# Simple Question on Earth Rotation involving Newton III

Simple question with a simple answer, just want a little more description.

Newton III states: For every action there is an equal and opposite reaction.

As the earth rotates one way, what's the opposite force that keeps it "balanced"? Inertia?

Arrived at the question when was thinking about a pendulum in SHM but could not
figure out why if, in a perfect system, why it kept coming back to the same position with the earth rotating only one way? Rationally thought there would be an over swing in the direction the earth was rotating. But Newton III states there must be a force acting on opposite end and after much thought, still stumped.

No force is needed to keep the earth rotating (remember Newton's 1st law) if you wanted to change the rotation then you would need something for the force to act against

BobG
Homework Helper
If an object is moving in a straight line at a constant velocity, what's the force that keeps it balanced?

It takes a force to change an object's motion. Regardless of whether it's moving in a straight line or rotating, momentum keeps it moving at the same rate, while a force changes that motion.

Just as you have conservation of linear momentum, you also have conservation of angular momentum.

Assume the earth is rotating left. Now, when watching a pendulum, swing left and right, shouldn't it swing farther left because the sum of all its forces, including the earth's rotation would be greater that way than the sum of all its forces in the right direction because the earth's rotation is missing in the equation?

Thanks for that response BobG. You posted when I was in the middle of writing this.

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Doc Al
Mentor
Newton III states: For every action there is an equal and opposite reaction.
Newton III applies to forces.

As the earth rotates one way, what's the opposite force that keeps it "balanced"? Inertia?
Rotation is not a force.

Arrived at the question when was thinking about a pendulum in SHM but could not
figure out why if, in a perfect system, why it kept coming back to the same position with the earth rotating only one way? Rationally thought there would be an over swing in the direction the earth was rotating. But Newton III states there must be a force acting on opposite end and after much thought, still stumped.
For a pendulum swinging back and forth, there are forces involved and thus Newton III applies. The result is that as the pendulum bob moves to the left of equilibrium, the support (and attached earth) moves ever so slightly to the right of equilibrium.