I was thinking that the reason behind the gravitational force is the magma, but after studying Newtonian model of gravity I am confused. It says that there can be gravitational force of attraction between any two masses. Then won't the gravitational force act only downwards? What actually causes the gravitational force of attraction?
[Physics] What causes gravitational force
forcesinteractionsnewtonian-gravity
Related Solutions
The statement
...the force between Any two rigid masses is only proportional to the product of their masses
is not true in general, or at least it is misleading. The shapes of the mass distributions and their relative positions matter when computing the gravitational force.
It is true that once you hold constant the shapes of the mass distributions and their relative positions, then the force will be proportional to the product of the total masses of the bodies.
There are certain situations where treating two extended massive bodies as point sources can be exactly correct (in the context of Newtonian gravity). For a spherically symmetric mass distribution, the gravitational potential outside of it is the same as that arising from a point source of the same mass. This is an application of Gauss' law.
In general, one can build up an increasingly good approximation of the gravitational potential arising from a given mass distribution via a multi-pole expansion. . The leading-order term, which drops off least rapidly with distance (force $\propto r^{-2}$), is that of a monopole like what arises for a point mass or outside a spherically symmetric system. But a general mass distribution will have contributions from higher-order terms (dipole, quadropole, octopole...), all of which drop off increasingly rapidly with distance. As one considers two bodies at increasing separation, reducing them both to their monopole terms becomes increasingly more accurate.
Finally, the fact that the gravitational force of attraction on an extended body due to another body can vary with position is essential when considering phenomena such as tidal forces.
Newton's Third Law tells us that the force on A due to B is equal (in magnitude, with opposite direction) to the force on B due to A. Therefore, in any interaction between a pair of objects it is sufficient to describe the force on just one of them, since the other can be deduced by Newton's Third Law. For this reason, it is common to refer to force acting on either object simply as the force "between" the objects. Thus, there should only be one pair of forces, with magnitude $\frac{Gm_Am_B}{r^2}$, to describe the gravitational interaction between a pair of masses.
Best Answer
In Newtonian mechanics, mass is what causes gravitational "pull". It is a pull,and only a pull, between any two masses and all masses. The magma is not itself a source of gravitational force, but the reason why it is so hot is a cause of the gravitational force. Planets form from relatively small space rocks, called asteroids, and smaller particles sometimes referred to as space dust. It is the gravitational pull that they create on each other that brings them together, and the collisions between them as they are "getting stuck" that heats them up. The reason why magma and the rest of the Earth's core are so hot is all the heat that was generated as the planet formed.