# Homework Help: Light Speed

1. Feb 20, 2008

### zachcumer

The Sun is 1.5 x 10^11 m from the Earth. How long does it take for the Sun's light to reach the Earth?

Above is the question I have to answer I was thinking....

300,000,000,000 meters/1.5*10^11....dunno?

2. Feb 20, 2008

### chroot

Staff Emeritus
Look at the units:

$1.5 \cdot 10^{11} \,m \cdot \frac{ 1 s }{ 3 \cdot 10^8 \,m }$

Note that the meters cancel, leaving you with seconds.

If you divide it your way, you get:

$\frac{ 3 \cdot 10^8 \, m }{ 1 s} \cdot \frac{1}{ 1.5 \cdot 10^{11} \,m}$

which has units of 1/s. This isn't a unit of time, so it's not what you want.

- Warren

3. Feb 20, 2008

### zachcumer

so.........

4. Feb 20, 2008

### chroot

Staff Emeritus
Yes?

- Warren

5. Feb 20, 2008

### zachcumer

I need it in minutes not seconds.?

6. Feb 20, 2008

### zachcumer

so the answer is 5 times 10 to the 18?

7. Feb 20, 2008

### Integral

Staff Emeritus
No, you may need to review how to multibly and divide exponents.

8. Feb 20, 2008

### zachcumer

so its 500 seconds...

9. Feb 20, 2008

### chroot

Staff Emeritus
Yes, that's correct.

- Warren