Chapter 5: Q. 20P (page 208)
A planet orbits a star in an elliptical orbit. At a particular instant the momentum of the planet is , and the force on the planet by the star is . Find and .
Short Answer
and
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Chapter 5: Q. 20P (page 208)
A planet orbits a star in an elliptical orbit. At a particular instant the momentum of the planet is , and the force on the planet by the star is . Find and .
and
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The radius of a merry-go round is , and it takes to go around one. What is the speed of an atom in the outer rim?
A proton moving in a magnetic field follows the curving path shown in Figure. The dashed circle is the kissing circle tangent to the path when the proton is at location A. The proton is traveling at a constant speed of 7.0 x 105 m/s, and the radius of the kissing circle is 0.08m . The mass of a proton is 1.7 x 10-27kg . Refer to the directional arrows shown at the right in Figure when answering the questions below.

(a) When the proton is at location A, what are the magnitude and direction of , the parallel component of ?
(b) When the proton is at location A, what are the magnitude and direction of , the perpendicular component of ?
An object is moving with constant momentum . What is the rate of change of momentum? What is the net force acting on the object? The object is acted on by three objects in the surroundings. Two of the forces are and . What is the third force?
In the 1970 sthe astronomer Vera Rubin made observations of distant galaxies that she interpreted as indicating that perhaps 90%of the mass in a galaxy is invisible to us ("dark matter"). She measured the speed with which stars orbit the center of a galaxy, as a function of the distance of the stars from the center. The orbital speed was determined by measuring the Doppler shift of the light from the stars, an effect that makes light shift toward the red end of the spectrum ("red shift") if the star has a velocity component away from us, and makes light shift toward the blue end of the spectrum if the star has a velocity component toward us.

She found that for stars farther out from the center of the galaxy, the orbital speed of the star hardly changes with distance from the center of the galaxy, as is indicated in Figure 5.79. The visible components of the galaxy (stars, and illuminated clouds of dust) are most dense at the center of the galaxy and thin out rapidly as you move away from the center, so most of the visible mass is near the center. (a) Predict the speed v ofa star going around the center of a galaxy in a circular orbit, as a function of the star's distance r from the center of the galaxy,assumingthat almost all of the galaxy's mass M is concentrated at the center.
(b) Construct a logical argument as to why Rubin concluded that much of the mass of a galaxy is not visible to us. Reason from principles discussed in this chapter, and your analysis of part (a). Explain your reasoning. You need to address the following issues: (i) Rubin's observations are not consistent with your prediction in (a). (ii) Most of the visible matter is in the center of the galaxy. (iii) Your prediction in (a) assumed that most of the mass is at the center.
This issue has not yet been resolved, and is still a current topic of astrophysics research. Here is a discussion by Rubin of her work: ''Dark Matter in Spiral Galaxies '' by Vera C. Rubin, Scientific American, June 1983 (96-108). You can find several graphs of the rotation curves for spiral galaxies on page 101 of this article.
When a particle with electric charge q moves with speed v in a plane perpendicular to a magnetic field B ,there is a magnetic force at right angles to the motion with magnetic qvB ,and the particle moves n a circle of radius r (see Figure 5.77). This equation for the magnetic force is correct even if the speed is comparable to the speed of light. Show that
even if is comparable to c.
This result is used to measure relativistic momentum: if the charge q is known, we can determine the momentum of a particle by observing the radius of a circular trajectory in a known magnetic field.

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