The gravity on a rocket planet three times the size of the earth...?

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Depends on the density of the planet. 

The further out from the center of gravity of the planet you are, the less you weigh;  however, the *more mass* there is, the *more* you'd weigh.  

So... Earth has a density of 5.5... Let's assume the volume of this new planet is 3 times that of the Earth, and maintains the same density - so, the mass of the new planet is also 3 times that of the Earth.

V = 4/3 x pi x r^3... 

For Earth, our radius is "1" - so:  
V = 4/3(3.141592)(1)^3
V = 4.189.  

With the new planet, 3V = 4/3 x pi x r^3... so:

3(4.189) = 4/3 x (3.141592) x r^3

which boils down to 

3 = r^3, so r = 1.442

The formula for finding the force of gravity between two objects is:

F = G(m1)(m2) / r^2, where the force F equals the gravitational constant G, times the mass of the first object (the planet) times the mass of the second object (say it's you or me), all divided by r^2. 

So, the force of gravity on the new planet is going to increase the mass of M1 by 3 times, and it's going increase the radius by 44.2%: 

F = G(3m1)(m2) / (1.442^2)   (since G and m2 don't change, let's remove them.)
F = 3/1.442^2 = 3/2.079 =  1.445

so - if you weigh 100 newtons on Earth, you'd weigh 144.5 newtons on the new planet....Show more

What exactly is a "rocket planet".

"3 times the size of the Earth" is meaningless.  3 times the mass, 3 times the diameter, both?
Surface gravity is a function of mass and radius.  We need both to determine surface gravity.
And if life evolved in that environment it would survive just fine....Show more

The size of the planet is not what is important. It's the MASS of the planet that determines  the magnitude of the force of gravity. 3 times the Mass would be?3 times the gravity. Most people would find that intolerable. ...Show more