# Q & A: Different gravity for different planets

Q:
Why does the acceleration due to gravity vary on different planets?
- alfred tsang (age 15)
A:
Because they have different sizes and masses.  According to Newton's law of universal gravitation two bodies exert a force on each other proportional to the product of their masses and inversely proportional to the square of the distance between them.
So big mass -->  big acceleration due to gravity.

LeeH

(published on 10/22/2007)

## Follow-Up #1: Is gravity weak?

Q:
If gravity is a "weak force" why then does gravity always "win" ? That is, when a massive star goes supernova it is because gravity has finally overcome the strong nuclear force. Why then is gravity considered a weak force ?
- Mike Richardson (age 63)
A:

This is a delightful question, and the answer is remarkably simple.

To understand intuitively how much weaker gravity is than other forces (like electromagnetism) consider a game of tug-of-war between a magnet and the earth. The magnetic force from even a small magnet easily picks up a nail against the force of gravity from the entire earth!

So, for most small-scale physical phenomena, electricity and magnetism dominate the forces we feel and observe. Two important examples are friction, which keeps objects from moving forever with respect to each other, and the normal force, which keeps us from falling through our chairs, the floor, and the earth!

However, on a larger scale (i.e. in astronomy), electromagnetic forces cancel out almost completely, and gravity becomes strong enough to dominate observed events. This is because gravity is always attractive; all masses are positive. The gravity effects from all the parts add up.  In contrast, electromagnetic charges come in positive and negative form, and virtually all large objects are made of similar quantities of each. Therefore, the net charge of celestial bodies is generally negligible.

The strong nuclear force (chromodynamic) is even stronger between individual particles (quarks) that have its "color charge" , and behaves quite strangely. Specifically, it gets stronger as you move the quarks further apart! If you try to separate two strongly-bound quarks, the force you need to apply quickly approaches infinity. For this reason, quarks are always bound in "color-neutral" bundles. Once in these neutral bundles, they don't exert much force at all over long ranges, so you can't see the effects on the scales we directly observe.

David

(published on 02/16/2013)