Centripetal Force and Swinging a Pail of Water Over Your Head

The Idea

If you fill a bucket with water and turn it upside down, the water will (of course) spill out. But, if you spin the bucket over your head fast enough, you may avoid getting wet. How fast do you have to swing a pail filled with water over your head so as to not get wet? In this project, you explore what it takes not to get soaked or, in other words, how fast is fast enough?

What You Need

• small bucket with a handle or string attached
• water
• stopwatch
• meterstick
• person willing to get wet
• another person willing to get the first person wet
• optional: paper towels or a mop to wipe up spills
• optional: raincoat or umbrella

Method

1. Put some water in the bucket.
2. Predict how fast you think you need to spin the bucket to avoid spilling its contents. This can be done qualitatively by spinning at a relatively fast rotation and pushing your luck by going progressively slower. One simple refinement would be to do this in terms of time (in seconds). A more quantitative prediction can be based on the linear or angular velocity, and it can be measured based on the person's arm length.)
3. Spin the bucket, as shown in Figure 17-1, and evaluate in terms of the predictions.

Expected Results

The slower you go, the greater the risk of soaking the spinner for a given radius. If your arm is shorter, you will have to complete the circle in less time.

The maximum time to go around a vertical circle of a given radius without spilling is shown in the table below. The maximum time to spin the bucket overhead is about half that time. Keep in mind these times are based on uniform velocity. The most critical point of course is at the top of the circle. (If you slow down there, you may need that raincoat identified in the what-you-need list.)

Why It Works

The person spinning the bucket will be spared a soaking as long as the bucket moves fast enough so the centripetal force is greater than the force of gravity.

The condition for this is:

Note, this result indicates it doesn't matter how much water is in the bucket as long as the spinner moves at a sufficient speed. The larger the radius, the faster you have to go. Too much water, however, may cause the spinner to slow down.

Other Things to Try

This can also be done using confetti instead of water.

Physics alert: there is really no such thing as a centrifugal force. The water is given a velocity and is forced into a circular path by the centripetal force exerted by the bottom of the bucket on the water. If the bucket is moving fast enough, the centripetal force of the bucket is needed to keep it going in a circle. If the bucket is not going fast enough, gravity would be great enough to cause the water to spill out.

The Point

The centripetal force on the water is provided by the bottom of the bucket. The handle of the bucket provides a centripetal force on the bucket itself. The water will not fall if the rate of rotation is high enough that the centripetal force is at least as great as gravity.

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125 Physics Projects for the Evil Genius
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