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Name:  JungHo N.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Can Earth's revolution affect Earth's accelation due to gravity ? I learned some factors affecting Acceleration due to Gravity are (1)Altitude,(2)Depth Below the Earth's Surface,(3)Rotation of the Earth etc... But I think Earth's revolution affect Earth's accelation due to gravity. That is to say (1)Sun is pulling the Earth -> (2)Earth go around the sun -> (3)Earth have centrifugal force -> (4)Earth's centrifugal force can affect Earth's accelation due to gravity. Is this Right ? I'm confusing. I want your advice.


Replies:
JungHo,

The first thing to realize is that "centrifugal force" is not real. A rotating object does not really experience a force away from the center of rotation. It is just the fact that EVERY object "prefers" to move in a straight line. When an object is pulled into a circle, there is no force pulling it out. That is just the object trying to continue in a straight line. The Earth does not have "centrifugal force".

The Earth does experience a centripetal force: the Sun pulling the Earth inward. This centripetal force is gravity. This gravitational pull from the Sun does NOT affect gravity at the surface of the Earth because it pulls on the Earth AND everything on the Earth. Both you and the Earth fall freely around the Sun together. Any effect it has on your body matches the effect it has on the Earth. As a result, you don't notice it. If the Earth were locked in place, not affected by the Sun, then you would easily notice the Sun's gravity.

Another example of such an event is the feeling of "weightlessness" on the space shuttle. The shuttle falls freely around the Earth due to gravitational pull. The astronauts experience the same effect, falling with the space shuttle. Since they fall together, gravity does not try to pull the astronauts through the floor of the shuttle. The astronauts do not notice their weight when on the shuttle. The astronauts don't notice the pull of the Earth's gravity. Since the shuttle orbits the Earth, falling with the Earth around the Sun, the astronauts don't notice the Sun's gravity either. To notice weight, gravity must pull you against something. Your body must press against a floor.

Dr. Ken Mellendorf
Illinois Central College


You are right, but the acceleration is very small -- around 0.006 m/s/s, compared to 9.8 m/s/s from gravity.

Tim Mooney


When doing problems involving multiple forces, sometimes it is easiest to work with one item at a time, then combining the resulting effects. In this case, isolate the gravity, because it is the item of interest. This would mean that the "acceleration due to gravity" would only be affected by gravity. Centripetal force, air resistance, etc., should be accounted separately from gravity.

-Wil Lam



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