Force which keeps the moon in orbit

The real difference between a satellite and an apple falling from a tree, is that for the fast sideways-moving satellite, the direction of "down" is always changing. But the satellite really is falling, and in fact a near-Earth satellite has almost the same acceleration that a falling apple has.

If it's above us now, then in about 45 minutes, for a low satellite, it will have fallen so far down that it'll be on the other side of Earth. By then, the direction of down has reversed completely, and the satellite will again fall down for those who live on the opposite side of Earth, returning to us about 90 minutes after we first saw it. Of course, it never hits Earth because of its ever-present sideways motion. The Moon is much farther away where gravity is weaker, so it takes fully two weeks to fall to the other side of Earth.

Centrifugal force was invented to allow us to do proper bookkeeping in a noninertial frame, if we insist on using such a frame to work with Newton's laws and there might be a good reason for wanting to do so. For a simple example of a noninertial frame, consider what happens when you stand in a bus while the driver brakes.

For a few moments, every passenger moves forward. The heavier ones feel a strong force that acts on their large mass, while the lighter ones feel a small force acting on their small mass. All feel the same acceleration. This is called a fictitious force, because it's a force that we invoke to explain why we are suddenly accelerated forward. In the almost inertial frame of the outside street, this force doesn't exist.

The real force there is a single simple one provided by the friction of the ground on the bus tyres, and transmitted through its brakes to its body. This force accelerates the bus backwards—or, to use the more intuitive expression, decelerates the bus.

Unless the passengers hang on, they will continue to move forwards until something inside the bus stops them. Depending on our choice of frame then, there are two forces to choose from when analysing why the passengers are impelled forward:. In the inertial street frame, there is a braking force that pushes backwards on the bus. This is a bona-fide force, in the sense that it's produced in an inertial frame.

It acts on the bus only, so unless we hold on, we'll continue to move forward at constant velocity. In the noninertial frame of the decelerating bus, the force is a mysterious force that acts on us, but not on the bus. It pushes us forwards, and has a strength that is proportional to how massive we are. This is certainly a more complicated force to explain than the simple one that pushes on the bus in the street frame.

Such a mass-dependent force is an indicator that something is awry—we have chosen to use a noninertial frame, and if we demand Newton's laws to hold in such a frame, then we'll have to invoke this fictitious force. It's worth pointing out that although the force impelling us forward in the bus's frame when the brakes are applied is conventionally called fictitious, it is real enough for the bus passengers! It might be a less natural force to use from a fundamental point of view of explaining why Nature works in the way that she does, but that does not mean that a fictitious force is not "real", that it is some kind of make-believe thing that will go away if we concentrate hard enough.

The word "fictitious" merely means that the force vanishes when we shift our frame of reference to one that is inertial, because inertial frames hold a very special position in physics. Like any object in orbit, an orbit is maintained, as according to Newton's first law of motion, by a combination of speed and gravity. The object, the moon, wants to move off in a straight line however the earth's gravity, outside force, moves it slightly out of line.

What keeps the moon in orbit around the earth? Feb 19, The earth 's gravity. In essence the moon is in continuous freefall towards the Earth.

It is the curvature of spacetime. A geodesic in curved spacetime is not a straight line, it is a curve. So, there is no force acting between the Earth and the Moon. A circle of dotted lines orbiting a point counter-clockwise. This shows the velocity and effects of centripetal force An object will only accelerate if a resultant force acts on it.

Example The Moon orbits the Earth - the diagram shows the force of gravity on the Moon, the direction the Moon would travel without Earth's gravity, and the Moon's actual orbit Gravitational attraction provides the centripetal force needed to keep planets in orbit around the Sun and all types of satellite in orbit around the Earth.