III - 1 II. Newton's Laws of Motion Concept Review Warm-Up ...

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He then says the class, “In a moment I am going to lift the hoop off the table and we are going to observe the motion of the marble afterward. But before I do that, I want to consider the possibilities of what might happen to the motion of the marble when I suddenly lift the hoop off the table. I will show you a diagram of four possible paths the marble might take when I lift the hoop. I want you to write down a reasonable argument to support each case. “Of course, only one of the answers will be correct, so not only will you have to give me a possible explanation for each case, you will also have to be able to tell me afterward what is the mistake in reasoning for each of the wrong explanations after we see what actually does happen.” Will you accept Mr. Nicholls challenge Can you give a reason for each prediction 4 3 2 1 The Predictions Here is the list of possibilities developed by the class (see the diagram): For Path 1: Path 2: A force supplied by the hoop is pushing the marble around its edge. When the hoop is lifted, that force is gone, so the marble could move opposite to the direction of the push, which is directly away from the center of the circle. (Everyone agrees that this possibility is very unlikely, since no one has ever seen an object released from a circular path behave that way.) It is true that the hoop is pushing on the marble, but it also has a velocity. So when the hoop is lifted, the marble should move away from the center of the circle but in a wide gentle arc until it finally travels in a straight line. Besides, that is the type of observation you would make if you were flinging a ball on a string in a circle over your head and suddenly let go of the string. Path 3: When the hoop is lifted, the marble moves in a straight line and tangent to the release point. That is what Newton’s First Law predicts. Path 4: Path 3 doesn’t take into account the forces acting on the ball after release. Although the push inward is gone, there is still a sideways push that kept it moving along the path, so the ball should travel in an arc but past the tangent line instead of before it, as with path 2. That path would also coincide with the arc that you would see if you let go of the ball on a string. III - 4
Which prediction do you believe is the correct one Or do you have a different prediction or reason The Answer Satisfied with their predictions and reasons, the class watches as Mr. Nicholls starts the motion and lifts the hoop. The ball travels in a straight line path, tangent to the circle at the last point of contact, which is Path 3. Now, can you explain what is wrong with the reasoning behind each of the other predictions Also, can you explain why the ball on the string appears from the point of view of the spinner to be traveling in an arc, when it fact it is traveling in a straight line 2. The Horse and Sled This is a very old physics brain-teaser. See if you can figure out where and how the reasoning made in the question becomes illogical. A horse is pulling a wooden sled across the ground. According to Newton’s third law of motion, the force of the horse acting on the sled pulling it forward must be equal and opposite to the force of the sled pulling backward on the horse. According to Newton’s second law of motion, only net forces cause accelerations. Since the two forces must add to zero by the third law, the acceleration of the horse and sled must also be zero. So how can the sled ever move, much less accelerate Can you explain this Hint: The brain-teaser refers to both the second and third laws of motion, and the statements made about the two laws of motion are perfectly correct. But under what conditions do each of the laws apply Follow-up question: It is true that an acceleration must be caused by a net force acting on an object. Can you identify the force or forces causing the acceleration of the horse and sled Hint: Under what conditions could the horse not cause the sled to move III - 5
Page 1 and 2: II. Newton’s Laws of Motion Conce
Page 3: 2. A 3 kg mass is accelerated by a
Page 7 and 8: QUALITATIVE REASONING General Hint:
Page 9 and 10: Would the spring scale read 0, 10,
Page 11: Pull the lower string quickly and t

III - 1 II. Newton's Laws of Motion Concept Review Warm-Up ...

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