# Centripetal Force vs. Gravity

1. Feb 22, 2009

### gamabunta

Hi,

I'm currently taking 1st semester Physics and am trying to understand centripetal force relative to the Earth's rotation.

From what I understand (using a bucket of water tied to a string as an example), swinging a bucket in a circular motion, when the bucket is completely above your head, gravity will pull the water towards the bottom of the bucket (in this case up, away from the string) while centripetal force (pointing inward towards the string) is what keeps the bucket from flying off in the direction of velocity.

If this is so does this mean that what we interpret as gravity (9.8 m/s^2) "pulling" us down towards the Earth in actuality is centripetal force while the "true" gravitational force points 180 degrees in the opposing direction (in this case up towards the sky?)

I spent a good hour looking online for answers and have reread my textbook 3 times and still need clarification. Any help is appreciated. Thanks in advance!

2. Feb 22, 2009

### Staff: Mentor

No. Gravity always acts towards the center of the earth--downwards. Since the bucket is being spun in a circle--and thus centripetally accelerating--there will be a net force acting on it towards the center (the centripetal force). When the bucket is overhead, the centripetal force is downward; the centripetal force is the sum of the weight and the string tension.
No.

3. Feb 22, 2009

### gamabunta

Crap, I just realized I was only thinking in terms of the Earth.

The centripetal force for the Earth is provided by its revolving around the Sun correct?

4. Feb 22, 2009

### Staff: Mentor

Whenever there is circular motion there is a centripetal force. When considering the Earth orbiting the Sun, the centripetal force is provided by gravity.

5. Feb 22, 2009

Thank you!

6. Mar 1, 2009

### ha9981

Sorry to bring this question back from the dead, but if tension and gravity are both causing centripetal acceleration and therefore a centripetal force toward the center than what keeps the bucket and water up? What is this force and is there a way to explain it.

7. Mar 1, 2009

### Staff: Mentor

What keeps the bucket and water "up" is their inertia. They are being swung in a circle. At the top of the swing the forces all act downward while the bucket and water move horizontally. Thus the change in the velocity is downward and the bucket turns in a circle as it moves.

8. Mar 1, 2009

### tiny-tim

Welcome to PF!

Hi gamabunta! Welcome to PF!
No, the string pulls the bucket down, and the bucket pulls the water down.

Nothing pulls the water up.

You could do away with the bucket if only someone would make string that could pull water.