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Radial and Transverse Components for Planar Motion

Dynamics · FE Reference Handbook section

Dynamics
7 formulas
10 exam-style examples
~59 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
Radial and transverse velocity components — solve for transverse velocity — Radial and Transverse Components for Planar Motion

A radar tracks an aircraft using radial and transverse velocity components. Given radial distance (r) = 13.1000 ft; angular rate (theta_dot) = 1.1000 rad/s; radial velocity (r_dot) = 19.0000 ft/s, determine the transverse velocity (v_theta) in ft/s.

Given

  • radialdistance(r)=13.1000ftradial distance (r) = 13.1000 ft
  • angularrate(thetadot)=1.1000rad/sangular rate (theta_dot) = 1.1000 rad/s
  • radialvelocity(rdot)=19.0000ft/sradial velocity (r_dot) = 19.0000 ft/s

Find

transverse velocity (v_theta), in ft/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except v_theta is given, so isolate v_theta 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 1 — schematic for Radial and transverse velocity components — solve for transverse velocity — Radial and Transverse Components for Planar Motion

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for v_theta:

    vtheta=rθ˙v_{theta} = r \dot{\theta}
  3. Step 3 — List the givens: radial distance (r) = 13.1000 ft, angular rate (theta_dot) = 1.1000 rad/s, radial velocity (r_dot) = 19.0000 ft/s.

  4. Step 4 — Substitute the given values:

    vtheta=13.1000θ˙v_{theta} = 13.1000 \dot{\theta}
  5. Step 5 — Evaluate:

    vtheta=14.4100 ft/sv_{theta} = 14.4100\ \text{ft/s}
  6. Step 6 — Check: returning v_theta = 14.4100 ft/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
vtheta=14.4100 ft/sv_{theta} = 14.4100\ \text{ft/s}

Why the other options are there

  • 28.8200 — kept a factor of two that cancels in the correct rearrangement.
  • 7.2050 — dropped that same factor in the other direction.
  • 15.8510 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 2
Radial and transverse velocity components — solve for radial distance — Radial and Transverse Components for Planar Motion (2)

A particle sliding in a rotating slot is analyzed with radial and transverse components. Given angular rate (theta_dot) = 4.8500 rad/s; transverse velocity (v_theta) = 4.3000 ft/s; radial velocity (r_dot) = 8.1000 ft/s, determine the radial distance (r) in ft.

Given

  • angularrate(thetadot)=4.8500rad/sangular rate (theta_dot) = 4.8500 rad/s
  • transversevelocity(vtheta)=4.3000ft/stransverse velocity (v_theta) = 4.3000 ft/s
  • radialvelocity(rdot)=8.1000ft/sradial velocity (r_dot) = 8.1000 ft/s

Find

radial distance (r), in ft

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except r is given, so isolate r 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 2 — schematic for Radial and transverse velocity components — solve for radial distance — Radial and Transverse Components for Planar Motion (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for r:

    r=vθθ˙r = \dfrac{v_\theta}{\dot{\theta}}
  3. Step 3 — List the givens: angular rate (theta_dot) = 4.8500 rad/s, transverse velocity (v_theta) = 4.3000 ft/s, radial velocity (r_dot) = 8.1000 ft/s.

  4. Step 4 — Substitute the given values:

    r=vθθ˙r = \dfrac{v_\theta}{\dot{\theta}}
  5. Step 5 — Evaluate:

    r=0.8866 ftr = 0.8866\ \text{ft}
  6. Step 6 — Check: returning r = 0.8866 ft to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
r=0.8866 ftr = 0.8866\ \text{ft}

Why the other options are there

  • 1.7732 — kept a factor of two that cancels in the correct rearrangement.
  • 0.4433 — dropped that same factor in the other direction.
  • 0.9753 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 3
Radial and transverse velocity components — solve for angular rate — Radial and Transverse Components for Planar Motion (3)

A satellite's planar motion is decomposed into radial and transverse components. Given radial distance (r) = 4.3000 ft; transverse velocity (v_theta) = 18.9000 ft/s; radial velocity (r_dot) = 11.5000 ft/s, determine the angular rate (theta_dot) in rad/s.

Given

  • radialdistance(r)=4.3000ftradial distance (r) = 4.3000 ft
  • transversevelocity(vtheta)=18.9000ft/stransverse velocity (v_theta) = 18.9000 ft/s
  • radialvelocity(rdot)=11.5000ft/sradial velocity (r_dot) = 11.5000 ft/s

Find

angular rate (theta_dot), in rad/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except theta_dot is given, so isolate theta_dot 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 3 — schematic for Radial and transverse velocity components — solve for angular rate — Radial and Transverse Components for Planar Motion (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for theta_dot:

    θdot=vθr\theta_{dot} = \dfrac{v_\theta}{r}
  3. Step 3 — List the givens: radial distance (r) = 4.3000 ft, transverse velocity (v_theta) = 18.9000 ft/s, radial velocity (r_dot) = 11.5000 ft/s.

