Gravity
Gravity is a universal force. It acts on all objects, and it's what
helps us stay on the Earth and not float out to space. On Earth, the
gravitational constant is about 9.81 m/s/s. Newton's Second Law states
that F=ma, where m is your mass in kg, and a in this case is the
gravitational constant. When you multiply your mass by 9.81 m/s/s, you
get the force of gravity that is acting on you.
Io and Jupiter
The moons Io, Europa, Ganymede, Callisto
are shown in order of distance from Jupiter in the image above. As you
can see, Ganymede is much larger than Io, so depending on where
Ganymede and Io are in their orbits, Ganymede is going to exert a
stronger force on Io. This helps greatly to give Io a more oval shaped
orbit path. The force of gravity from Jupiter acts greatest on Io,
since Jupiter is the largest and most dense object close to Io.
Jupiter's gravitational pull on Io causes Io to lie just within
Jupiter's magnetic field, which generates an enormous electric current
of about 3 million amps. Io's current then runs directly to Jupiter,
causing intense lightening storms in Jupiter's atmosphere.