1A_week_4_friction_rev3 - Physics 1AL Introduction Testing...

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Physics 1AL Testing a Model for Normal Forces and Friction Sum05 Rev 3 Introduction The study of physics is an attempt to make sense of the measurements we make. In these labs so far you have made measurements of distance, speed, acceleration, and you have used “theories” to interpret and understand your data. A theory is just a model of “reality”, that is a model helping you to understand your measurements. Your model should be able to explain all the data available and it should allow you to make testable predictions about different situations. If you then go and make new measurements and find that they do not agree with the model that you used, then the theory must be changed or replaced by another. For example, Einstein showed that Newton’s Laws do not agree with observational data on motions close to the speed of light or objects near large masses. He developed the theory of relativity to explain these new data. Many experiments done in the last hundred years have confirmed relativity theory, so for objects moving in these situations we rely on Einstein’s relativistic mechanics. It replaces Newton’s theory, which has been shown to be invalid at high speeds. It turns out that the differences between relativity and Newton’s Laws is very small at lower speeds, so we can continue to use Newton’s Laws for most all our every day life, including almost all of the Physics 1 class. (Even if your particle is moving at one tenth of the speed of light [that is a speed of 0.1* 3*10 8 m/s = 3* 10 7 m/s] the difference between Newtonian mechanics and relativity is just a half of one percent. At half the speed of light the difference would be 15 %.) But not all the “theories” presented to you in a physics class are as simple, elegant, and well established as Einstein’s relativity or its subset - Newton’s Laws. One of these is the way introductory physics text books treat frictional forces. In this lab you will be able to measure how friction forces behave in a variety of situations. You will be able to test the assertions made in Chapter 4 of Serway and Faughn (bottom of page 99) against your own data. Before you start, read the section of your textbook (Ch 4.6) that covers frictional forces and how they behave. You should use the model of friction presented in the text book to answer the pre-lab questions. Pre-lab Questions: (there are 6 questions) 1. A wooden crate of mass 15 kg is at rest on a ramp with slope of 20 o . a) Calculate the component of the crate’s weight in the direction pointing down the ramp. b) Calculate the force provided by friction. c) Calculate the normal force of the ramp on the crate. d) Draw the force diagram for the crate. Include all the forces at the correct angles and label their sizes. e) Calculate the minimum value of the coefficient of friction that will just be able to
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1A_week_4_friction_rev3 - Physics 1AL Introduction Testing...

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