s=−4.8157 1/ss = -4.8157\ \text{1/s}
  6. Step 6 — Check: returning s = -4.8157 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
s=−4.8157 1/ss = -4.8157\ \text{1/s}

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

Example 6
Laplace transform of an exponential — solve for decay constant (case 2) — Laplace transform by replacing s with jω provided (6)

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

  • complexfrequency(s)=2.80001/scomplex frequency (s) = 2.8000 1/s
  • transformvalue(F)=1.1150stransform value (F) = 1.1150 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for a:

    a=1F−sa = \dfrac{1}{F} - s
  3. Step 3

    Listthegivens:complexfrequency(s)=2.80001/s,transformvalue(F)=1.1150sList the givens: complex frequency (s) = 2.8000 1/s, transform value (F) = 1.1150 s
  4. Step 4 — Substitute the given values:

    a=11.1150−2.8000a = \dfrac{1}{1.1150} - 2.8000
  5. Step 5 — Evaluate:

    a=−1.9031 1/sa = -1.9031\ \text{1/s}
  6. Step 6 — Check: returning a = -1.9031 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=−1.9031 1/sa = -1.9031\ \text{1/s}

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

Example 7
Laplace transform of an exponential — solve for transform value (case 3) — Laplace transform by replacing s with jω provided (7)

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

  • decayconstant(a)=2.70001/sdecay constant (a) = 2.7000 1/s
  • complexfrequency(s)=2.00001/scomplex frequency (s) = 2.0000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=2.70001/s,complexfrequency(s)=2.00001/sList the givens: decay constant (a) = 2.7000 1/s, complex frequency (s) = 2.0000 1/s
  4. Step 4 — Substitute the given values:

    F=12.0000+2.7000F = \dfrac{1}{2.0000+2.7000}
  5. Step 5 — Evaluate:

    F=0.2128 sF = 0.2128\ \text{s}
  6. Step 6 — Check: returning F = 0.2128 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.2128 sF = 0.2128\ \text{s}

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

Example 8
Laplace transform of an exponential — solve for complex frequency (case 3) — Laplace transform by replacing s with jω provided (8)

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

  • decayconstant(a)=2.60001/sdecay constant (a) = 2.6000 1/s
  • transformvalue(F)=0.4310stransform value (F) = 0.4310 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for s:

    s=1F−as = \dfrac{1}{F} - a
  3. Step 3

    Listthegivens:decayconstant(a)=2.60001/s,transformvalue(F)=0.4310sList the givens: decay constant (a) = 2.6000 1/s, transform value (F) = 0.4310 s
  4. Step 4 — Substitute the given values:

    s=10.4310−2.6000s = \dfrac{1}{0.4310} - 2.6000
  5. Step 5 — Evaluate:

    s=−0.2798 1/ss = -0.2798\ \text{1/s}
  6. Step 6 — Check: returning s = -0.2798 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
s=−0.2798 1/ss = -0.2798\ \text{1/s}

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

Example 9
Laplace transform of an exponential — solve for decay constant (case 3) — Laplace transform by replacing s with jω provided (9)

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

  • complexfrequency(s)=10.00001/scomplex frequency (s) = 10.0000 1/s
  • transformvalue(F)=0.6200stransform value (F) = 0.6200 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for a:

    a=1F−sa = \dfrac{1}{F} - s
  3. Step 3

    Listthegivens:complexfrequency(s)=10.00001/s,transformvalue(F)=0.6200sList the givens: complex frequency (s) = 10.0000 1/s, transform value (F) = 0.6200 s
  4. Step 4 — Substitute the given values:

    a=10.6200−10.0000a = \dfrac{1}{0.6200} - 10.0000
  5. Step 5 — Evaluate:

    a=−8.3871 1/sa = -8.3871\ \text{1/s}
  6. Step 6 — Check: returning a = -8.3871 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=−8.3871 1/sa = -8.3871\ \text{1/s}

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

Example 10
Laplace transform of an exponential — solve for transform value (case 4) — Laplace transform by replacing s with jω provided (10)

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

  • decayconstant(a)=6.70001/sdecay constant (a) = 6.7000 1/s
  • complexfrequency(s)=9.80001/scomplex frequency (s) = 9.8000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=6.70001/s,complexfrequency(s)=9.80001/sList the givens: decay constant (a) = 6.7000 1/s, complex frequency (s) = 9.8000 1/s
  4. Step 4 — Substitute the given values:

    F=19.8000+6.7000F = \dfrac{1}{9.8000+6.7000}
  5. Step 5 — Evaluate:

    F=0.0606 sF = 0.0606\ \text{s}
  6. Step 6 — Check: returning F = 0.0606 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.0606 sF = 0.0606\ \text{s}

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

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