# Homework Help: Universal Gratvitation Problems

1. Nov 12, 2012

### s-win

1. An object weighing 90 N on the earth's surface would weigh only 10 N when its distance from the center of the earth is ____?

Do I solve for mass and try to find r?

2. An astronaut has a mass of 90 kg on the earth's surface. If he moves to a height of 6 earth radii above the earth's surface, how much will be his weight?

Last edited by a moderator: Nov 12, 2012
2. Nov 12, 2012

### Staff: Mentor

Welcome to the PF.

I fixed your post up a bit, and moved it to Intro Physics. The Advanced Physics forum is for upper-division and graduate-level questions.

When you fill out the Homework Help Template, take care to post your text between the bold lines you are given, instead of trying to replace that text with yours. The HH Template lines should stay visible and bold, and your text that you fill out should be plain text without any bolding.

When you fill out the HH Template, you list the Relevant Equations and your Attempt at a Solution.

What is the Relevant Equation for your first question? You don't need to solve for any mass -- the answer is independent of the mass of the object they are asking about...

3. Nov 12, 2012

### s-win

I'm kinda lost since I only learned this lesson in less than an hour and I have a test tomorrow so... I don't know what equation to use?

4. Nov 12, 2012

### Staff: Mentor

What is the equation that relates the force of gravitational attraction to the mass of the Earth and the mass of the object and the distance separating their centers of mass? It's pretty much the only equation that applies, and I'm pretty confident you were shown in in the lesson. If not, check wikipedia.org ...

We do not give out answers here at the PF. The Rules link at the top of the page is pretty clear about how we handle Homework Help. Please show your work...

5. Nov 12, 2012

### s-win

10N/90N = 1/9th of the force

1/9 = 1/r^2

6. Nov 12, 2012

### Staff: Mentor

You are on the right track. Hopefully that means you found the right equation. So the ratio of the two radii is __________, which means that the object would be how far away?

7. Nov 12, 2012

### s-win

8. Nov 12, 2012

### Staff: Mentor

Correct. Is that the form of the answer they want, or do they want a quantitative number?

9. Nov 12, 2012

### s-win

Just like that. I'm new to the website, what if I want to delete posts or adjust it to a different section?

10. Nov 12, 2012

### Staff: Mentor

Just click the Report button on your post and ask the Mentors to help. Ignore the messsage that says Reports are only for spam, etc. That's how you ask for help from the Mentors.

Keep in mind that we do not delete posts or threads that have replies.