How to Use Newton's Laws to Explain a Garden Sprinkler
Rotating garden sprinklers are excellent demonstrations of Newton's third law of motion, the law of interaction. The law states that, for each action, there is an equal and opposite reaction. To put it another way, if you push on something, it pushes back on you. In explaining to students the application of this principle to a rotary garden sprinkler, a teacher can elucidate the nature of the law with an appliance they can easily relate to.
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Instructions
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Make sure that what type of sprinkler you are talking about is clear to the students. Perhaps bring in an example. Sprinklers that are spring-loaded or make a fan-shaped spray may be too complex to help clarify the third law, so stick to a rotary kind with nozzles on the end of multiple, curved arms.
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Clarify in the students' minds that, though the water imparts energy to the sprinkler arms, both exert force on each other. Therefore, the water sprays in the opposite direction that the arm travels. They are pushing off of each other. This is an opportunity to mention Newton's first law, that objects in motion tend to stay in motion. Since the water is in motion, it tends to keep going, right out the nozzle. The result is that the inertia of the armatures is overcome and the arms spin.
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Take advantage of any opportunity to use memorable phrases such as "action-reaction," "for each action, there is an equal and opposite reaction," and "you can't push without being pushed back just as hard."
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Test the students with this especially subtle question. Ask them, if water is sucked into an underwater sprinkler using a vacuum, which direction does the third law say the sprinkler will spin? Since the surrounding water pushes on the wall, the incoming water counters that from inside, and the arms won't tend to move. Both "blobs" of water on either side of the curve in the armature start out with the same pressure, so they counter each other equally. (This is quite the opposite of what the case was when the water was pushing out of the armatures, having far greater pressure than the air on the outside of the armature curve.)
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Tips & Warnings
There are conditions under which the submerged arms will move though, so be prepared for student responses. Specifically, if the vacuum is made powerful enough and the arms are wide enough, the incoming fluid could hit the opposite inner wall only after being sucked close to the center of the armatures, where the incoming fluid's leverage against the fluid outside the armature wall is greatly reduced. This however gets into the question of angular momentum, straying from the third law, the original point of the lesson. See the video demonstrations in the Resources section below.
