Chapter 8: 8-14Q (page 198)
We claim that momentum and angular momentum are conserved. Yet most moving or rotating objects eventually slow down and stop. Explain.
Due to the presence of the frictional force, the net force on the object is not equal to zero. That’s why most moving or rotating objects slow down and stop.
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A potter is shaping a bowl on a potter's wheel rotating at a constant angular velocity of 1.6 rev/s (Fig. 8–48). The frictional force between her hands and the clay is 1.5 N. (a) How large is her torque on the wheel if the diameter of the bowl is 9.0 cm? (b) How long would it take for the potter's wheel to stop if the only torque acting on it is due to the potter's hands? The moment of inertia of the wheel and the bowl is \(0.11\;{\rm{kg}} \cdot {{\rm{m}}^{\rm{2}}}\).
FIGURE 8-48
Problem 40
A centrifuge rotor rotating at 9200 rpm is shut off and eventually brought uniformly to rest by a frictional torque of \({\bf{1}}{\bf{.20}}\;{\bf{m}} \cdot {\bf{N}}\). If the mass of the rotor is 3.10 kg, and it can be approximated as a solid cylinder of radius 0.0710 m, through how many revolutions will the rotor turn before coming to rest, and how long will it take?
The bolts on the cylinder head of an engine require tightening to a torque of 95 m N. If a wrench is 28 cm long, what force perpendicular to the wrench must the mechanic exert at its end? If the six-sided bolt head is 15 mm across (Fig. 8–44), estimate the force applied near each of the six points by a wrench.
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