WEEK 4
Newton's Three Law of Motions
(SU ch. 5, OM ch. 3)
Photograph of Newton's Principia
Used with permission.
Photograph © Andrew Dunn, 5 November 2004. Website: http://www.andrewdunnphoto.com/
Used with permission.
Photograph © Andrew Dunn, 5 November 2004. Website: http://www.andrewdunnphoto.com/
DAY 1: Newton's First Law
READING
From "beginning to chapter" to "...someone stops them."
NOTES
There are great videos to go with this reading:
Amazon Alexa Moments: Kid Magician. I'm not sure where my kids first saw this; most likely watching sports with Dad. Boy, did we laugh and laugh and laugh. My kids' comment, "If only he pulled down!" Tablecloth Trick: Cool Science Experiment. For those of you who do not want to try this at home. |
DAY 2: Newton's Second LawREADING
From "Newton's Second Law..." to "...that mass moving!"
NOTES
None yet.
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DAY 3: Newton's Third Law
READING
Do not read SU. Please, watch this Khan video instead.
NOTES
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Mr. Fleisher falls into an unfortunate but all too common misapplication and misunderstanding of Newton's Third Law. We are not going to read this section of SU. Rather, we will watch a Khan video. Do not watch the second Khan video on Newton's Third Law. It involves objects which are not in contact and the Law of Universal Gravitation which we have not yet covered.
Newton's Third Law: For every action, there is an equal and opposite reaction.
Though this is the most common way to phrase Newton's Third Law, it often leads to great confusion, which is seen in this chapter.
There is a better way to phrase Newton's Third Law: Between two bodies, for every force applied by the first body on the second, there is an equal and opposite force of the same type applied by the second body on first.
Take the simple case of a hand pushing on a wall.
Taken alone, Newton's Third Law does not explain motion. It always deals with two bodies and two forces. While Newton's Third Law accounts for one of the forces acting on a body, we can only understand motion by looking at all the forces acting on a single body. When all of the forces acting on a single body do not cancel each other out, we have motion. This is Newton's First Law. For those interested in learning more about why I recommend not reading this section, please read on. The example of the canoe is very misleading because, while the canoe moving forward is a result of the paddle pushing on the water, the paddle pushing on the water and the canoe moving forward do not form a Newton's Third Law pair.
Why, then, does the canoe move? To answer this question, we must look at the forces acting only on the canoe. We are now applying Newton's First Law. The canoe moves forward because the force of the water pushing forward on the paddle (and transferred to the canoe through a complex interaction between the paddle, the human, and the canoe) is greater than the force due to friction from the water pulling backward on the canoe. Imagine trying to paddle in sand. The paddle may push on the sand with the same exact force with which it previously pushed on the water but the canoe most likely will not move. In this sand example, the force pushing the canoe forward is equal to the force due to friction pulling the canoe backward, and it stays in the same place. Now, what about the rifle and the bullet? While this is a great example of the conservation of linear momentum, it also falls short as an application of Newton's Third Law. At what point did the bullet push back on the rifle? It didn't; there was an explosion. I will leave you to try puzzle that one out. A rocket is the classic example of Newton's Third Law. The two bodies are the rocket and the gas. The rocket pushes the gas out and the gas pushes back on the rocket. But the actual motion of the rocket is explained by Newton's First Law. Because the force of the gas pushing on the rocket is greater than the weight of the rocket, the rocket takes off. The process of walking is such a complex kinematic example, that we are just going to leave it. Suffice it to say that our muscles pushing on the earth and our body moving forward do not form a Newton's Third Law pair. |