Helicopters Over a Traffic JamDate: 09/22/98 at 20:29:50 From: CWS Subject: Relative speeds This is a math problem related to dealing with relative speeds: A helicopter flies over a traffic jam with a velocity of 180 km/h. If it flies in the same direction the cars move, it needs 2 minutes to overfly the cars. Flying back against the direction in which the cars are moving takes 80 seconds. How long is the traffic jam and with what speed do the cars move? Date: 09/23/98 at 09:21:55 From: Doctor Peterson Subject: Re: Relative speeds Hi, Martin. Probably the hardest part about this problem is working with the units, since you have km/h and minutes and seconds all mixed together. Let's start by getting everything in terms of meters per second: km 1000 m 1 hr 1 min m 180 -- * ------ * ------ * ------ = 50 --- hr 1 km 60 min 60 sec sec Now suppose the traffic is moving at V m/sec. If the helicopter flies in the same direction as the cars, it is moving at (50 - V) relative to the cars. It takes 120 sec to travel D (the length of the line), so: 120 * (50 - V) = D If it flies back, it is moving at (50 + V) relative to the cars. This takes 80 seconds, so: 80 * (50 + V) = D See if you can solve those two equations. One way is just to set the two D's equal to each other and solve for V, then find D from either of the two equations. You'll probably want to change V back into km/hr when you have it. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 09/23/98 at 14:32:14 From: CWS Subject: Re: Relative speeds Hi, First of all, thanks for answering my question. Second, I'd like to ask if I solved the equation correctly: D = 120*(50-V) D = 80*(50+V) Substituting: 120*(50-V) = 80*(50+V) Thus: V = 10 km/h And: D = 120*(50-10) = 340 meters A: The traffic jam is 340 meters long and it moves at 10km/h. Is this right? Sincerely, Martin Date: 09/23/98 at 16:48:40 From: Doctor Peterson Subject: Re: Moving-problem (puzzle?) Hi, Martin. You're almost right! But you missed my reminder to change V back to km/h when you are done. It's 10 m/s. In calculating D you must have done something wrong, because I get 4800 m immediately. (Note that you do use the 10 in the formula for D, but you must remember again that the variable is in meters.) When you get your final answers in a problem like this, it would be good to check them not only in the equations you wrote, but also in the actual problem as originally stated, using the actual units of your answer. How long will a helicopter going 180 km/h over 4.8 km of traffic moving 36 km/h take in each direction? Then you don't have to ask me if you are right. (And I don't have to check my own answers for you, which could be wrong.) - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 09/24/98 at 21:07:29 From: CWS Subject: Moving-Problem Greetings, I'd just like to say that I'm thankful to you for helping me with this puzzle. It really helped me a lot. Sincerely, CWS |
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