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Water Pressure

1. Drill three 5 mm diameter holes in the side of a 2-L plastic bottle. The first hole is 5 cm above the bottom of the bottle, the second hole is 10 cm above the bottom of the bottle, and the third hole is 15 cm above the bottom of the bottle. The three holes are drilled along the same vertical line. A single strip of masking tape is placed over all three holes, leaving a tab so that the tape can be pulled off quickly.

2. Fill the bottle with water to a height of 20 cm above the bottom of the bottle. Call this height H. If you do this, then the holes are located at heights of 1/4 H, 1/2 H and 3/4 H precisely.

3. Place the bottle on a large, flat pan (such as an aluminum oven pan) into which the water can flow and be collected without spillage. (You might also do this outdoors on a flat surface.)

4. Predict what will happen when the tape is pulled off the holes, and the water is allowed to stream out of the three holes.

Out of which hole will the water stream go furthest?

(a) the hole at 1/4 H?
(b) the hole at 1/2 H? or
(c) the hole at 3/4 H?

Give a reason for your prediction!

5. Test your prediction!

The result might surprise you. Water pressure is greatest at the lowest hole, so the water comes out of the lowest hole at the highest speed. However, this stream has the least amount of time to reach the flat surface at the bottom of the bottle. Did you notice what happens to the highest and lowest holes?

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