Hello all,
I have a simple question, that has challenged my mind for quite a while now.
The question is: since the moon is in orbit due to the Earth's gravitational force, why doesn't the moon crash into the Earth(and the Earth crash into the sun for that matter)?
I have the assumption that the resultant of the gravitational froces from other solar bodies keeps the moon in orbit. Please correct me if I'm wrong.

D H
Staff Emeritus
When you throw a ball, does it fall straight to the Earth the instant it leaves your hand? Ignoring air resistance, the ball is in free-fall (i.e., it is falling) the instant it leaves your hand. The ball also has a horizontal component to its velocity. Gravity only affects the vertical velocity. It does not affect the component of the velocity vector normal to the gravitational acceleration (i.e., the horizontal component).

A body in orbit about another more massive object also has a component of its velocity that is normal to the gravitational acceleration. Without gravity, this normal component velocity would make the distance between the two bodies increase. With just the right velocity, the inward acceleration balances this tangential velocity to make the distance constant (a circular orbit).

Bottom line: The moon is constantly falling toward the Earth. The reason it doesn't ever collide with the Earth is because it has a huge velocity that is nearly normal to the acceleration vector.

Isnt it because when the moon orbits the earth it experiences a force of mv^2/r in the direction opposite to GMemm/r2..? correct me if im wrong.

A.T.
Isnt it because when the moon orbits the earth it experiences a force of mv^2/r in the direction opposite to GMemm/r2..? correct me if im wrong.
This is true in a rotating reference frame, where earth and moon are both at rest. In this non inertial frame centrifugal forces (mv^2/r) and gravitation add up to zero. In any inertial frame, gravitation is the only force the moon experiences.

jtbell
Mentor
Bottom line: The moon is constantly falling toward the Earth. The reason it doesn't ever collide with the Earth is because it has a huge velocity that is nearly normal to the acceleration vector.

In other words, it's constantly missing the Earth.

D H
Staff Emeritus