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Equations of Motion

Dynamics · FE Reference Handbook section

Dynamics
3 formulas
10 exam-style examples
~51 min
All Dynamics 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
Equation of motion (Newton's second law) — solve for net force — Equations of Motion

A block sliding on a frictionless surface is analyzed with equations of motion. Given mass (m) = 36.8000 slug; acceleration (a) = 31.2000 ft/s^2, determine the net force (F) in lbf.

Given

  • mass(m)=36.8000slugmass (m) = 36.8000 slug
  • acceleration(a)=31.2000ft/s2acceleration (a) = 31.2000 ft/s^2

Find

net force (F), in lbf

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 1 — schematic for Equation of motion (Newton's second law) — solve for net force — Equations of Motion

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for F:

    F=maF = m a
  3. Step 3

    Listthegivens:mass(m)=36.8000slug,acceleration(a)=31.2000ft/s2List the givens: mass (m) = 36.8000 slug, acceleration (a) = 31.2000 ft/s^2
  4. Step 4 — Substitute the given values:

    F=36.800031.2000F = 36.8000 31.2000
  5. Step 5 — Evaluate:

    F=1148 lbfF = 1148\ \text{lbf}
  6. Step 6 — Check: returning F = 1,148 lbf to

    ∑F=ma\sum F = m a

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

Answer:
F=1148 lbfF = 1148\ \text{lbf}

Why the other options are there

  • 2,296 — kept a factor of two that cancels in the correct rearrangement.
  • 574.1 — dropped that same factor in the other direction.
  • 1,263 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 2
Equation of motion (Newton's second law) — solve for mass — Equations of Motion (2)

A cart towed by a cable is solved using the equations of motion. Given acceleration (a) = 7.5000 ft/s^2; net force (F) = 356.0 lbf, determine the mass (m) in slug.

Given

  • acceleration(a)=7.5000ft/s2acceleration (a) = 7.5000 ft/s^2
  • netforce(F)=356.0lbfnet force (F) = 356.0 lbf

Find

mass (m), in slug

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • Everything except m is given, so isolate m 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 2 — schematic for Equation of motion (Newton's second law) — solve for mass — Equations of Motion (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for m:

    m=Fam = \dfrac{F}{a}
  3. Step 3

    Listthegivens:acceleration(a)=7.5000ft/s2,netforce(F)=356.0lbfList the givens: acceleration (a) = 7.5000 ft/s^2, net force (F) = 356.0 lbf
  4. Step 4 — Substitute the given values:

    m=356.07.5000m = \dfrac{356.0}{7.5000}
  5. Step 5 — Evaluate:

    m=47.4667 slugm = 47.4667\ \text{slug}
  6. Step 6 — Check: returning m = 47.4667 slug to

    ∑F=ma\sum F = m a

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

Answer:
m=47.4667 slugm = 47.4667\ \text{slug}

Why the other options are there

  • 94.9333 — kept a factor of two that cancels in the correct rearrangement.
  • 23.7333 — dropped that same factor in the other direction.
  • 52.2133 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 3
Equation of motion (Newton's second law) — solve for acceleration — Equations of Motion (3)

An elevator cab's equations of motion determine cable tension during acceleration. Given mass (m) = 26.5000 slug; net force (F) = 727.0 lbf, determine the acceleration (a) in ft/s^2.

Given

  • mass(m)=26.5000slugmass (m) = 26.5000 slug
  • netforce(F)=727.0lbfnet force (F) = 727.0 lbf

Find

acceleration (a), in ft/s^2

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 3 — schematic for Equation of motion (Newton's second law) — solve for acceleration — Equations of Motion (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for a:

    a=Fma = \dfrac{F}{m}
  3. Step 3

    Listthegivens:mass(m)=26.5000slug,netforce(F)=727.0lbfList the givens: mass (m) = 26.5000 slug, net force (F) = 727.0 lbf
  4. Step 4 — Substitute the given values:

    a=727.026.5000a = \dfrac{727.0}{26.5000}
  5. Step 5 — Evaluate:

    a = 27.4340\ \text{ft/s^2}
  6. Step 6 — Check: returning a = 27.4340 ft/s^2 to

    ∑F=ma\sum F = m a

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

Answer:
a = 27.4340\ \text{ft/s^2}

Why the other options are there

  • 54.8679 — kept a factor of two that cancels in the correct rearrangement.
  • 13.7170 — dropped that same factor in the other direction.
  • 30.1774 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 4
Equation of motion (Newton's second law) — solve for net force (case 2) — Equations of Motion (4)

A block sliding on a frictionless surface is analyzed with equations of motion. Given mass (m) = 36.5000 slug; acceleration (a) = 11.7000 ft/s^2, determine the net force (F) in lbf.

