Warm-Up Exercise 3

Due 12:15 pm, Fri 31 Aug 2012

Physics 123, Fall 2012
Reading Assignment: 14.5-14.7

You have two identical buckets. You are filling them with water from the garden hose. It takes 30 seconds to fill the first one. You notice that the water comes out "faster" when you put your thumb over the end of the hose a little bit. How long do you think it will take to fill the second bucket? Explain how you know.

The *velocity* of the water exiting the hose is faster with your thumb over the end, but the total *volume* exiting the hose each second is the same. So covering the opening with your thumb will cause it to take just as long to fill the bucket as when the hose is running "slow" and unobstructed.

In your own words, without parroting the text, explain what the "equation of continuity" for fluids means. Interpret it in physical terms. Imagine you were explaining it to your brother/sister/mother who hasn't taken physics at all.

One way to think about it is that fluid cannot be lost or created, and it cannot "pile up" anywhere. The continuity equation means that in any region of space, the same amount of liquid that flows into that place must also flow out (and at the same rate).

If the airstream from a hair dryer is directed ove a ping pong ball, the ball can be levitated. Explain how this works.

The faster moving air on top of the ball is lower in pressure than the slower moving air below it. So the higher pressure under the ball will lift it up.

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