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Unformatted text preview: Oregon State University Physics 201 , Fall 2009 HW1-2 (due Oct. 12 at 5:00 p.m.) Page 1 Oregon State University Physics 201 Fall Term, 2009 HW1-2 Solutions In these solutions, I try to think out loud for you, so I use more words and explanation than you need to put in your own so- lutions. The points awarded (shown only for the problems scored this week) are noted near the vital steps that you should have in some recognizable form; these add up to 10 points. Note that diagrams are often part of the points for set-up. Note also that I try to avoid putting in most numbers as long as I canonly about 1 point in 10 is awarded for the correct answer, including units and signi cant gures. If you put numbers in earlier, you should keep MORE than 3 signi cant digits in your intermediate results. (If you dont, then youll propagate error by using a rounded intermediate result.) Youll see that when I do use intermediate results (because sometime it is more convenient), I keep a couple of extra digits. Finally, keep in mind that with kinematics, there is always more than valid path to the solution . I have chosen one I thought was convenient. 1. Youre driving at a speed of 28.0 m/s eastward along a straight, level road at night. Suddenly you see the shining eyes of a deer standing in the roadway ahead. You slam on the brakes and manage to stop your car just 6.40 m west of the terriF ed deer. If your reaction time was 0.365 s, and you F rst saw the deer from a distance of 120 m, what was the average acceleration ap- plied by your brakes? You MUST break the analysis up into two partsbecause the cars acceleration changes . There are two intervals in which the acceleration is constant, but that constant value is different in the F rst interval than in the second. The F rst interval is the time it takes you to react to seeing the deerjust get your foot to the brake pedal. During this time, the car maintains its previous velocity; the ac- celeration is zero. During the second interval, the brakes are actually applied, and the car is slowing. The braking acceleration is constant (assumed herean average); this is what you want to calculate. Interval 1 x i = 0 (Traveling at velocity v i , you see the deer here.) x f = ? (Your foot hits the brake here.) (Deer is here at x = 120.) x Interval 2: x i = ? (You brake steadily from an initial velocity v i , starting there... ... until you stop here (x f = 113.6 ). irst, do a quick and easy analysis of Interval 1, measuring time, t (and displacement, x ) from the moment you see the deer (120 m away) until your foot hits the brake pedalyour reaction distance. a = 0 in this interval, so x = v i ( t ) + ( / 2 ) a ( t ) 2 reduces to x = v i ( t ) (distance = rate x time)....
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This note was uploaded on 01/13/2010 for the course PH 201 taught by Professor Staff during the Fall '08 term at Oregon State.
- Fall '08