The figure above shows the geological time scale of Earth. Life has existed for ________ of the Earth's history.
A) all
B) about 90%
C) about 50%
D) about 25%
B) about 90%
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Sunspots are known to be magnetic phenomena because
a. Doppler shifts in spectral lines are observed. b. the Zeeman effect is observed in sunspots. c. collisional broadening is observed in spectral lines. d. infrared observations indicate that the sunspots are cooler than their surroundings. e. observations during eclipses reveal a very extensive photosphere.
Astronaut Rob leaves Earth in a spaceship at a speed of 0.960c relative to an observer on Earth
Rob's destination is a star system 14.4 light-years away (one light-year is the distance light travels in one year). Relative to a frame of reference that is fixed with respect to Earth, how long does it take Rob to complete the trip? A) 14.4 years B) 15.0 years C) 10.8 years D) 4.20 years E) 22.7 years
Suppose you wanted to hold up an electron against the force of gravity by the attraction of a fixed proton some distance above it
How far above the electron would the proton have to be? (k = 1/4??0 = 9.0 × 109 N ? m2/C2, e = 1.6 × 10-19 C, mproton = 1.67 × 10-27 kg, melectron = 9.11 × 10-31 kg) A) 4.6 m B) 3.7 m C) 5.1 m D) 2.3 m E) 1.5 m
It is desired to use an electron microscope to study features in a virus that are about 0.25 nm. What accelerating voltage is needed for the electrons used in this microscope?
A. 96 V B. 48 V C. 24 V D. 5 V