What characteristic of a star cluster is used to determine its age?
A) the ratio of main sequence to white dwarfs stars
B) the number of red giants
C) the faintest stars seen in the cluster
D) the main sequence turnoff
E) the total number of stars in the cluster
D
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Equilibrium: A store's sign has a mass of 20 kg and is 3.0 m long. It is uniform, so its center of gravity is at the center of the sign. It is supported horizontally by a small loose bolt attached to the wall at one end and by a wire at the other end, as shown in the figure. What is the magnitude of the net force that the bolt exerts on the sign?
A. 460 N B. 230 N C. 200 N D. 120 N E. 300 N
A variable resistor is connected to a constant voltage source. When the resistance is increased from 5.5 
to 9.3
what happens to the power dissipated?
A. no change B. a 35% decrease C. a 41% decrease D. a 59% decrease E. a 69% decrease
How many radioactive atoms are present in a sample that has an activity of 0.5 mCi and a half-life of 10 years? (1 curie = 3.7 × 10^10 decays/s)
a. 9.5 × 10^12 atoms b. 8.4 × 10^12 atoms c. 7.3 × 10^12 atoms d. 6.5 × 10^12 atoms e. 2.7 × 10^5 atoms
Mercury has a radius of about 0.38 Earth radii and a mass of only 0.055 Earth masses. Estimate the acceleration due to gravity on Mercury.
A. 1.45 m/s2 B. 3.81 m/s2 C. 26.3 m/s2 D. 69.1 m/s2