# Kepler's third law help.

1. Sep 18, 2007

### kylewoodloveast

A comet has a perihelion distance of 0.29 A.U. and an orbital period of 77 years. To the nearest thousandth of an A.U., what is its aphelion distance from the Sun?

I know that keplers third law plays a part in this p2=a3 but I just can't plug it in right. I know the answer is 35.908 but I don't know how to get to that point.

2. Sep 18, 2007

### kylewoodloveast

Taking the mass of Earth as 6 x 1024 kg and its radius to be 6400 km, (corresponding to a value for g of 9.77 m/s2 at the Earth's surface) what is the value for g to 2 decimal places at an elevation of 1700 km?

I know that newtons laws fall into here somewhere but I am having some trouble realizing how. I know that G'6.67 x 10-11...

3. Sep 18, 2007

### hage567

It's best to start a new thread for a new problem.

For the question in the original post: show your work, so we can see what you're doing.

4. Sep 18, 2007

### kylewoodloveast

what i did was square 77 which is 5929 and then I got the cube root of that which is 18.

5. Sep 18, 2007

### hage567

OK, so what is that number? What does it represent in the shape of the orbit?

6. Sep 18, 2007

### hage567

For the second question: yes, you need Newton's laws here. Two of them. Do you know which ones?

7. Sep 18, 2007

### kylewoodloveast

I have no idea. I have the law of centrpetal acc. and law of universal gravitation right here beside me.

8. Sep 18, 2007

### hage567

Let's concentrate on the Kepler's law question, so things don't get confused!

9. Sep 18, 2007

### kylewoodloveast

10. Sep 18, 2007

### hage567

What does Kepler's law tell you? What does the number 18 represent? Do you know about ellipses? Do you know what aphelion and perihelion mean?

11. Sep 18, 2007

### kylewoodloveast

keplers law tells me that the square of the period is equal to the cube of the au. aphelion is the farthest distance from the sun and perihelion is the closest.

12. Sep 18, 2007

### hage567

"cube of au"? OK, but do you know what that is? It's the length of the semi-major axis of the orbit. So draw out a picture of an ellipse, and label it. Put the aphelion and perihelion on it. All you need to solve this problem is to understand the geometry of the orbit. Your calculation of "a" is correct (but watch the rounding). But "a" is not what the final answer is. You now need to use this value to get the aphelion distance.