Posted: 2/13/2007 5:33:13 PM EDT
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I’ve noticed that there is a lot of information, misinformation, and disinformation generated in the physics related threads. I, myself, have not always been thorough in my answers to these threads, and have been mistaken at times. So, to generate discussion and ultimately create some education of physics related topics I decided to create this thread. What I am going to do is present some basic physics problems. When these are answered well enough, I’ll choose what I think is the best correct answer to each problem, and edit the original post indicating the best answer, who posted it, and on which page it appears. There are no prizes, only notoriety. Also, just the answer without discussion as to why will not receive credit, you need to justify your answers. I’ll add additional problems as I think of them, and to keep enough problems going. I’ll also edit the original post and state where a new problem appears. I’ll also accept problems from others. When you respond please indicate by stating the title of the problem. All problems will have a brief title with which to distinguish them. If you create a problem, please follow the format, thanks. 1. Balls Rolling Down a Ramp. Given: Three solid balls made of the following materials: aluminum (2.7 g/cc), steel (6.9 g/cc), brass (8.6 g/cc). These balls are rolled down a flat rectangular ramp which makes a 30 degree angle with the horizontal plane. They roll to a finish line that is parallel to the starting line and which is also drawn on the ramp. The ramp is in a vacuum. The three balls are started at the same line (where they make contact with the ramp) and at the same time. Part a) If the balls are the same diameter, which one reaches the finish-line first? Part b) If the balls are the same weight (mass) which one reaches the finish-line first? Part c) What would the difference (proportionally) need to be for the three balls to reach the finish line at the same time? 2. Ball, Solid Cylinder, Ring, and Spool Rolling Down a Ramp. Given: There are four objects, a solid ball, a solid cylinder, a round tube (ring) and a spool which are started at the same time on the same line on the same ramp in the first problem. All objects have the same external diameter, and are the same density and weight (mass) The ring has an inside diameter that is 75% of the outside diameter, the spool has two solid wheels that are 1/4 the thickness of the solid cylinder and a solid axle that is 1/4 the diameter of the wheels. Which of the four objects reaches the finish-line first, second, third, and fourth? 3. Falling Spheres in the Atmosphere. Given: Three solid balls made of the following materials: aluminum (2.7 g/cc), steel (6.9 g/cc), brass (8.6 g/cc). They are released from the same height as measured from the center of gravity. These are dropped from a height in which the terminal velocity of the fastest one will just be achieved. (Ignore any buoyancy effects.) Part a) If the three balls are the same diameter which hits the ground first? Part b) If the three balls are the same weight (mass) which one hits the ground first? Part c) What would the difference (proportionally) need to be for the three balls to reach the ground at the same time? What would that quality be called if it has a common name? 4. Model Airplane on a Conveyor. (Not that problem, but inspired by it.) Given: A plastic model airplane (or car—it doesn’t matter), whose wheels weigh 1/10th the total weight of the model. The wheels are solid thin cylinders except for a small hole (0.4 mm or 0.016 inches) through the center on which a small steel wire acts as the axle. The wheels are 25 mm (1 inch) in diameter and 3mm (1/8th inches). The kinetic friction coefficient between the steel axle and the wheels is 0.1 (ignore the coefficient of static friction for this problem). The friction coefficient (static) between the rubber belt and the wheels is 3.0; there will be no slipping. Part a) If the model is placed on the conveyor while the belt is moving at a constant velocity then released, what will be the model’s acceleration before it reaches the speed of the belt? Does it matter what the initial speed of the belt is? If so, solve in terms of the initial velocity of the belt. Part b) If the belt is accelerating at a rate of 0.5 m/s^2 what will be the model’s acceleration? Assume the belt starts from stationary with the model sitting on it before the belt is started. Have fun guys. Dave. P.S. I'm probably going to hell (if it exists) for this. Edit: made titles bold for easier identification of the different problems. |