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Physics 20A: Introduction to Astronomy
Homework assignment #1
Solutions
1. Textbook, p. 20, question #6.
Answer:
(a) 4
.
642
×
10
3
; 7
×
10
4
; 3
.
47
×
10
1
.
(b) 2
.
54
×
10

1
; 4
.
6
×
10

3
; 1
.
0243
×
10

1
.
(c) 2
,
543
,
000; 200
.
43; 0
.
0007673.
2. Textbook, p. 20, question #8.
Answer:
(a) Find the time it would take to travel a distance of 4.2 AU, traveling at a constant speed of 20 km/sec.
First, convert 4.2 AU into kilometers. 1 AU is equal to about 1
.
5
×
10
11
meters (rounding o± to 1 decimal
place accuracy), which is equal to 1
.
5
×
10
8
kilometers. So, 4.2 AU is equal to 6
.
3
×
10
8
km.
Then, using
the fact that for travel at constant speed, (speed
×
time elapsed) = (distance travelled), we can divide the
distance by the speed to obtain the time the asteroid takes for the journey, which is 3
.
15
×
10
7
seconds.
Now: how long is that? Let’s compare it to one year. There are 60 seconds in a minute, 60 minutes in an
hour, 24 hours in a day, and 365 days in a year. So the number of seconds in a year is 60
×
60
×
24
×
365,
which is equal to 3
.
15
×
10
7
seconds.
So, in the time it takes for the asteroid to reach the earth’s current position, the earth would go 1 complete
orbit around the sun and return to its current position so we might have to worry about a collision. (However,
in reality the asteroid wouldn’t be traveling at a constant speed, and the chances of a collision would be very
small.)
3. An electromagnetic wave has a frequency of 300 Megahertz. What is its wavelength?
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