HW #4
9.12 (9.12)
9.20 (9.21)
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9.24
High-speed photos of a 220 microgram flea jumping vertically show that the jump lasts 1.4 milliseconds and involves an average vertical acceleration of 110g. (a) What average force does the ground exert on the flea during its jump? (b) What impulse does the ground exert on the flea during its jump?

9.29 (9.29)
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9.36
Wildlife biologists fire 21 g rubber bullets to stop a rhinoceros charging at 0.81 m/s. The bullets strike the rhino and drop vertically to the ground. The biologists' gun fires 15 bullets each second, at 76 m/s, and it takes 31 s to stop the rhino. (a) What impulse does each bullet deliver? (b) What's the rhino's mass? Neglect forces between rhino and ground.

9.54 (9.51)
9.63 (9.57)
9.76 (9.69)
10.18 (10.19)
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10.33 (10.34)
10.37 (10.39)
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10.43
An eagle with 2.0 m wingspan flaps its wings 21 times per minute, each stroke extending from 46 degrees above the horizontal to 46 degrees below. Downward and upward strokes take the same time. (a) On a given downstroke, what's the average angular velocity of the wing? (b) What's the average tangential velocity of the wingtip?

10.53
The cellular motor driving the flagellum in E. coli exerts a typical torque of 390 pN-nm on the flagellum. If this torque results from a force applied tangentially to the outside of the 13 nm-radius flagellum, what's the magnitude of that force?

10.55 (10.55)
10.58 (10.58)
10.77
You're skeptical about a new hybrid car that stores energy in a flywheel. The manufacturer claims the flywheel stores 12 MJ of energy and can supply 40 kW of power for 5 minutes. You dig deeper and find that the flywheel is a 35 cm-diameter ring with mass 45 kg that rotates at 30,000 rpm. Are the specs correct?

11.13 (11.12)
11.19
When you hold your arm outstretched, it's supported primarily by the deltoid muscle. The given figure shows a case in which the deltoid exerts a 67-N force at 15 degree to the horizontal. If the force-application point is 18 cm horizontally from the shoulder joint, what torque does the deltoid exert about the shoulder?

11.22 (11.22)
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11.35
Biomechanical engineers have developed micromechanical devices for measuring blood flow as an alternative to dye injection following angioplasty to remove arterial plaque. One experimental device consists of a 300 micron-diameter, 2.1 micron-thick silicon rotor inserted into blood vessels. Moving blood spins the rotor, whose rotation rate provides a measure of blood flow. This device exhibited an 810 rpm rotation rate in tests with water flows at several m/s. Treating the rotor as a disk, what was its angular momentum at 810 rpm? (Hint: The density of silicon is 2.33 g/cm3.)

11.41 (11.43)
11.48
You're a civil engineer for an advanced civilization on a solid spherical planet of uniform density. Running out of room for the expanding population, the government asks you to redesign your planet to give it more surface area. You recommend reshaping the planet, without adding any material or angular momentum, into a hollow shell whose thickness is one-fifth its outer radius. (a) How much will your design increase the surface area? (b) How will it change the length of the day?

11.54 (11.58)