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Name: Emily P.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 2001-2002


Question:
If a baster is tied closed, floated in water, and is suddenly untied, so that it takes in water, I have found that it will not move. A baster that squirts water out will move backwards according to Newton's third law. Why will it not move if it takes water in?


Replies:
Dear Emily,

This is a good question illustrating some basic ideas on Newton's Laws which are rather subtle.

When you first untie the baster, I presume water is accelerated into the baster because the water pressure is higher than the partial vacuum inside the baster. It is, however, pushed in by the water in the pot, not pulled in by the baster. (If you do not believe this, try the experiment in a vacuum). If the water pressure on the outside of the nozzle is reduced, there may be a small force pushing the baster in the direction the nozzle points. However, when the water rushing in is stopped, perhaps by hitting the back wall of the baster, Newton's 3rd law says the baster will experience a backward force.

So I believe the baster will not move, although the argument is subtle and not all my colleagues agree with me! Note also that the center of mass of the water does not change, which should also prove that the baster does not move.

A similar problem concerns a normal lawn sprinkler which rotates due to reaction to the force exerted on the water sprayed out. Note that the water changes direction as it approaches the nozzle and keeps going forever, at least as far as the sprinkler is concerned.

If you now put that sprinkler under water and attach it to a pump so water is sucked INTO the nozzles, what does it do? I believe the correct answer is that it does not turn. The water also changes direction as before, which would tend to make the sprinkler turn in the same direction, but the water, which was initially at rest, goes to the center of the sprinkler perpendicular to the direction the sprinkler might turn, so there is no change in momentum in that direction. The reduction in pressure at the entrance to the nozzle must balance the 3rd law force as the water goes around the corner.

I hope this is helpful; it is very lengthy, but it a puzzling phenomena. If you can find a sprinkler, a tub, and a water pump, please let me know what you learn!

Best, Dick Plano...



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