Chapter 7 review


    Conceptual questions
      Section 7.2
      1. Medium The International Astronomical Union in 2006 adopted new definitions for planets and dwarf planets:
        “(1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
        (2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
        (3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as Small Solar System Bodies.”
        1. Is the Moon a dwarf planet? Why or why not?
        2. If the dust and rocks near a Solar System object have been swept up into its rings, is the object more likely to be a planet or a dwarf planet?
        3. Write one or more sentences communicating what you think is meant by “hydrostatic equilibrium” as it is written in the IAU’s text.
        4. Provide an example of an object that is not in hydrostatic equilibrium and communicate why you think so using one or more sentences.
        5. The Earth bulges out slightly along its equator. Is the Earth a planet?
        6. Research and write a one-page analysis of the IAU’s decision to reclassify Pluto as a dwarf planet. Cite specific data corroborating or challenging this decision.
      Quantitative problems
        Section 7.1
        1. What is the largest diameter of circular pipe around which you can just wrap a 10 m length of string?
        2. If a wheel has a radius of 2 m, how far does it roll in two complete rotations?
        3. Easy Robbie is swinging his favorite yo-yo in circles on a meter-long string.
          1. His classmate, Lucy, measures the instantaneous velocity of the yo-yo as 12 m/s. What is the centripetal acceleration of the yo-yo?
          2. The mass of the yo-yo is 60 g. What is the centripetal force on the yo-yo?
        4. Medium David is swinging his 0.5 m long sling and exerting a centripetal force of 1,250 N. Soon he will hurl the 250 g rock in the sling at Goliath. What is the velocity of the rock as it leaves the sling?
          Rotation of the Earth seen from above the north pole
        1. Easy Seen from a point above the north pole, the Earth spins (rotates) in a counterclockwise direction, and every point on its surface (other than the poles) traces a circular path around the planet’s axis. The radii of these paths are indicated here for two locations.
          1. With what approximate speed (in meters per second) does the equatorial spot go around the Earth’s spin axis?
          2. With what approximate speed (in meters per second) does the spot at 40°N latitude go around the Earth’s spin axis?
          3. Speculate on the reason why the USA launches most of its space probes from Florida and the Europeans launch most of theirs from the northern coast of South America.
        2. Easy On a calm day, a windmill blade 4 m long turns 0.75 rad in 3 s. What is the linear velocity of the tip of the blade?
        3. Easy A dragster is speeding down the track at 150 m/s. Its rear wheels are 2 m in diameter, and its front wheels are 40 cm in diameter. What are the angular velocities of the front and rear wheels, respectively?
        4. Easy A 1,400 kg car traveling at 35 m/s enters a curve with a radius of 100 m.
          1. How much centripetal force does it take to keep the motion of a car following the curve?
          2. Calculate the ratio of the centripetal force from Part a to the car’s weight.
          3. Will the car make it around the curve? Explain.
        5. Easy John is swinging a 100 g mass on a string around his head. On one end of the string is a spring scale that says he is exerting 5 N of centripetal force. Attached to the mass is a speedometer that tells him the mass is moving at 4 m/s. How long is the string?

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