Pink: greater Yellow: smaller Blue: the same as the acceleration of the Moon



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CT9-4.A rock is released from rest at a point in space far from Earth, beyond the orbit of the Moon. The rock falls toward the Earth and crosses the orbit of the Moon. When the rock is the same distance from the Earth as the Moon, the magnitude of the acceleration of the rock is .. (Ignore the gravitational force between the rock and the Moon.)
Pink: greater Yellow: smaller Blue: the same as the acceleration of the Moon.
A
nswer: The acceleration of the rock and the moon have the same magnitude. If the they are the same distance from the center of Earth, they experience the same acceleration since the acceleration is

and this depends only on the distance r from the Earth's center.


As the rock falls toward the Earth, its acceleration is..


Pink: constant. Green: not constant.
Answer: Not constant. As the rock falls, r decreases, and g increases.


CT9-5. Two satellites, A and B, are in circular orbit around the earth. The distance of satellite B from the center of the Earth is twice that of satellite A. What is the ratio of the magnitudes of the accelerations of A to B?
.... Pink: 1 Green: 2 Yellow: 4 Blue: 1/2 Purple: 1/4

Answer: Qualitatively, the closer satellite will feel a larger force of gravity and will thus have a larger acceleration. Both the force (F) and the acceleration (F/m) of gravity vary as "one-over-r-squared" 1/r2. So if r doubles, the acceleration changes by a factor of 4.



CT9-6. A planet in elliptical orbit around a star moves from the point in its orbit furthest from the star to the closest point. The work done by the force of gravity during this movement is:
Pink: zero Green: Positive Yellow: Negative.



T
he work is positive. The force of gravity from the Sun is centripetal (toward the center). The force has a component along the direction of displacement.

Also, the work-energy theorem saysWnet = KE. The work must be positive, since by Kepler's second law, the planet moves faster (has higher KE) when it is nearer the Sun, so KE>0.

The planet executes one complete orbit starting from point A and returning to A. The work done by the force of gravity during this orbit is:
Pink: zero Green: Positive Yellow: Negative.

Answer: zero. The positive work done in the first half of the orbit is cancelled by the negative work in the second half.



CT9-7 Kepler's third law states that the ratio is a constant for all the planets. The period T of the Earth is 1 year. An astronomical unit (1 A.U.) is defined as the mean distance from the Earth to the Sun, therefore the mean Earth-Sun distance is 1A.U.
Consider an asteroid in circular orbit around the Sun with radius r = 2A.U. The period of the asteroid is..
Pink: 2 years. Green: 3 years Blue: 23/2  2.83 years.
Yellow: 22/3  1.59 years Purple: None of these.
Answer: The constant (T2/r3) = (1yr)2/(1 AU)3= 1 yr2 AU-3 .
The point is that since T2/r3 is the same for all planets (says Kepler III), if we know the value of T2/r3 for any planet, then we know it for all the planets. For the Earth, T2/r3 = 1, so T2/r3 must equal 1 for all the planets.

CT9-8.
At time t=0, a satellite in circular orbit about the Earth is directly over Denver, 300 miles above the city, and traveling eastward at 16,000 mph. At the same time, a rock is released from rest 300 miles above the city, very near the satellite.



True or False: at t=0+, the accelerations of the rock and the satellite are identical in magnitude and direction.


Green: true Pink: False

Answer: True! Remember: acceleration is not velocity; velocity is not acceleration. The satellite and the rock have very different velocities, but that has NOTHING to do with the acceleration. The acceleration of gravity = g = Fgrav/m = GME/r2 is the same for both the rock and the satellite because they at the same location, and g only depends on the location.
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