# Homework Help: Time for light to reach earth homework

1. Oct 17, 2009

### phys1120

QUESTION 1 (3 points):
a) Given that the Sun is 150,000,000 km from Earth, how long does it take
light to travel from the Earth to the Sun. Give your answer in minutes (min)

b) The Cassini spacecraft, currently in orbit about Saturn, sends its data to
Earth using radio waves. Assume that Saturn is at its closest approach to
Earth. How long does it take the signals to reach Earth from Saturn? Give

QUESTION 2 (3 points):
A regular incandescent light bulb radiates most strongly at a wavelength of
3000 nm.
a) How hot is the filament? Give your answer in Kelvins (K)

b) In which part of the spectrum does it emit the most radiation: visible, or
infrared (heat)?

QUESTION 3 (6 points):
Use the orbit of Ganymede to calculate the mass of Jupiter in kilograms. You
will need to use the size of the orbit and orbital period, which can be found in
Table 8.1 on page 216 of the 6th edition of the text. (Any other text boox will
have a table of appendix with this information. Remember that the units to
calculate the mass have to be in metres, and seconds if you want to get the
mass in kilograms.

2. Oct 17, 2009

### ideasrule

Generally, we expect people who want help with homework to list all relevant equations and show their attempt at all questions. Please do this and we'll help you.

BTW, this should really go in the "Homework and Coursework" section.

3. Oct 17, 2009

### phys1120

#1a) time = distance
speed
time = 150,000,000
299,792.458m/s
time = 150,000,000
482,469,193.528km/s
time = 0.3109km/s
time = 18.654km/m
b) NO IDEA!

#2a) wavelength = 1
temperature
3000nm = 2.9 x10^6
temperature
3000nm = 2,900,000
temperature
temperature = 966.667
b)infrared

#3) M = 4pie^2a^3
GP^2
M = 39.478 x 1634.206m^3
6.7 7.15^2
M = 5.892 x 2,417,217.876
51.123
M = 5.892 x 47,285.17
M = 278,604.222

4. Oct 18, 2009

### ideasrule

I think there was a problem that messed up your formatting and deleted some of your text, but I'll help the best I can.

For 1a), all you need to know is that time equals distance divided by speed. This should be obvious; if you're driving at 40 km/h, you'd take one hour to cover 40 km, 2 hours to cover 80 km, and so on. The speed of light is 299 792.458 km/s (not m/s) and the distance from the Sun to the Earth is 150 000 000 km. So how many seconds does it take sunlight to reach earth?

For 1b), try looking at the back of your textbook for the orbital parameters of Saturn and Earth. How far is Saturn from Earth at closest approach?

All of this is correct.
I can't follow this, but 278 604 kg can't possibly be right. That's 278 tons, about the weight of a Boeing 747, but Jupiter is a planet much more massive (by far) than the Earth.