
Concept explainers
(a)
The number of revolutions of gear C required for its angular velocity to increase from 100 to 450 rpm.
(a)

Answer to Problem 17.11P
The number of revolutions of gear C required for its angular velocity to increase from 100 to 450 rpm is 6.35 rev_.
Explanation of Solution
Given information:
The mass (mA) of gear A is 2.4 kg.
The mass (mB) of gear B is 2.4 kg.
The mass (mC) of gear C is 12 kg.
The radius of gyration (kA) of gear A is 60 mm.
The radius of gyration (kB) of gear B is 60 mm.
The radius of gyration (kC) of gear C is 150 mm.
The radius (rA) of gear A is 80 mm.
The radius (rB) of gear B is 80 mm.
The radius (rC) of gear C is 200 mm.
The initial angular velocity (ωC)1 of gear C is 100 rpm.
The final angular velocity (ωC)2 of gear C is 450 rpm.
The couple acting on gear C is 10 N⋅m.
Calculation:
Find the mass moment of inertia (IA) of gear A using the equation:
IA=mAk2A
Substitute 2.4 kg for mA and 60 mm for kA.
IA=(2.4)(60 mm×1 m1,000 mm)2=8.64×10−3 kg⋅m2
Find the mass moment of inertia (IB) of gear B using the equation:
IB=mBk2B
Substitute 2.4 kg for mB and 60 mm for kB.
IB=(2.4)(60 mm×1 m1,000 mm)2=8.64×10−3 kg⋅m2
Find the mass moment of inertia (IC) of gear C using the equation:
IC=mCk2C
Substitute 12 kg for mC and 150 mm for kA.
IC=(12)(150 mm×1 m1,000 mm)2=270×10−3 kg⋅m2
Find the initial angular velocity (ωA)1 of gear A using the kinematics:
rA(ωA)1=rC(ωC)1(ωA)1=rCrA(ωC)1
Substitute 200 mm for rC, 80 mm for rA, and 100 rpm for (ωC)1.
(ωA)1=20080(100 revmin×2π rad1 rev×1 min60 s)=26.18 rad/s
Find the initial angular velocity (ωB)1 of gear B using the kinematics:
rB(ωB)1=rC(ωC)1(ωB)1=rCrB(ωC)1
Substitute 200 mm for rC, 80 mm for rB, and 100 rpm for (ωC)1.
(ωB)1=20080(100 revmin×2π rad1 rev×1 min60 s)=26.18 rad/s
Find the initial kinetic energy (T1) of the system using the equation:
T1=(T1)A+(T1)B+(T1)C=12IA(ωA)21+12IB(ωB)21+12IC(ωC)21
Here, (T1)A is the initial angular velocity of gear A, (T1)B is the initial angular velocity of gear B, and (T1)C is the initial angular velocity of gear C.
Substitute 8.64×10−3 kg⋅m2 for IA, 26.18 rad/s for (ωA)1, 8.64×10−3 kg⋅m2 for IB, 26.18 rad/s for (ωB)1, 270×10−3 kg⋅m2 for IC, and 100 rpm for ωC.
T1={12[(8.64×10−3)(26.18)2+12(8.64×10−3)(26.18)2+12(270×10−3)](100 revmin×2π rad1 rev×1 min60 s)2}=2.9609+2.9609+14.8044=20.726 J
Find the final angular velocity (ωA)2 of gear A using the kinematics:
rA(ωA)2=rC(ωC)2(ωA)2=rCrA(ωC)2
Substitute 200 mm for rC, 80 mm for rA, and 450 rpm for (ωC)2.
(ωA)2=20080(450 revmin×2π rad1 rev×1 min60 s)=117.81 rad/s
Find the final angular velocity (ωB)2 of gear B using the kinematics:
rB(ωB)2=rC(ωC)2(ωB)2=rCrB(ωC)2
Substitute 200 mm for rC, 80 mm for rB, and 450 rpm for (ωC)2.
(ωB)2=20080(450 revmin×2π rad1 rev×1 min60 s)=117.81 rad/s
Find the initial kinetic energy (T2) of the system using the equation:
T2=(T2)A+(T2)B+(T2)C=12IA(ωA)22+12IB(ωB)22+12IC(ωC)22
Here, (T2)A is the final angular velocity of gear A, (T2)B is the final angular velocity of gear B, and (T2)C is the final angular velocity of gear C.
Substitute 8.64×10−3 kg⋅m2 for IA, 117.81 rad/s for (ωA)1, 8.64×10−3 kg⋅m2 for IB, 117.81 rad/s for (ωB)1, 270×10−3 kg⋅m2 for IC, and 450 rpm for ωC.
T2={[12(8.64×10−3)(117.81)2+12(8.64×10−3)(117.81)2+12(270×10−3)](450 revmin×2π rad1 rev×1 min60 s)2}=59.958+59.958+299.8=419.7 J
Find the work done (U1→2) due to couple acting on gear C using the equation:
U1→2=MCθC
Substitute 10 N⋅m for MC.
U1→2=10θC
Find the number of revolutions of gear C (θC) required for its angular velocity to increase from 100 to 450 rpm.
Apply Principle of work and energy for system.
T1+U1→2=T2
Substitute 20.726 J for T1, 419.7 J for T2, and 10θC for U1→2.
20.726+10θC=419.7θC=39.897 rad×1 rev2π radθC=6.35 rev
Thus, the number of revolutions of gear C required for its angular velocity to increase from 100 to 450 rpm is 6.35 rev_.
(b)
The tangential force acting on gear A.
(b)

Answer to Problem 17.11P
The tangential force acting on gear A is 7.14 N_.
Explanation of Solution
Calculation:
Find the rotation of gear A using the equation:
rAθA=rCθCθA=rCrAθC
Substitute 200 mm for rC, 80 mm for rA, and 39.897 rad for θC.
θA=20080(39.897)=99.743 rad
Find the tangential force (Ft) acting on gear A.
Apply Principle of work and energy for gear A.
(T1)A+MAθA=(T2)A12IA(ωA)21+MAθA=12IA(ωA)22
Substitute 8.64×10−3 kg⋅m2 for IA, 26.18 rad/s for (ωA)1, 99.743 rad/s for θA, and 117.81 rad/s for (ωA)2.
12(8.64×10−3)(26.18)2+MA(99.743)=12(8.64×10−3)(117.81)22.9609+99.743MA=59.958MA=0.57144 N⋅m
Find the tangential force (Ft) acting on gear A using the equation:
Ft=MArA
Substitute 0.57144 N⋅m for MA and 80 mm for rA.
Ft=0.57144(80 mm×1 m1,000 mm)=7.14 N
Thus, the tangential force acting on gear A is 7.14 N_.
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