91Ó°ÊÓ

A small mass m on a string is rotating without friction in a circle. The string is shortened by pulling it through the axis of rotation without any external torque, Fig. 8–39. What happens to the angular velocity of the object?

(a) It increases.

(b) It decreases.

(c) It remains the same.

FIGURE 8-39

Mis-Conceptual Questions 10 and 11.

A rotating merry-go-round makes one complete revolution in 4.0 s (Fig. 8–41). (a) What is the linear speed of a child seated 1.2 m from the center? (b) What is her acceleration (give components)?

FIGURE 8-41 Problem 10

A small mass m on a string is rotating without friction in a circle. The string is shortened by pulling it through the axis of rotation without any external torque, Fig. 8–39. What happens to the tangential velocity of the object?

(a) It increases.

(b) It decreases.

(c) It remains the same.

FIGURE 8-39

MisConceptual Questions 10 and 11.

Two spheres look identical and have the same mass. However, one is hollow and the other is solid. Describe an experiment to determine which is which.

If there were a great migration of people toward the Earth's equator, the length of the day would

(a) increase because of conservation of angular momentum.

(b) decrease because of conservation of angular momentum.

(c) decrease because of conservation of energy.

(d) increase because of conservation of energy.

(e) remain unaffected.

Suppose you are sitting on a rotating stool holding a 2-kg mass in each outstretched hand. If you suddenly drop the masses, your angular velocity will

(a) increase.

(b) decrease.

(c) stay the same.

How fast (in rpm) must a centrifuge rotate if a particle 8.0 cm from the axis of rotation is to experience an acceleration of 100,000 g’s?

Why do tightrope walkers (Fig. 8–34) carry a long, narrow rod?

FIGURE 8-34 Question 13.

In traveling to the Moon, astronauts aboard the Apollo spacecraft put the spacecraft into a slow rotation to distribute the Sun’s energy evenly (so one side would not become too hot). At the start of their trip, they accelerated from no rotation to 1.0 revolution every minute during a 12-min time interval. Think of the spacecraft as a cylinder with a diameter of 8.5 m rotating about its cylindrical axis. Determine (a) the angular acceleration, and (b) the radial and tangential components of the linear acceleration of a point on the skin of the ship 6.0 min after it started this acceleration.

Can the diver of Fig. 8–28 do a somersault without having any initial rotation when she leaves the board? Explain.