# Time

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**Time**

10E. Physicist Enrico Fermi once pointed out that a standard lecture period (50 Olin) is close to I microcentury. (a) How long is a rnicrocentury in minutes? (b) Using find the percentage difference from Fermi’s approximation.

liE. Express the speed of light, 3.0 X 108 m/s, in (a) feet per nanosecond and (b) millimeters per picosecond.

12E. A unit of time sometimes used in microscopic physic is the shake. One shake equals 10-8 s. (a) Are there more shakes in a second than there are seconds in a year? (b) Humans have existed for about 106 years, whereas the universe is about 1010 years old. If the age of the universe now is taken to be I universe day for how many “universe seconds have humans existed?

13P. Five clocks are being tested in a laboratory. Exactly at noon, as determined by the WWV time signal, on successive days of a week the clocks read as in the following table. Rank the five clocks according to their relative value as good timekeepers, best to worst. Justify your choice.

14P. Three digital clocks A, B. and C run at different rates and do not have simultaneous readings of zero. Shows simultaneous readings on pairs of the clocks for four occasions. (At the earliest occasion. for example, Breads 25.0 sand C reads 92.0 s.) If two events are 600 s apart on clock A, how far apart are they on (a) clock B and (b) clock C? (c) When clock A reads 400 s, what does clock B read? (d) When clock C reads 15.0 s, what does clock B read? (Assume negative readings for prezero times.).

15P. An astronomical unit (AU) is the average distance of Earth from the Sun. approximately 1.50 X 10 km. The speed of light is about 3.0 X 10 m/s. Express the speed of light in terms of astronomical units per minute.

16P. Until 1883. every city and town in the United States kept its own local time. Today. travelers reset their watches only when the time change equals 1.0 h. How far, on the average. must you travel in degrees of longitude until your watch must be reset by 1.0 h? (Him: Earth rotates 360° in about 24 h.)

17P. Assuming the length of the day uniformly increases by 0.00 10 s per century. calculate the cumulative effect on the measure of time over 20 centuries. (Such slowing of Earth’s rotation is indicated by observations of the occurrences of solar eclipses during this period.)

18P. Time standards are now based on atomic clocks. A promising second standard is based on pulsars. which are rotating neutron stars (highly compact stars consisting only of neutrons). Some rotate at a rate that is highly stable. sending out a radio beacon that sweeps briefly across Earth once with each rotation. like a light house beacon.Pular PSR 1937~21 is an example; it rotates once every 1.557 806 448 872 75 = 3 min where the trailing ±3 indithe cates the uncertainly in the last decimal place (it does not mean many times does PSR 1937:+-21 rotate in Ho much time does the pulsar take to rotate and (c) what is the associated uncertainty?