a) The distance to a star is approximately 9.50 1018 m. If this star were to bur

Question

a) The distance to a star is approximately 9.50 1018 m. If this star were to burn out today, in how many years would we see it disappear?
years

(b) How long does it take for sunlight to reach Uranus?
minutes

(c) How long does it take for a microwave radar signal to travel from Earth to the Moon and back?
s

(d) How long does it take for a radio wave to travel once around the Earth in a great circle, close to the planet's surface?
s

(e) How long does it take for light to reach you from a lightning stroke 8.2 km away?
s

Explanation / Answer

All of these questions simply use V=d/t. The speed of light, v ( or c ) is 3e8 m/s.

a) V = d/t

3e8 m/s = 9.50e18 m / t

t = 3.1667e10 sec. To convert to years, we divide by 60, then 60, then 24, 365.25 ( the .25 accounts for a leap year...) so we have 1003.46 years. Not all textbook use the .25 leap year, so if the answer is wrong, thy just 365

b) distance from sun to uranus ( look it up in book or google ) is 2.88e12 m, so

v=d/t

3e8 m/s = 2.88e12 m / t

t = 9600 sec, divide by 60 to get minutes: t = 160 minutes

c) distance from Eart to moon is 3.84e8 m

v=d/t

3e8 m/s =3.84e8 m / t

t = .78125 sec BUT it has to come back, and travel double the distance, so double the time = 1.5625 sec

d) We need the circumference of Earth, using 2r. The radius of Earth is 6.38e6 m, so

C = 2r

C = 2(6.38e6 m) = 4.01e7 m. Now, we use:

v=d/t

3e8 m/s =4.01e7 m / t

t = .1336 sec

e) 8.2 km is 8200 m, so:

v=d/t

3e8 m/s =8200 m / t

t = 2.73e-5 sec

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