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Chapter 12: Q. 20 (page 331)
The 20-cm-diameter disk in FIGURE EX12.20 can rotate on an axle through its center. What is the net torque about the axle?
The net torque on the axle is , the direction is clockwise.
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The bunchberry flower has the fastest-moving parts ever observed in a plant. Initially, the stamens are held by the petals in a bent position, storing elastic energy like a coiled spring. When the petals release, the tips of the stamen act like medieval catapults, flipping through a 60o angle in just 0.30 ms to launch pollen from anther sacs at their ends. The human eye just sees a burst of pollen; only high-speed photography reveals the details. As FIGURE CP12.85 shows, we can model the stamen tip as a 1.0mm long, 10 μg rigid rod with a 10 μg anther sac at the end. Although oversimplifying, we’ll assume a constant angular
acceleration.
a. How large is the “straightening torque”?
b. What is the speed of the anther sac as it releases its pollen?
The 2.0 kg, 30-cm-diameter disk in Figure P12.66 is spinning at 300 rpm. How much friction force must the brake apply to the rim to bring the disk to a halt in 3.0 s?
An athlete at the gym holds a 3.0 kg steel ball in his hand. His BI0 arm is 70 cm long and has a mass of 3.8 kg , with the center of mass 40% at of the arm length. What is the magnitude of the torque about his shoulder due to the ball and the weight of his arm if he holds his arm
a. Straight out to his side, parallel to the floor?
b. Straight, but below horizontal?
A 10 g bullet traveling at 400 m/s strikes a 10 kg, 1.0-m-wide door at the edge opposite the hinge. The bullet embeds itself in the door, causing the door to swing open. What is the angular velocity of the door just after impact?
Vector and vector . What is the cross product ?
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