The Demonstration: A bucket of water is spun in a vertical circle and the water stays in the bucket. Why?

Quick Physics: This is an example of centripetal (circular) motion. The water stays in the bucket because of inertia. The water wants to fly off from the circle, but the bucket gets in the way and keeps it in place. This is the same effect you feel when you go around a tight corner in the car and get squished against the door.

The Details:
This demonstration illustrates Newton’s first law of motion: Objects either remain still, or move in straight lines. This property is called inertia. In order to make something move in a circle, you have to use force (a push or a pull). For example, take a yoyo or tie a ball on the end of a string. Now spin it around in a circle holding one end of the string in front of you. You will notice that the yoyo wants to move in a straight line and you have to supply a pulling force to make it move in a circle. If you let go, the yoyo flies off tangent to the circle, in a straight line. This particular kind of force is called a centripetal force.

The same is true for the pail of water. When you swing the pail back and forth, you can feel the pail and the water inside pulling on your arm because it wants to continue to move in a straight line and you want to make it go in a circle. You hold on to the bucket so it doesn’t fly away and the water pushes on the bottom of the bucket. As you swing higher and higher, the water pushes harder on the bottom of the pail. But when you swing the bucket upside down, gravity tries to pull the water out of the bucket. Whichever force, the gravity or the centripetal, is stronger wins. As long as you swing the bucket fast enough, the force of the water pushing on the bottom of the bucket (centripetal force) will be stronger than gravity and the water will stay in the pail. Of course, if you swing too slowly, you’ll get wet