PHYSICS 1403 QUIZ 3, Chapter 5, March 6, 2006
Partial credit will be LIBERAL, provided that you show your work! USE BACK IF NECESSARY!
NOTE: In the following, PLEASE use scientific (power of 10) notation for large & small numbers!
Name: ______________________
1.
(10 points) CONCEPTUAL QUESTION. See figure. A ball, mass m, is twirled at the
end of a string in a vertical circle of radius r at constant speed v. Free body diagrams for
the ball at the top & at the bottom of the circle are shown. Is the string tension FTA that
the string exerts on the ball at the top (point A) less than, more than, or the same as the
tension FTB at the bottom (point B)? Why? Explain (using WORDS, as well as
equations!) your answer using Newton’s 2nd Law with centripetal acceleration!
2.
(20 points). See figure. A planet, mass m = 8.0 x 1024 kg, is in a circular orbit at constant speed
around a star. The orbit radius (measured from the star’s center) is r = 1.8 x 1011 m. The mass of
the star is M = 2.8 x 1030 kg. G = 6.67 x 10-11 N m2/kg2. In the following, PLEASE be careful in
calculating with large & small numbers!
a. Compute the gravitational force of attraction between the planet and the star. (Hint: This
is very large!)
circular orbit!
b.
Compute the centripetal force and the centripetal acceleration experienced by the planet. What
is their direction? (Hint: You don’t need to know the planet’s speed to answer this! Use the
results of part a along with Newton’s 2nd Law!)
c.
Use the results of part b to compute the planet’s speed in orbit. Note: Einstein told us that
NOTHING can move faster than the speed of light, c = 3 x 108 m/s! If you get a v greater
than this (or a significant fraction of it!) SOMETHING IS WRONG with your calculation!
d.
Compute the period of the planet’s orbit. Note: Your answer should be something reasonable
for a planet’s period! If you get a period of only a few minutes or seconds (or less!) or one of
billions of years, SOMETHING IS WRONG with your calculation!