6. "The Moon is an average distance of 3.8108m from Earth. It circles Earth once each 27.3 days. (a) What is its average speed? (b) What is its acceleration? (c) How does this acceleration compare to the acceleration of free fall on Earth?"

8. "BIO Ultracentrifuge You are working in a biology lab and learning to use a new ultracentrifuge for blood tests. The specifications for the centrifuge say that a red blood cell rotating in the ultracentrifuge moves at 470ms and has a radial acceleration of 150,000 g's (that is, 150,000 times 9.8ms2). The radius of the centrifuge is 0.15 m. You wonder if this claim is correct. Support your answer with a calculation."

25. "* Roller coaster ride A roller coaster car travels at speed 8.0ms over a 12-m-radius vertical circular hump. What is the magnitude of the upward force that the coaster seat exerts on a 48-kg woman passenger?"

28. "* A 20.0-g ball is attached to a 120-cm-long string and moves in a horizontal circle (see Figure P5.28). The string exerts a force on the ball that is equal to 0.200 N. What is the angle ?"

41. " EST The average radius of Earth's orbit around the Sun is 1.5108km. The mass of Earth is 5.971024kg, and it makes one orbit in approximately 365 days. (a) What is Earth's speed relative to the Sun? (b) Estimate the Sun's mass using Newton's law of universal gravitation and Newton's second law. What assumptions did you need to make?"

52. "* A spaceship in outer space has a doughnut shape with 500-m outer radius. The inhabitants stand with their heads toward the center and their feet on an outside rim (see Figure Q5.21). Over what time interval would the spaceship have to complete one rotation on its axis to make a bathroom scale have the same reading for the person in space as when on Earth's surface? Indicate any assumptions that you made."

4. "A rope attached to a truck pulls a 180-kg motorcycle at 9.0ms. The rope exerts a 400-N force on the motorcycle at an angle of 15 above the horizontal. (a) What is the work that the rope does in pulling the motorcycle 300 m? (b) How will your answer change if the speed is 12ms? (c) How will your answer change if the truck accelerates?"

18. "* BIO EST Heart pumps blood The heart does about 1 J of work while pumping blood into the aorta during each heartbeat. (a) Estimate the work done by the heart in pumping blood during a lifetime. (b) If all of that work was used to lift a person, to what height could an average person be lifted? Indicate any assumptions you used for each part of the problem."

21. "* BIO EST Climbing Mt. Everest In 1953 Sir Edmund Hillary and Tenzing Norgay made the first successful ascent of Mt. Everest. How many slices of bread did each climber have to eat to compensate for the increase of the gravitational potential energy of the system climbers-Earth? (One piece of bread releases about 1.0106J of energy in the body.) Indicate all of the assumptions used. Note: The body is an inefficient energy convertersee the reading passage at the end of this section."

22. "A door spring is difficult to stretch. (a) What maximum force do you need to exert on a relaxed spring with a 1.2104-Nm spring constant to stretch it 6.0 cm from its equilibrium position? (b) How much does the elastic potential energy of the spring change? (c) Determine its change in elastic potential energy as it returns from the 6.0-cm stretch position to a 3.0-cm stretch position. (d) Determine its elastic potential energy change as it moves from the 3.0-cm stretch position back to its equilibrium position."