  4. Step 4 — Substitute the given values:

    θdot=vθ4.3000\theta_{dot} = \dfrac{v_\theta}{4.3000}
  5. Step 5 — Evaluate:

    θdot=4.3953 rad/s\theta_{dot} = 4.3953\ \text{rad/s}
  6. Step 6 — Check: returning theta_dot = 4.3953 rad/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
θdot=4.3953 rad/s\theta_{dot} = 4.3953\ \text{rad/s}

Why the other options are there

  • 8.7907 — kept a factor of two that cancels in the correct rearrangement.
  • 2.1977 — dropped that same factor in the other direction.
  • 4.8349 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 4
Radial and transverse velocity components — solve for transverse velocity (case 2) — Radial and Transverse Components for Planar Motion (4)

A radar tracks an aircraft using radial and transverse velocity components. Given radial distance (r) = 4.0000 ft; angular rate (theta_dot) = 2.8000 rad/s; radial velocity (r_dot) = 12.7000 ft/s, determine the transverse velocity (v_theta) in ft/s.

Given

  • radialdistance(r)=4.0000ftradial distance (r) = 4.0000 ft
  • angularrate(thetadot)=2.8000rad/sangular rate (theta_dot) = 2.8000 rad/s
  • radialvelocity(rdot)=12.7000ft/sradial velocity (r_dot) = 12.7000 ft/s

Find

transverse velocity (v_theta), in ft/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except v_theta is given, so isolate v_theta 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 4 — schematic for Radial and transverse velocity components — solve for transverse velocity (case 2) — Radial and Transverse Components for Planar Motion (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for v_theta:

    vtheta=rθ˙v_{theta} = r \dot{\theta}
  3. Step 3 — List the givens: radial distance (r) = 4.0000 ft, angular rate (theta_dot) = 2.8000 rad/s, radial velocity (r_dot) = 12.7000 ft/s.

  4. Step 4 — Substitute the given values:

    vtheta=4.0000θ˙v_{theta} = 4.0000 \dot{\theta}
  5. Step 5 — Evaluate:

    vtheta=11.2000 ft/sv_{theta} = 11.2000\ \text{ft/s}
  6. Step 6 — Check: returning v_theta = 11.2000 ft/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
vtheta=11.2000 ft/sv_{theta} = 11.2000\ \text{ft/s}

Why the other options are there

  • 22.4000 — kept a factor of two that cancels in the correct rearrangement.
  • 5.6000 — dropped that same factor in the other direction.
  • 12.3200 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 5
Radial and transverse velocity components — solve for radial distance (case 2) — Radial and Transverse Components for Planar Motion (5)

A particle sliding in a rotating slot is analyzed with radial and transverse components. Given angular rate (theta_dot) = 1.7500 rad/s; transverse velocity (v_theta) = 52.8000 ft/s; radial velocity (r_dot) = 1.6000 ft/s, determine the radial distance (r) in ft.

Given

  • angularrate(thetadot)=1.7500rad/sangular rate (theta_dot) = 1.7500 rad/s
  • transversevelocity(vtheta)=52.8000ft/stransverse velocity (v_theta) = 52.8000 ft/s
  • radialvelocity(rdot)=1.6000ft/sradial velocity (r_dot) = 1.6000 ft/s

Find

radial distance (r), in ft

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except r is given, so isolate r 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 5 — schematic for Radial and transverse velocity components — solve for radial distance (case 2) — Radial and Transverse Components for Planar Motion (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for r:

    r=vθθ˙r = \dfrac{v_\theta}{\dot{\theta}}
  3. Step 3 — List the givens: angular rate (theta_dot) = 1.7500 rad/s, transverse velocity (v_theta) = 52.8000 ft/s, radial velocity (r_dot) = 1.6000 ft/s.

  4. Step 4 — Substitute the given values:

    r=vθθ˙r = \dfrac{v_\theta}{\dot{\theta}}
  5. Step 5 — Evaluate:

    r=30.1714 ftr = 30.1714\ \text{ft}
  6. Step 6 — Check: returning r = 30.1714 ft to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
r=30.1714 ftr = 30.1714\ \text{ft}

Why the other options are there

  • 60.3429 — kept a factor of two that cancels in the correct rearrangement.
  • 15.0857 — dropped that same factor in the other direction.
  • 33.1886 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 6
Radial and transverse velocity components — solve for angular rate (case 2) — Radial and Transverse Components for Planar Motion (6)

A satellite's planar motion is decomposed into radial and transverse components. Given radial distance (r) = 8.4000 ft; transverse velocity (v_theta) = 20.7000 ft/s; radial velocity (r_dot) = 2.8000 ft/s, determine the angular rate (theta_dot) in rad/s.

