HELP with planetay physics problem about Neptune and ones weight on it.?

Question

Neptune has a mass 17.2 times larger than that of Earth and a radius 3.88 times larger. A person weighing 128 lb on the Earth would weigh how much on Neptune?

HELP: Begin with Weight=GmpMe/re2 and alter this to take into account Neptune's mass and radius.

HELP: It's a simple ratio. Remember, it's asking for weight, not mass.

After solving the problem could you please include a short explanation of how you got the answer and also round to 5 decimal places. The answer should be around 144lbs. Tahnks for the help.

Answer 1

You've got the correct equation, the key is to know what's changing and what's not.

On the right hand side of the equation, you have 4 terms, right?  But on both planets, G is the same because it's a universal constant for that equation, and mp (mass of the person) is the same because mass is also something that is unchanging no matter where you are in the universe.

Therefore, the only things that change are Me (mass of the planet) and R (radius of the planet).

You can plug in the numbers you have into the equation and find out what your weight would be on neptune, but you'd have to find out what the mass of earth is (and multiply it by 17.2) and find out the radius of earth (and multiply it by 3.88).  Those values are easily found in any astronomy book or by googling, but there is an easier way.

The second hint says that the ratio is all you need- that's because of what I said in my first paragraph about only two things changing.  So just calculate the things that change between the planets (mass of planet and radius of planet).  Then all you need to do 17.2 / (3.88)^2, and that's your ratio: 1.142523.  Multiply that by 128 and you've got your answer: 146.24

Hope that helps.