Question

The distance to a star is approximately 8.60  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 Mars?
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 8.7 km away?

Answer 1

a)
Distance from the star, d = 8.60 * 10¹⁸ m
Speed of light, c = 3.00 * 0⁸
Time taken, t = d / c
= (8.60 * 10¹⁸) / (3.00 * 10⁸)
= 2.87 * 10¹⁰ s

1 year = 365 * 24 * 60* 60 s
= 3.15 * 10⁷ s

Number of years, n = (2.87 * 10¹⁰) / (3.15 * 10⁷)
= 909 years

b)
Distance from sun to mars, d = 207.94 * 10⁹ m
Speed of light, c = 3.00 * 0⁸
Time taken, t = d / c
= (207.94 * 10⁹) / (3.00 * 10⁸)
= 693 s
= 693 / 60 min
= 11.6 min

c)
Distance between earth and moon, d = 3.844 * 10⁸ m
Speed of microwave, c = 3.00 * 0⁸
Time taken, t = 2d / c
= (2 * 3.844 * 10⁸) / (3.00 * 10⁸)
= 2.56 s

d)
Circumference of earth, d = 2 * R
Where R is the radius of earth
= 2 * 6.371 * 10⁶ = 0.40 * 10⁸ m
Speed of radiowave, c = 3.00 * 0⁸
Time taken, t = d / c
= (0.40 * 10⁸) / (3.00 * 10⁸)
= 0.13 s

e)
Distance from the lightning source, d = 8.7 * 10³ m
Speed of light, c = 3.00 * 0⁸
Time taken, t = d / c
= (8.7 * 10³) / (3.00 * 10⁸)
= 2.9 * 10^-5 s