Physics Help Forum Help with some Basic Word problems Ch 2 book Physics

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Jan 19th 2010, 11:53 AM #11 Junior Member   Join Date: Jan 2010 Posts: 12 "It is simply n / (sum(1/vi)) where sum(1/vi) is the sum of the inverse of (in this case speed) if the speed is slower, the speed is traveled at for a longer time hence this type of weighting is often used when calculating average speeds." Maybe this is the rule that allows you to flip a fraction over.... I am not sure I cannot understand what is stated there very well... but will try to study that sentence and look up information on it online
 Jan 19th 2010, 12:00 PM #12 Junior Member   Join Date: Jan 2010 Posts: 11 OK, i see. from the beginning then: A-»B, we say is distance "D" 1)a»b, 5m/s 2)b»a, 3 m/s V=Distance/time time=D/V «» time(1)=d/5m/s time(2)=d/3m/s Total time= d/5 + d/3= 5d+3d/15= 8d/15. Total distance=d+d=2d V(average)=total D/total T =2d/(8d/15) = 30d/8d = 3,75. That help? =)
 Jan 19th 2010, 12:16 PM #13 Senior Member   Join Date: Jul 2009 Location: Kathu Posts: 132 Average speed = Total Distance / Total Time Total Distance = 2*d where d is the 1 wat distance Total time = t1 + t2 where the time is unknown Average speed = 2*d / (t1 + t2) Substituting t1 = d1/v1 and t2 = d2 / v2 Average speed = 2*d / ( d1/v1 + d2/v2) Average speed = 2*d / ((d1v2+d2v1)/(v1*v2)) Average speed = 2 *v1 * v2 / (v1 + v2) Average speed = 2 * 1 / (1/v1+1/v2) which is the harmonic mean. The other easier way to do it is simply to work out the unknown for time which is what physicsquest did earlier. The only other advice I can give you is to repeat the problem over and over again, eventually it will become clear to you.
 Jan 19th 2010, 12:45 PM #14 Junior Member   Join Date: Jan 2010 Posts: 12 Thanks Thanks! Stuck on a call right now at work (I work in PC IT HelpDesk) but I will read these soon as I can I get off work in 30 mins (thanks god!) Appreciate taking the time to work with me on this as its been driving me absolutely crazy. Brian
 Jan 19th 2010, 09:37 PM #15 Junior Member   Join Date: Jan 2010 Posts: 12 Like to think everyone! I like to thank everyone for the help! I think I have this figured out. I will be around for a lot more questions I am rusty on math and physics is very hard for me as I am not very good with word problems. Only thing I have going for me is I love to learn and I am a very curious person and I actually get excited about physics and math. I did buy the solution guide to my text book today and getting it sent to me air mail so that will help some! Thanks, Brian

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