What will the traveler weigh on another planet?

[pookisantoki]

A Space traveler weighs 695N on Earth. What will the traveler weigh on another planet whose radius is 3 times that of Earth and whose mass is 2 times that of earth?

So for this problem I got the mass and radius of the earth
Mass of earth= (5.98 *10^24)
Radius of Eath- (6.38 *10^6)
Since it says mass of Earth is twice of Earth I multipled it by two (5.98 * 10^24)*2= 1.196*10^25
and I multiplied the radius of Earth by three (6.38*10^6)*3=19140000

Then I plugged it into the Newton's law of universal gravitation formula
F=G(m1*m2/R^2)
G=6.674*10^-11

F=(6.674*10^-11)((1.196*10^25)/(19140000^2))=2.898*10^-10
but its wrong what did I do wrong. Thank you!

 

[Doc Al]

F=(6.674*10^-11)((1.196*10^25)/(19140000^2))=2.898*10^-10
but its wrong what did I do wrong. Thank you!

Two problems:
(1) You forgot the traveler's mass.
(2) You messed up the exponent.

Rather than do all that number crunching, just use ratios. Much less chance of error that way.

 

[pookisantoki]

Ratios...How would i set that up?

Would i just put (695*2)/3?

 

[Doc Al]

Ratios...How would i set that up?

Would i just put (695*2)/3?

Almost, but not exactly. (The radius should be squared.)

Try this:
Weight on Earth = GmM/R^2 = 695 N
Weight on planet = Gm(2M)/(3R)^2 = ?

Compare those two expressions.

 

[pookisantoki]

so I plugged the formula in:
((6.67*10^-11)(695)((5.98*10^24)*2))/(((6.38*10^6)^2)*3)=4540.244
I plugged that in as the answer but it's still wrong...Am i missing a step? or is my calculation wrong?

 

[Doc Al]

so I plugged the formula in:
((6.67*10^-11)(695)((5.98*10^24)*2))/(((6.38*10^6)^2)*3)=4540.244
I plugged that in as the answer but it's still wrong...Am i missing a step? or is my calculation wrong?

Two new problems:
(1) 695 is the traveler's weight, not his mass. Find his mass.
(2) You multiplied the demoninator by 3 after you squared the radius instead of before.

But this is still the hard way. I'd still play around with the suggestion for using ratios that I gave in my last post:

Hint: Weight on planet = Gm(2M)/(3R)^2 = 2/(3^2) * GmM/R^2
(And you already know what GmM/R^2 equals.)