Centripetal force demo.... how? why?

Question

I know that if you fill a bucket with water and rotate it quickly in a circle, the water doesn't fall out. I know that this is due to centripetal force. But could someone explain how the force is created, and how it acts on the water?

Answer 1

You're thinking of centrifugal force. Centripetal force is force directed to the center of the system where as centrifugal force is directed away from the center.

For example, if you are on a merry-go-round you feel like you are being pushed to the outside and if you didn't hold on to something it would throw you off.

The reason the water stays in the bucket is because it is being pushed to the bottom of the bucket with a force greater than that of gravity. If you had a bucket with a bottom that could open, when the bottom was removed the water would fly out of the bucket but would immediately start falling towards the ground as the centrifugal force has been removed from the equation.

By the way, centrifugal force is not really a force like gravity or the weak nuclear force. It's called a fictitious or inertial force as it does not rely on interactions between objects like the other ones mentioned do.

Answer 2

There is this thing called Critical Speed. It is when the speed of the object at the "top"(the bucket itself) is least than a critical value(Vc), the bucket handle goes and the upper part of the object's path flattens out so the water stays in the bucket.

Answer 3

I'm afraid Jeanne C's answer is wrong and confusing. Ignore centrifugal force - its fictitious (unless in a moving frame of reference). I've met plenty of physics teachers who still don't understand this!

f42 gives a good answer.

A Centripetal force always acts to the centre of a circle and is necessary to cause things to move in a circle. Anything can provide this force - sometimes it is the friction between a car tyre and road surface, sometimes the tension in string, sometimes gravity.

When the bucket is at the top, the centripetal force is provided by gravity. If you spin a bit faster, the centripetal force required will be a bit higher and the extra bit will be provided by the pull of your hand (tension).

Take a tennis ball on a string. Rotate the ball round. Imagine doing this in space. When you cut the string, the ball moves off in a tangent.

Cheers,

Ben

Answer 4

The force is in the tension in the string. This is why if you swing a thin piece of string with a weight on the end fast enough the string will break- because the force acting on it is too great.

The force itself (and therefore the acceleration) acts towards the centre, so technically the force is pulling the water downwards when it is upside-down. This means that it's not the force that stops the water falling out, but the velocity of the bucket. I suppose the velocity is caused by the force, but they act in different directions. The force acts towards the centre of the circle, but the velocity is perpendicular (at right-angles) to this. I know it's a bit confusing in writing, sometimes a diagram can help: http://content.answers.com/main/content/...