Lab#4 - Physics 1AL Introduction Testing a Model for...

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Physics 1AL Testing a Model for Friction Winter 2009 1 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 nature, 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, it has been shown that Newton’s Laws do not agree with observations of objects moving near the speed of light. Einstein’s theory of relativity explains these new data. It turns out that the differences between relativity and Newton’s Laws are very small at lower speeds, so we can continue to use Newton’s Laws for our everyday lives, including almost all of Physics 1. (Even if your particle is moving at one tenth of the speed of light [that is a speed of 3x10 7 m/s = 6.7x10 7 mph] the difference between Newtonian mechanics and relativity is just a half of one percent. At half the speed of light the difference is 15 %.) But not all of 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 4 questions!) 1. Suppose you press a book against the wall with your hand. The book is not moving and you are pushing horizontally. Do this experiment for yourself at home before answering these questions. a) Identify all the forces on the book and draw a free body diagram b) Now suppose you slightly increase your push, but the book still remains at rest. What happens to each of the following forces? Do they increase in magnitude, decrease or remain the same? F push __________________ Weight __________________ Normal __________________ Friction force __________________
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Physics 1AL Testing a Model for Friction Winter 2009 2 2. Consider a box in the back of a pickup truck that is headed due east. a)
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Lab#4 - Physics 1AL Introduction Testing a Model for...

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