yang_dong's Physics Question Thread

[yang_dong]

Apparently Gravity holds solar systems together. As gravity is the only force acting on a planet, it is the net force.
However isn't their weight as well... cause W = mg and it there is g... I'm not sure... please clarify me

Thank you

[EspoirTron]

If I am not mistaken the planet's gravity wouldn't act on itself. 'W=mg' is referring to the gravitational force that would be acting on a body of mass m, given that the body lies within the gravitational field of the planet. That 'Weight' does not act on the planet itself, but rather an object within its field. So for example, a human has 'Weight' since we all live within the gravitational field of Earth.

[yang_dong]

what about the gravity of the sun? does that exist? the gravitational pull of the sun to keep it in orbit?

[rhinwarr]

Gravitational forces from anything with a mass acts on planets but most of these are negligible. The net force would be the sum of all these gravitational forces. The gravitational pull from the sun keeps Earth in orbit because it is the most significant gravitational force (it depends on the mass of both objects and their distance from each other).

[Alwin]

Apparently Gravity holds solar systems together. As gravity is the only force acting on a planet, it is the net force.
However isn't their weight as well... cause W = mg and it there is g... I'm not sure... please clarify me

Thank you

Hi

So first up, we consider what W = mg actually means. W is just a special type of force the weight force, m is a mass and g is the acceleration due to gravity in a constant gravitational field.

What we can do is 'prove' that W is a gravity force using Newton's law of universal gravitation:


So as you can see, this is where 'g=9.8' comes from (alternate calculations can be made and this is only one of them)

In space, there is no W=mg because as you can see, g only applies for objects close to the surface of earth. The only force that would act is the force of gravity (also a centripetal force fyi )

This force can also be calculated by use of Newton's law of universal gravitational constant

[yang_dong]

thanks for all the help

I was also wondering:

can a geostationary satellite remain above melbourne. why or why not?

[availn]

Nope. An unpowered satellite can only orbit around the centre of the earth, because this is the earth's centre of mass, which is where earth's gravity pulls the satellite. No way for a satellite to be geostationary over any points on earth except the equator.