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exam1_sp10_sol

# exam1_sp10_sol - Physics 101 Classical Physics Spring 2010...

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Physics 101 Classical Physics Spring 2010 Exam 1 Instructions : The answer sheets must be handed in as soon as time is called. You have until 10:20am. Answer the following four multiple choice questions. Each question is worth 4 points. 1. Which of the following is a necessary condition for a ‘theory’ to be considered a scientific theory? (a) It is based on observations and measurements of the natural world (b) It is believed by most scientists (c) It makes predictions that can be tested (d) It describes the true causes natural phenomena (e) It has a mathematical basis which obeys all known physical and mathematical laws A scientific theory must be falsifiable—it must make predictions that can be tested which, if it fails, mean the theory is wrong. So the answer is c . 2. An object is measured with a ruler. Which of the following could reduce the systematic error on the length of the object? (a) Making the measurement many times with the same ruler and averaging the results (b) Using a ruler that has more finely-spaced grid lines (c) Using a ruler that has more widely-spaced grid lines (d) Using several different rulers made by other manufacturers and averaging the results (e) Having different people make the same measurement with the same ruler and averaging the results A systematic error is basically a ‘mistake’ in a measurement (as distinct from a systematic uncertainty ) that is caused by the way in which the measurement is made. For example, if a ruler has rulings that are incorrect—say, 1 cm reads as 1.1 cm on the ruler. No matter how many times you make the measurement with that ruler it will be ‘systematically’ (always) wrong. Using a ruler with more finely or widely spaced rulings only improves the precision on the measurement, it doesn’t correct an error. Having different people make the same measurement with the same ruler also doesn’t fix the problem if the ruler is wrong, though if the person making the measurement is incompetent or biased, and that is what causes the systematic uncertainty, then that could reduce the systematic error. So there were two acceptable answers, d and e . 1

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3. The space shuttle is traveling upward at a constant velocity of 100 m/s. When it reaches a height of 1000 m, one of its tiles falls off. If air resistance is neglected, what is the highest point above the Earth that the tile reaches? (a) 10 4 m, (b) 500 m, (c) 1500 m, (d) 1000 m, (e) 995 m When the tile falls off of the space shuttle, it is also moving upward at 100 m/s. Therefore the tile first goes upward, before coming to rest, and then falls back downward. So we can use: v ( t ) 2 = v ( t 0 ) + 2 a ( y f - y 0 ) . At the very top of its trajectory, v ( t ) = 0, and it starts with y 0 = 1000 m, and v 0 = 100m/s, so 0 = (100m / s) 2 - 2 g ( y f - 1000m) . and solving for y f we get y f = 10 4 m 2 / s 2 + 2(10m / s 2 )(1000 m) 2(10m / s 2 ) = 1500m .
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