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August 16th, 2015, 11:40 PM  #1 
Newbie Joined: Aug 2015 From: finland Posts: 9 Thanks: 0  How far truck travels
Hi I have (hopefully) simple question . I'm using units and frames instead of miles and hours. I hope that is ok. truck goes 50 units per frame. truck is slowing down 5 units per frame. how long the truck travels until it stops? I got help to this question from here but I couldn't get the right answer, maybe because I work on units/frames instead. Last edited by skipjack; August 17th, 2015 at 06:20 AM. 
August 17th, 2015, 06:22 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 17,912 Thanks: 1382 
After 10 frames, the truck has stopped. During those 10 frames, it averages 25 units per frame. Hence it covers 250 units while slowing to a stop.

August 17th, 2015, 04:32 PM  #3 
Math Team Joined: Jan 2015 From: Alabama Posts: 2,647 Thanks: 680 
Actually you got exactly the same answer where you posted it before.

August 18th, 2015, 12:41 AM  #4 
Newbie Joined: Aug 2015 From: finland Posts: 9 Thanks: 0 
For some reason I do not get satisfactory result. here's another example, the truck drives 10 units per frame and slows down 5 units per frame. with the math given the result should be 10 / 5 * 5 = 10 but if I calculate each frame by hand I get this: totalTravel = 0 speed = 10 totalTravel + speed = 10 speed  5 = 5 speed = 5 totalTravel + speed = 15 speed  5 = 0 speed = 0 totalTravel = 15 so the answer when calculated like this is 15. which is 5 units off from the result calculated with the given formula 10 
August 18th, 2015, 02:23 AM  #5 
Global Moderator Joined: Dec 2006 Posts: 17,912 Thanks: 1382 
Your method of calculation is incorrect. The truck presumably slows down at a constant rate. This is not the same as the truck slowing instantaneously by 5 units/per frame at the end of each frame. In the first frame, the truck slows from 10 units per frame to 5 units per frame, so it covers 7.5 units in that frame. In the second frame it slows from 5 units per frame to 0 units per frame, so it covers 2.5 units in that frame. You could alternatively obtain that by calculating 7.5  5 = 2.5. The total number of units for the two frames is therefore 7.5 + 2.5 = 10. 
August 18th, 2015, 03:22 AM  #6 
Newbie Joined: Aug 2015 From: finland Posts: 9 Thanks: 0  Good point. I didn't think of that because I'm doing computer simulations which work quite differently from the real world. In my simulation, the decrease in speed is, in fact, instant.
Last edited by skipjack; August 18th, 2015 at 03:30 AM. 

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