# Is the force that keeps the Earth spinning

1. Apr 5, 2015

### quincy harman

Is the the force that keeps the Earth spinning the same force that moves the moon farther away from us? Spinning objects are accelerating and it takes a force to accelerate an object so the force that keeps the Earth spinning is centrifugal force right?

2. Apr 5, 2015

### Staff: Mentor

There is no "force that keeps the Earth spinning" because the earth spins at a constant rate (to a very good approximation).
Not if they spin at a constant angular velocity.
Points on the object are accelerating - but Earth is held together by gravity.

The tides induced by the moon slow down the rotation frequency a tiny bit, and they also move the moon outwards very slowly.

3. Apr 5, 2015

### quincy harman

so it's a combination of centrifugal force and gravity? and if you're spinning at a constant speed but changing direction that is acceleration right?

4. Apr 6, 2015

### Staff: Mentor

On the surface of earth you feel both together, yes. Gravity is much stronger.
That depends on what you mean with "acceleration". The individual parts of you are accelerating, but I would not say you as a whole are accelerating.

5. Apr 6, 2015

### CWatters

An object on the surface of the earth is moving at a constant speed and it's direction is changing so yes the object is accelerating. The force that causes it to accelerate is gravity. Without gravity the object would move in a straight line and fly off the earth at a tangent to the surface.

There is no "force" that keeps the earth spinning. It's inertia that keep it spinning.

The reason the moon is moving further away from us is quite complicated. The moon has gravity which causes a tidal bulge on earth. A combination of the earths rotation and friction means the tidal bulge isn't directly below the moon, but slightly ahead or leading the moon. The gravitational pull between the tidal bulge and the moon cause the moon to gain energy and move into a higher orbit. It's a bit like swinging a ball on a string around your head...by moving your hand around in a circle slightly ahead of the ball you can make it go faster.

6. Apr 6, 2015

### quincy harman

But it has it is constantly changing direction and it takes force to change direction and what is a tidal bulge is the Earth actually misshaped by the moon?

7. Apr 6, 2015

### Staff: Mentor

The centripetal force is perpendicular to the direction of motion, so it (gravity) is a force that holds the Earth together, not a force that changes or works to maintain the rotation rate.

8. Apr 6, 2015

### PWiz

The Earth has non-zero angular momentum, which causes the Earth to rotate. Momentum is always conserved, so unless a significantly strong force acts on Earth, its rotational rate will remain more or less constant.

9. Apr 6, 2015

### CWatters

Velocity has components speed and direction. In the context of the moon... Inertia ensures the speed stays constant. Gravity ensures the direction changes.

The most noticeable and significant effect are the water tides (many meters in height) but yes the solid surface of the earth is also distorted (about 40cm)..

http://en.wikipedia.org/wiki/Earth_tide

10. Apr 6, 2015

### quincy harman

and how does gravity change the shape of Earth?

11. Apr 6, 2015

### Staff: Mentor

It keeps earth in the approximate shape of a sphere.

12. Apr 6, 2015

### CWatters

http://en.wikipedia.org/wiki/Tide

13. Apr 6, 2015

### quincy harman

I'm referring to the tidal bulges.

sorry i didn't see that

Thank you for the help.

Last edited by a moderator: Apr 6, 2015