# Closing SpeedsWatch

#1
Hey guys,

I read through the speed / distance / time thread but couldn't find anything to help on this subject.

Can anybody shed some light on how to work out closing speeds please?

Here is an example which I have just made up:

'2 aircraft depart at 1000hrs and head towards each other. 'Aircraft A' is traveling at 120mph and 'Aircraft B' is traveling at 300mph. The distance between them is 50 miles. At what time and distance do they meet?'.

Any advice, examples, etc greatly appreciated!
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9 years ago
#2
In the SDT thread there is a link to a website, which very clearly explains it all. Can't remember what the link is, but if you look through the thread you'll find it.
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9 years ago
#3
1007hrs

A would travel 14.3miles, B would travel 35.7 miles.

Absolutely no idea if that is right, or how i worked it out other than just thinking about.
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9 years ago
#4
Unsure if you're allowed a calculator for this kind of thing but the way i do these is as follows:

Find a common multiple (like most other speed/time/distance problems):
60mph

Find what factor of the speeds 60mph is:
AircraftA: 120/60 = 2
AircraftB: 300/60 = 5

This means the Journey can be split into 7 (2+5) parts where AircraftA covers 2 sectors AircraftB covers 5 sectors.

AircraftA covers 2 sevenths of 50miles.
AircraftB covers 5 sevenths of 50miles.

Knowing this use one aircraft to find how far is travelled (smaller numbers are easiest, so use AircraftA)

It covers (50/7)*2 miles at 120mph:

S = D/T
T = D/S = [(50/7)*2]/120 = 0.12 of an hour

Distance: (50/7)*2 miles = 14.28 miles From AircraftA's takeoff point.
Time: 1000hrs + 7.2minutes = 1007hrs

My apologies if this was too long or anything, but that's how i do maths, blame my professors
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9 years ago
#5
Because I'm good like that:

http://mathforum.org/dr.math/faq/faq.two.trains.html
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#6
thanks a lot!
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9 years ago
#7
The best way I find to tackle the questions involving "Aircraft hading towards each other" is to add the speeds together and imagine one of the aircraft is stationary, then its just a simple SDT question. so

120+300= 420mph (closing speed between two aircraft)

then 50 miles / 420 mph = (approx 0.1 hours) (more accurately 0.12 hours)

Then if one hour= 60 mins the answer in mins is 0.12 X 60 = 7.2 mins

Hence they will meet at 11.07 after 7 mins of flight. You can then work out how far they will each travel by taking their individual speeds and using 7mins in a simple SDT calculation.

I find its best to tackle them this way, firstly work out the "closing speed" between the aircraft and do simple SDT calculations. I am also sure that using the estimate of 0.1 hours = 6 mins, will give you a close enough answer for the tests.

Hope this helps a bit.

Richard
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9 years ago
#8
Even easier way.....!

120 + 300 = 420

42/6 = 7 therefore 7 minutes because there are 6 lots of 10 minutes in 1hr and 7/6 = 7 out of 10 which is 7. Boom chikka wah 1007, none of this decimal pointy stuff. Make it easy kids, it's the only way I can do it the EASY way!
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