Homework Help: Weight on hypothetical planet

1. Jul 15, 2008

Ready2GoXtr

1. The problem statement, all variables and given/known data

The mass of a hypothetical planet is 1/100 that of Earth and its radius is 1/4 that of Earth. If a person weighs 150 lb on earth, what would he weigh on this planet

2. Relevant equations

I think F = G*m1*m2 / r^2

3. The attempt at a solution

I believe F = 150lbs (im not sure)

So in that problem do i do G (Me/100)(Me)/ (Re/4)^2

no im not doign something right. I really wish my professor would have covered this.

Last edited: Jul 15, 2008
2. Jul 15, 2008

rock.freak667

You have his weight on Earth, so you'd need to find his mass by dividing by acc. due to gravity on Earth.

Then use the info to find the accel. due to gravity on the planet,g'.

New weight will be mg'.

Remember how to find gravitational field strength?

3. Jul 15, 2008

Ready2GoXtr

Since my proffessor is way behind, he said Gravitational fields is one thing that we will not cover on test/final. Does this mean this question is invalid?

4. Jul 15, 2008

rock.freak667

ah my bad. You don't need to go through all of this.

Just use

$$F=\frac{GM_1m}{r^2}$$

which is what you were doing. But one of the masses is the planets mass while the other is the mass of the person (this is what you need to find).

5. Jul 15, 2008

Ready2GoXtr

150lb = 68 kg

F = G*5.98x10^24*68 / 1592500^2 = 107 N ?

6. Jul 15, 2008

rock.freak667

One mass is the mass of the planet= 5.98E24 /100

for r did you put (radius of the earth)/4 and then square?

7. Jul 15, 2008

Ready2GoXtr

yes1234

8. Jul 15, 2008

rock.freak667

Then it should be correct.

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