Given

  • mass(m)=36.5000slugmass (m) = 36.5000 slug
  • acceleration(a)=11.7000ft/s2acceleration (a) = 11.7000 ft/s^2

Find

net force (F), in lbf

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 4 — schematic for Equation of motion (Newton's second law) — solve for net force (case 2) — Equations of Motion (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for F:

    F=maF = m a
  3. Step 3

    Listthegivens:mass(m)=36.5000slug,acceleration(a)=11.7000ft/s2List the givens: mass (m) = 36.5000 slug, acceleration (a) = 11.7000 ft/s^2
  4. Step 4 — Substitute the given values:

    F=36.500011.7000F = 36.5000 11.7000
  5. Step 5 — Evaluate:

    F=427.0 lbfF = 427.0\ \text{lbf}
  6. Step 6 — Check: returning F = 427.0 lbf to

    ∑F=ma\sum F = m a

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

Answer:
F=427.0 lbfF = 427.0\ \text{lbf}

Why the other options are there

  • 854.1 — kept a factor of two that cancels in the correct rearrangement.
  • 213.5 — dropped that same factor in the other direction.
  • 469.8 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 5
Equation of motion (Newton's second law) — solve for mass (case 2) — Equations of Motion (5)

A cart towed by a cable is solved using the equations of motion. Given acceleration (a) = 11.8000 ft/s^2; net force (F) = 811.0 lbf, determine the mass (m) in slug.

Given

  • acceleration(a)=11.8000ft/s2acceleration (a) = 11.8000 ft/s^2
  • netforce(F)=811.0lbfnet force (F) = 811.0 lbf

Find

mass (m), in slug

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • Everything except m is given, so isolate m 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 5 — schematic for Equation of motion (Newton's second law) — solve for mass (case 2) — Equations of Motion (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for m:

    m=Fam = \dfrac{F}{a}
  3. Step 3

    Listthegivens:acceleration(a)=11.8000ft/s2,netforce(F)=811.0lbfList the givens: acceleration (a) = 11.8000 ft/s^2, net force (F) = 811.0 lbf
  4. Step 4 — Substitute the given values:

    m=811.011.8000m = \dfrac{811.0}{11.8000}
  5. Step 5 — Evaluate:

    m=68.7288 slugm = 68.7288\ \text{slug}
  6. Step 6 — Check: returning m = 68.7288 slug to

    ∑F=ma\sum F = m a

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

Answer:
m=68.7288 slugm = 68.7288\ \text{slug}

Why the other options are there

  • 137.5 — kept a factor of two that cancels in the correct rearrangement.
  • 34.3644 — dropped that same factor in the other direction.
  • 75.6017 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 6
Equation of motion (Newton's second law) — solve for acceleration (case 2) — Equations of Motion (6)

An elevator cab's equations of motion determine cable tension during acceleration. Given mass (m) = 6.6000 slug; net force (F) = 886.0 lbf, determine the acceleration (a) in ft/s^2.

Given

  • mass(m)=6.6000slugmass (m) = 6.6000 slug
  • netforce(F)=886.0lbfnet force (F) = 886.0 lbf

Find

acceleration (a), in ft/s^2

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 6 — schematic for Equation of motion (Newton's second law) — solve for acceleration (case 2) — Equations of Motion (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for a:

    a=Fma = \dfrac{F}{m}
  3. Step 3

    Listthegivens:mass(m)=6.6000slug,netforce(F)=886.0lbfList the givens: mass (m) = 6.6000 slug, net force (F) = 886.0 lbf
  4. Step 4 — Substitute the given values:

    a=886.06.6000a = \dfrac{886.0}{6.6000}
  5. Step 5 — Evaluate:

    a = 134.2\ \text{ft/s^2}
  6. Step 6 — Check: returning a = 134.2 ft/s^2 to

    ∑F=ma\sum F = m a

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

Answer:
a = 134.2\ \text{ft/s^2}

Why the other options are there

  • 268.5 — kept a factor of two that cancels in the correct rearrangement.
  • 67.1212 — dropped that same factor in the other direction.
  • 147.7 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 7
Equation of motion (Newton's second law) — solve for net force (case 3) — Equations of Motion (7)

A block sliding on a frictionless surface is analyzed with equations of motion. Given mass (m) = 25.9000 slug; acceleration (a) = 23.1000 ft/s^2, determine the net force (F) in lbf.

Given

  • mass(m)=25.9000slugmass (m) = 25.9000 slug
  • acceleration(a)=23.1000ft/s2acceleration (a) = 23.1000 ft/s^2

Find

net force (F), in lbf

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 7 — schematic for Equation of motion (Newton's second law) — solve for net force (case 3) — Equations of Motion (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for F:

    F=maF = m a
  3. Step 3

    Listthegivens:mass(m)=25.9000slug,acceleration(a)=23.1000ft/s2List the givens: mass (m) = 25.9000 slug, acceleration (a) = 23.1000 ft/s^2
  4. Step 4 — Substitute the given values:

    F=25.900023.1000F = 25.9000 23.1000
  5. Step 5 — Evaluate:

    F=598.3 lbfF = 598.3\ \text{lbf}
  6. Step 6 — Check: returning F = 598.3 lbf to

    ∑F=ma\sum F = m a

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

Answer:
F=598.3 lbfF = 598.3\ \text{lbf}

Why the other options are there

  • 1,197 — kept a factor of two that cancels in the correct rearrangement.
  • 299.1 — dropped that same factor in the other direction.
  • 658.1 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 8
Equation of motion (Newton's second law) — solve for mass (case 3) — Equations of Motion (8)

A cart towed by a cable is solved using the equations of motion. Given acceleration (a) = 1.4000 ft/s^2; net force (F) = 138.0 lbf, determine the mass (m) in slug.

