Equations of Motion
Dynamics · 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.
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
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.
Figure 1 — schematic for Equation of motion (Newton's second law) — solve for net force — Equations of Motion
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 = 1,148 lbf to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
Figure 2 — schematic for Equation of motion (Newton's second law) — solve for mass — Equations of Motion (2)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for m:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning m = 47.4667 slug to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
Figure 3 — schematic for Equation of motion (Newton's second law) — solve for acceleration — Equations of Motion (3)
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:
a = 27.4340\ \text{ft/s^2}Step 6 — Check: returning a = 27.4340 ft/s^2 to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
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
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 = 427.0 lbf to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
Figure 5 — schematic for Equation of motion (Newton's second law) — solve for mass (case 2) — Equations of Motion (5)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for m:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning m = 68.7288 slug to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
Figure 6 — schematic for Equation of motion (Newton's second law) — solve for acceleration (case 2) — Equations of Motion (6)
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:
a = 134.2\ \text{ft/s^2}Step 6 — Check: returning a = 134.2 ft/s^2 to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
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
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 = 598.3 lbf to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
Figure 8 — schematic for Equation of motion (Newton's second law) — solve for mass (case 3) — Equations of Motion (8)
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for m:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning m = 98.5714 slug to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
Figure 9 — schematic for Equation of motion (Newton's second law) — solve for acceleration (case 3) — Equations of Motion (9)
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:
a = 31.7266\ \text{ft/s^2}Step 6 — Check: returning a = 31.7266 ft/s^2 to
reproduces the given quantities, and both sides carry the same units.
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
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
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.
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
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 = 197.6 lbf to
reproduces the given quantities, and both sides carry the same units.
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