# Really weird physics problem, can you help me?

1. May 27, 2012

### TrevorRyan

Suppose that, while lying on a beach near the equator watching the Sun set over a calm ocean, you start a stopwatch just as the top of the Sun disappears. You then stand, elevating your eyes by a height H = 1.70 m, and stop the watch when the top of the Sun again disappears. If the elapsed time is t = 11.1 s, what is the radius r of Earth?

2. May 27, 2012

### shreyakmath

Use the celestial sphere to solve the problem.

3. May 27, 2012

### tiny-tim

Hi TrevorRyan!

Show us what you've tried, and where you're stuck, and then we'll know how to help!

4. May 27, 2012

### shreyakmath

Yes, an accurate diagram would really benifit

5. May 27, 2012

### sacscale

Looks like a trigonometry problem. You are given two sides and the angle between them.

6. May 27, 2012

### Nessdude14

How so? You can figure the angle that the earth rotates in the problem, but you aren't given the sides that enclose that angle.

7. May 27, 2012

### sacscale

I'd like to see a better description on that first sentence too.

8. May 27, 2012

### sacscale

One side is the radius the other is the radius plus 1.70m.

9. May 27, 2012

### Nessdude14

Those sides aren't given, the radius of the earth is an unknown in this problem. If you try to apply the law of cosines, you get 2 unknowns and only one equation.

10. May 27, 2012

### sacscale

At the tangent to the surface is a right angle.

Last edited: May 27, 2012
11. May 27, 2012

### Redbelly98

Staff Emeritus
Moderator's note

At this point, please let TrevorRyan respond with an attempt toward working out the problem, in accordance with our forum guidelines on Homework Help. Please do not post further hints and help until we here from the OP.

12. May 28, 2012

### TrevorRyan

I always get to the this equation, R+1.7cos0.04625=R, and Have no idea what to do from there.

13. May 28, 2012

### ehild

Add parentheses at the proper places? Your equation means that 1.7 cos0.04625=0 which is obviously wrong.

For better understanding, I attach the missing figure. One sees the sunset when the ray arriving to the eye from the sun is tangent to the Earth.

ehild

#### Attached Files:

• ###### sunset2.JPG
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14. May 28, 2012

### TrevorRyan

wow im retarded lol. Thx ehild, now i get it