Pail of Water, 1D50.40

Topic and Concept:

• Motion in Two Dimensions, 1D50. Central Forces

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Location:

• Cabinet: Mechanic (ME)

• Bay: (A9)

• Shelf: #2

Abstract:

A partially filled pail of water is whirled around in a vertical circle without spilling any water, illustrating centripetal force in conjunction with Newton's first and second laws.

Important Setup Notes:

• Make sure the handle on the bucket is in good condition so as to not detach during spinning.

Setup and Procedure:

1. Partially fill the bucket with water.
2. Start with the bucket about an eighth of a turn in the direction opposite to the rotation.
3. Swing the bucket in a vertically-oriented circle with enough speed so that all the water remains in the bucket.

Cautions, Warnings, or Safety Concerns:

• Make sure not to swing the bucket in the direction of the audience in case the handle would detach from the bucket.
• Demonstration may require practice.
• If water does spill, clean it up promptly to avoid any slips/falls.

Discussion: According to Newton's first law, objects in motion tend to continue moving at constant velocity in a straight line unless acted upon by a force. The water is always moving tangentially to the buckets trajectory due to a central force. Thus, this demo is also good for showing central or, more specifically, centripetal force. At the top of the trajectory, the force of gravity and the centripetal force are in the same direction. The water is falling due to gravity, but the bucket is falling faster. This is why there is a critical rotational speed that the bucket must attain. Newton's second law can be thrown into the discussion here. The total force on the water will always be the sum of the centripetal and gravitational forces.

Videos:

• Pail of Water, 1D50.40

• Lecture Demonstration's Youtube Channel

References:

• Sprott, Julien Clinton. Physics Demonstrations: A Sourcebook for Teachers of Physics. The University of Wisconsin Press (2006).

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