Given

  • radialdistance(r)=8.4000ftradial distance (r) = 8.4000 ft
  • transversevelocity(vtheta)=20.7000ft/stransverse velocity (v_theta) = 20.7000 ft/s
  • radialvelocity(rdot)=2.8000ft/sradial velocity (r_dot) = 2.8000 ft/s

Find

angular rate (theta_dot), in rad/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except theta_dot is given, so isolate theta_dot 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 6 — schematic for Radial and transverse velocity components — solve for angular rate (case 2) — Radial and Transverse Components for Planar Motion (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for theta_dot:

    θdot=vθr\theta_{dot} = \dfrac{v_\theta}{r}
  3. Step 3 — List the givens: radial distance (r) = 8.4000 ft, transverse velocity (v_theta) = 20.7000 ft/s, radial velocity (r_dot) = 2.8000 ft/s.

  4. Step 4 — Substitute the given values:

    θdot=vθ8.4000\theta_{dot} = \dfrac{v_\theta}{8.4000}
  5. Step 5 — Evaluate:

    θdot=2.4643 rad/s\theta_{dot} = 2.4643\ \text{rad/s}
  6. Step 6 — Check: returning theta_dot = 2.4643 rad/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
θdot=2.4643 rad/s\theta_{dot} = 2.4643\ \text{rad/s}

Why the other options are there

  • 4.9286 — kept a factor of two that cancels in the correct rearrangement.
  • 1.2321 — dropped that same factor in the other direction.
  • 2.7107 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 7
Radial and transverse velocity components — solve for transverse velocity (case 3) — Radial and Transverse Components for Planar Motion (7)

A radar tracks an aircraft using radial and transverse velocity components. Given radial distance (r) = 5.7000 ft; angular rate (theta_dot) = 2.4000 rad/s; radial velocity (r_dot) = 17.7000 ft/s, determine the transverse velocity (v_theta) in ft/s.

Given

  • radialdistance(r)=5.7000ftradial distance (r) = 5.7000 ft
  • angularrate(thetadot)=2.4000rad/sangular rate (theta_dot) = 2.4000 rad/s
  • radialvelocity(rdot)=17.7000ft/sradial velocity (r_dot) = 17.7000 ft/s

Find

transverse velocity (v_theta), in ft/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except v_theta is given, so isolate v_theta 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 7 — schematic for Radial and transverse velocity components — solve for transverse velocity (case 3) — Radial and Transverse Components for Planar Motion (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for v_theta:

    vtheta=rθ˙v_{theta} = r \dot{\theta}
  3. Step 3 — List the givens: radial distance (r) = 5.7000 ft, angular rate (theta_dot) = 2.4000 rad/s, radial velocity (r_dot) = 17.7000 ft/s.

  4. Step 4 — Substitute the given values:

    vtheta=5.7000θ˙v_{theta} = 5.7000 \dot{\theta}
  5. Step 5 — Evaluate:

    vtheta=13.6800 ft/sv_{theta} = 13.6800\ \text{ft/s}
  6. Step 6 — Check: returning v_theta = 13.6800 ft/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
vtheta=13.6800 ft/sv_{theta} = 13.6800\ \text{ft/s}

Why the other options are there

  • 27.3600 — kept a factor of two that cancels in the correct rearrangement.
  • 6.8400 — dropped that same factor in the other direction.
  • 15.0480 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 8
Radial and transverse velocity components — solve for radial distance (case 3) — Radial and Transverse Components for Planar Motion (8)

A particle sliding in a rotating slot is analyzed with radial and transverse components. Given angular rate (theta_dot) = 3.2000 rad/s; transverse velocity (v_theta) = 47.0000 ft/s; radial velocity (r_dot) = 11.9000 ft/s, determine the radial distance (r) in ft.

Given

  • angularrate(thetadot)=3.2000rad/sangular rate (theta_dot) = 3.2000 rad/s
  • transversevelocity(vtheta)=47.0000ft/stransverse velocity (v_theta) = 47.0000 ft/s
  • radialvelocity(rdot)=11.9000ft/sradial velocity (r_dot) = 11.9000 ft/s

Find

radial distance (r), in ft

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except r is given, so isolate r 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 8 — schematic for Radial and transverse velocity components — solve for radial distance (case 3) — Radial and Transverse Components for Planar Motion (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for r:

    r=vθθ˙r = \dfrac{v_\theta}{\dot{\theta}}
  3. Step 3 — List the givens: angular rate (theta_dot) = 3.2000 rad/s, transverse velocity (v_theta) = 47.0000 ft/s, radial velocity (r_dot) = 11.9000 ft/s.

