Friday, June 1, 2012

About Question about Newtons, Joules, and Watts

Question about Newtons, Joules, and Watts?
So I have this question in my school work that goes like this: "A motorcycle breaks down FIVE KM from its destination. You use 400 N for the entire 60 minutes that it takes you to push this motorcycle to its destination. How powerful were you? How many Watts?" It doesn't make sense to me. Can someone answer this question and show how you arrived at your answer? I want to know the steps to figure it out so I'll be able to do it myself. Thanks, Aurora This question is multiple choice in my book, and the answers given are as follows: A. 6.67 W B. 0.111 W C. 0.150 W D. 400 W E. 25 KW I followed the instructions oldprof gave and I arrived at 555 W. Two other people gave me the answers 555 and 560. What's with that? That's not in the multiple choice. So am I still doing something wrong or is their course wrong...? >_> P = E/t E = F x D F = 400N D = 5km (5,000m) 5,000m x 400N = 2,000,000 J (or N-m) 2,000,000 J / 3600 sec. = 555 W Something is rotten in Denmark, because I keep getting that answer yet it's not in the multiple choice.
Physics - 1 Answers
Random Answers, Critics, Comments, Opinions :
1 :
The measure of "powerful" is a power metric. Power, by definition, is the expended energy over an interval of time. In math talk that's P = E/t = ? Watts, the answer you are looking for. Energy is in Joules (J) and time is in seconds when using the kms SI units. A Watt, by an arcane definition, is 1 J/sec. You have t = 60 min, it must be changed into seconds. So to get P, you must find E, the energy expended. That we get from the work function WE = Fd; where F = 400 Newtons and d = 5 km, the distance the cycle was pushed. Note that work is a special kind of energy. Thus P = E/t = WE/t = Fd/t = ? in Watts when you use consistent kms SI units for F, distance, and time. F is already in kms SI units... Newtons. But the distance must be converted into meters. And time t must be in seconds. You have everything given, you can do the math once you make the proper unit conversions.
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