This week's CarTalk Puzzler has a physics theme that may be appropriate for 1st year physics students. Those teaching them may want to give this out as an extra credit or in-class discussion type of problem.
Below is a verbatim quote of it from the CarTalk Puzzler email from their website.
_____________________________________________
<Charming Balloon
Now it's time for our charming new puzzler. For lack of a better word, I guess I can call it charming, but you all be the judge.
Here it is.
You have a balloon that is filled with air. Like a balloon, you'd get at the circus. You tie the knot with a string at the end, and you also tie a weight to the string. The purpose of the weight is because you're going to place the balloon in water, like a swimming pool, such that the weight will keep the balloon's top just even with the top of the water as we describe in geometry, tangential to the surface.
The entire balloon is submerged in the water, except for the very top. All right, and the weight is keeping the balloon from rising beyond the surface of the water. We can all imagine this would work right? Balloons want to float. If the weight were not there, or if you came by with your scissors and snipped the string, it would pop right up.
For the sake of this puzzler, you don't have to know what the amount of the weight is. The weight of weight could be anything. It doesn't matter, for the sake of this puzzler.
Okay now, you take your hand and push down on the balloon. You push it down all the way into the water, one foot down. So, when you push the balloon down one foot and of course, it will go down nice and smoothly as you push it down.
And then, when you take your hand away, what happens?
What happens and why?>
_____________________________________________