  4. Step 4 — Substitute the given values:

    r=vθθ˙r = \dfrac{v_\theta}{\dot{\theta}}
  5. Step 5 — Evaluate:

    r=14.6875 ftr = 14.6875\ \text{ft}
  6. Step 6 — Check: returning r = 14.6875 ft to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
r=14.6875 ftr = 14.6875\ \text{ft}

Why the other options are there

  • 29.3750 — kept a factor of two that cancels in the correct rearrangement.
  • 7.3438 — dropped that same factor in the other direction.
  • 16.1563 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 9
Radial and transverse velocity components — solve for angular rate (case 3) — Radial and Transverse Components for Planar Motion (9)

A satellite's planar motion is decomposed into radial and transverse components. Given radial distance (r) = 8.3000 ft; transverse velocity (v_theta) = 35.3000 ft/s; radial velocity (r_dot) = 16.7000 ft/s, determine the angular rate (theta_dot) in rad/s.

Given

  • radialdistance(r)=8.3000ftradial distance (r) = 8.3000 ft
  • transversevelocity(vtheta)=35.3000ft/stransverse velocity (v_theta) = 35.3000 ft/s
  • radialvelocity(rdot)=16.7000ft/sradial velocity (r_dot) = 16.7000 ft/s

Find

angular rate (theta_dot), in rad/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except theta_dot is given, so isolate theta_dot 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 9 — schematic for Radial and transverse velocity components — solve for angular rate (case 3) — Radial and Transverse Components for Planar Motion (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for theta_dot:

    θdot=vθr\theta_{dot} = \dfrac{v_\theta}{r}
  3. Step 3 — List the givens: radial distance (r) = 8.3000 ft, transverse velocity (v_theta) = 35.3000 ft/s, radial velocity (r_dot) = 16.7000 ft/s.

  4. Step 4 — Substitute the given values:

    θdot=vθ8.3000\theta_{dot} = \dfrac{v_\theta}{8.3000}
  5. Step 5 — Evaluate:

    θdot=4.2530 rad/s\theta_{dot} = 4.2530\ \text{rad/s}
  6. Step 6 — Check: returning theta_dot = 4.2530 rad/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
θdot=4.2530 rad/s\theta_{dot} = 4.2530\ \text{rad/s}

Why the other options are there

  • 8.5060 — kept a factor of two that cancels in the correct rearrangement.
  • 2.1265 — dropped that same factor in the other direction.
  • 4.6783 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

Example 10
Radial and transverse velocity components — solve for transverse velocity (case 4) — Radial and Transverse Components for Planar Motion (10)

A radar tracks an aircraft using radial and transverse velocity components. Given radial distance (r) = 9.7000 ft; angular rate (theta_dot) = 4.0000 rad/s; radial velocity (r_dot) = 6.8000 ft/s, determine the transverse velocity (v_theta) in ft/s.

Given

  • radialdistance(r)=9.7000ftradial distance (r) = 9.7000 ft
  • angularrate(thetadot)=4.0000rad/sangular rate (theta_dot) = 4.0000 rad/s
  • radialvelocity(rdot)=6.8000ft/sradial velocity (r_dot) = 6.8000 ft/s

Find

transverse velocity (v_theta), in ft/s

Start with the thinking

  • The governing relation printed in this handbook section is Radial and transverse velocity components.
  • Everything except v_theta is given, so isolate v_theta 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.
  • Radial and transverse components for planar motion describe a particle's velocity in polar coordinates.
v_rv_\thetaParticle

Figure 10 — schematic for Radial and transverse velocity components — solve for transverse velocity (case 4) — Radial and Transverse Components for Planar Motion (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}
  2. Step 2 — Rearrange symbolically for v_theta:

    vtheta=rθ˙v_{theta} = r \dot{\theta}
  3. Step 3 — List the givens: radial distance (r) = 9.7000 ft, angular rate (theta_dot) = 4.0000 rad/s, radial velocity (r_dot) = 6.8000 ft/s.

  4. Step 4 — Substitute the given values:

    vtheta=9.7000θ˙v_{theta} = 9.7000 \dot{\theta}
  5. Step 5 — Evaluate:

    vtheta=38.8000 ft/sv_{theta} = 38.8000\ \text{ft/s}
  6. Step 6 — Check: returning v_theta = 38.8000 ft/s to

    vr=r˙,vθ=rθ˙v_r = \dot{r}, \quad v_\theta = r \dot{\theta}

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

Answer:
vtheta=38.8000 ft/sv_{theta} = 38.8000\ \text{ft/s}

Why the other options are there

  • 77.6000 — kept a factor of two that cancels in the correct rearrangement.
  • 19.4000 — dropped that same factor in the other direction.
  • 42.6800 — rounded an intermediate value before the final step.

Reference: FE Handbook — Dynamics: Radial and Transverse Components for Planar Motion

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