Concept explainers
Determine the kinetic energy of the 100-kg object.
a)

The kinetic energy of the object:
Answer to Problem 1PP
The kinetic energy of the disk is 900 J.
Explanation of Solution
Given:
The mass of disk is 100 kg.
The angular velocity of the disk is ω=3 rad/s.
The radius of the disk is 2 m.
Draw the free body diagram of the rod as shown in Figure (1a).
Refer Figure (1a).
Write the formula for mass moment of inertia (I) of the disk about it is center.
I=12mr2
Here, m is the mass and r is the radius of the disk.
Write the formula for kinetic energy (T)
(Rotation about fixed axis).
T=12Iω2
Substitute 12mr2 for I.
T=12(12mr2)ω (I)
Here, ω is the angular velocity.
Conclusion:
Calculate the kinetic energy of the disk.
Substitute 100 kg for m, 2 m for r and 3 rad/s for ω in Equation (I).
T=12[12(100 kg)(2 m)2](3 rad/s)2=12(200)(9)=900 J
Thus, the kinetic energy of the disk is 900 J.
b)

The kinetic energy of the object:
Answer to Problem 1PP
The kinetic energy of the disk is 800 J.
Explanation of Solution
Given:
The mass of rod is 100 kg.
The angular velocity of the rod is ω=2 rad/s.
The length of the rod is 6 m.
Draw the free body diagram of the rod as shown in Figure (1b).
Refer Figure (1b).
Write the formula for mass moment of inertia (IO) of the rod about the point O.
IO=IG+mlOG2 (I)
Here, IG is the moment of inertial about the centroid G and IG=112ml2.
Substitute 112ml2 for IG in Equation (I).
IO=112ml2+mlOG2
Here, IO is the mass moment of inertia about point O, m is the mass, l is the total length of the rod and lOG, distance between the point O and G.
Write the formula for kinetic energy (T).
T=12IOω2
Substitute 112ml2+mlOG2 for IO.
T=12(112ml2+mlOG2)ω2 (I)
Here, ω is the angular velocity.
Conclusion:
Refer Figure (1b).
Calculate the kinetic energy of the rod.
Substitute 100 kg for m, 6 m for l, 1 m for lOG and 2 rad/s for ω in Equation (I).
T=12[112(100 kg)(6 m)2+(100 kg)(1 m)2](2 rad/s)2=12(400)(4)=800 J
Thus, the kinetic energy of the rod is 800 J.
c)

The kinetic energy of the object:
Answer to Problem 1PP
The kinetic energy of the disk is 1200 J.
Explanation of Solution
Given:
The mass of disk is 100 kg.
The angular velocity of the disk is ω=3 rad/s.
The radius of the disk is 2 m.
Draw the free body diagram of the disk as shown in Figure (1c).
Refer Figure (1c).
Write the formula for mass moment of inertia (I) of the disk about it is center.
I=12mr2
Here, m is the mass and r is the radius of the disk.
Write the formula for kinetic energy (T).
T=12mv2+12Iω2
Here, v=rω.
Substitute rω for v and 12mr2 for I.
T=12m(rω)2+12(12mr2)ω2 (I)
Here, m is the mass, r is radius, I is the mass moment of inertia and ω is the angular velocity.
Conclusion:
Refer Figure (1c).
Calculate the kinetic energy of the disk.
Substitute 100 kg for m, 2 m for r and 3 rad/s for ω in Equation (I).
T=[12(100 kg)(2 m×2 rad/s)2]+12[12(100 kg)(2 m)2](2 rad/s)2=800+400=1200 J
Thus, the kinetic energy of the disk is 1200 J.
d)

The kinetic energy of the object:
Answer to Problem 1PP
The kinetic energy of the rod is 600 J.
Explanation of Solution
Given:
The mass of rod is 100 kg.
The angular velocity of the rod is ω=2 rad/s.
The length of the rod is 3 m.
Draw the free body diagram of the rod as shown in Figure (1d).
Refer Figure (1d).
Write the formula for mass moment of inertia (IO) of the rod having the rotation axis at one end.
IO=13ml2
Here, m is the mass and l is the length of the rod about mass center O.
Write the formula for kinetic energy (T)
(Rotation about fixed axis).
T=12IOω2
Substitute 13ml2 for IO.
T=12(13ml2)ω2 (I)
Here, IO is the mass moment of inertia and ω is the angular velocity.
Conclusion:
Refer Figure (1d).
Calculate the kinetic energy of the rod.
Substitute 100 kg for m, 3 m for l and 2 rad/s for ω in Equation (I).
T=12[13(100 kg)(3 m)2](2 rad/s)2=12(300)(4)=600 J
Thus, the kinetic energy of the rod is 600 J.
e)

The kinetic energy of the object:
Answer to Problem 1PP
The kinetic energy of the disk is 4800 J.
Explanation of Solution
Given:
The mass of disk is 100 kg.
The angular velocity of the disk is ω=4 rad/s.
The radius of the disk is 2 m.
Draw the free body diagram of the disk as shown in Figure (1e).
Refer Figure (1e).
Write the formula for mass moment of inertia (I) of the disk about it is center.
I=12mr2
Here, m is the mass and r is the radius of the disk.
Write the formula for kinetic energy (T).
T=12mv2+12Iω2
Here, v=rω.
Substitute rω for v and 12mr2 for I.
T=12m(rω)2+12(12mr2)ω2 (I)
Here, m is the mass, r is radius, I is the mass moment of inertia and ω is the angular velocity.
Conclusion:
Refer Figure (1e).
Calculate the kinetic energy of the disk.
Substitute 100 kg for m, 2 m for r and 4 rad/s for ω in Equation (I).
T=[12(100 kg)(2 m×4 rad/s)2]+12[12(100 kg)(2 m)2](4 rad/s)2=3200+1600=4800 J
Thus, the kinetic energy of the disk is 4800 J.
f)

The kinetic energy of the object:
Answer to Problem 1PP
The kinetic energy of the disk is 3200 J.
Explanation of Solution
Given:
The mass of rod is 100 kg.
The angular velocity is ω=4 rad/s.
Draw the free body diagram of the object as shown in Figure (1f).
Refer Figure (1f).
Here, the ends of the rod are connected to two rods of same length. Hence the rod travels in circular motion.
Consider as the mass travels in a radius of r=2 m.
Write the formula for kinetic energy (T)
(Rotation about fixed axis).
T=12mv2
Here, v=rω.
Substitute rω for v.
T=12m(rω)2 (I)
Here, m is the mass, r is radius and ω is the angular velocity.
Conclusion:
Refer Figure (1f).
Here r=2 m.
Calculate the kinetic energy of the disk.
Substitute 100 kg for m, 2 m for r and 4 rad/s for ω in Equation (I).
T=12(100 kg)[(4 rad/s)(2 m)]2=50(64)=3200 J
Thus, the kinetic energy of the disk is 3200 J.
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Chapter 18 Solutions
ENGINEERING MECH DYNAMICS W/MASTREV
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