Given

  • acceleration(a)=1.4000ft/s2acceleration (a) = 1.4000 ft/s^2
  • netforce(F)=138.0lbfnet force (F) = 138.0 lbf

Find

mass (m), in slug

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • Everything except m is given, so isolate m 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 8 — schematic for Equation of motion (Newton's second law) — solve for mass (case 3) — Equations of Motion (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for m:

    m=Fam = \dfrac{F}{a}
  3. Step 3

    Listthegivens:acceleration(a)=1.4000ft/s2,netforce(F)=138.0lbfList the givens: acceleration (a) = 1.4000 ft/s^2, net force (F) = 138.0 lbf
  4. Step 4 — Substitute the given values:

    m=138.01.4000m = \dfrac{138.0}{1.4000}
  5. Step 5 — Evaluate:

    m=98.5714 slugm = 98.5714\ \text{slug}
  6. Step 6 — Check: returning m = 98.5714 slug to

    ∑F=ma\sum F = m a

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

Answer:
m=98.5714 slugm = 98.5714\ \text{slug}

Why the other options are there

  • 197.1 — kept a factor of two that cancels in the correct rearrangement.
  • 49.2857 — dropped that same factor in the other direction.
  • 108.4 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 9
Equation of motion (Newton's second law) — solve for acceleration (case 3) — Equations of Motion (9)

An elevator cab's equations of motion determine cable tension during acceleration. Given mass (m) = 13.9000 slug; net force (F) = 441.0 lbf, determine the acceleration (a) in ft/s^2.

Given

  • mass(m)=13.9000slugmass (m) = 13.9000 slug
  • netforce(F)=441.0lbfnet force (F) = 441.0 lbf

Find

acceleration (a), in ft/s^2

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 9 — schematic for Equation of motion (Newton's second law) — solve for acceleration (case 3) — Equations of Motion (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for a:

    a=Fma = \dfrac{F}{m}
  3. Step 3

    Listthegivens:mass(m)=13.9000slug,netforce(F)=441.0lbfList the givens: mass (m) = 13.9000 slug, net force (F) = 441.0 lbf
  4. Step 4 — Substitute the given values:

    a=441.013.9000a = \dfrac{441.0}{13.9000}
  5. Step 5 — Evaluate:

    a = 31.7266\ \text{ft/s^2}
  6. Step 6 — Check: returning a = 31.7266 ft/s^2 to

    ∑F=ma\sum F = m a

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

Answer:
a = 31.7266\ \text{ft/s^2}

Why the other options are there

  • 63.4532 — kept a factor of two that cancels in the correct rearrangement.
  • 15.8633 — dropped that same factor in the other direction.
  • 34.8993 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

Example 10
Equation of motion (Newton's second law) — solve for net force (case 4) — Equations of Motion (10)

A block sliding on a frictionless surface is analyzed with equations of motion. Given mass (m) = 22.2000 slug; acceleration (a) = 8.9000 ft/s^2, determine the net force (F) in lbf.

Given

  • mass(m)=22.2000slugmass (m) = 22.2000 slug
  • acceleration(a)=8.9000ft/s2acceleration (a) = 8.9000 ft/s^2

Find

net force (F), in lbf

Start with the thinking

  • The governing relation printed in this handbook section is Equation of motion (Newton's second law).
  • 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 equations of motion apply Newton's second law to relate the net force on a body to its mass and acceleration.
FmgBlock

Figure 10 — schematic for Equation of motion (Newton's second law) — solve for net force (case 4) — Equations of Motion (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∑F=ma\sum F = m a
  2. Step 2 — Rearrange symbolically for F:

    F=maF = m a
  3. Step 3

    Listthegivens:mass(m)=22.2000slug,acceleration(a)=8.9000ft/s2List the givens: mass (m) = 22.2000 slug, acceleration (a) = 8.9000 ft/s^2
  4. Step 4 — Substitute the given values:

    F=22.20008.9000F = 22.2000 8.9000
  5. Step 5 — Evaluate:

    F=197.6 lbfF = 197.6\ \text{lbf}
  6. Step 6 — Check: returning F = 197.6 lbf to

    ∑F=ma\sum F = m a

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

Answer:
F=197.6 lbfF = 197.6\ \text{lbf}

Why the other options are there

  • 395.2 — kept a factor of two that cancels in the correct rearrangement.
  • 98.7900 — dropped that same factor in the other direction.
  • 217.3 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Equations of Motion

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