Physics Help Forum Constant Speed Problem

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 10th 2008, 06:03 PM #1 Junior Member   Join Date: Sep 2008 Posts: 4 Constant Speed Problem hey, i am gonna just write the problem just as it is in the textbook. I have thought about this one for a while, and yet have no idea where to start. I figured out the answer (I think) by guess and check, but i would really like to UNDERSTAND why the answer is what it is. Thanks, here it is: Two runners approaching each other on a straight track have constant speeds of 4.50m/s and 3.50m/s, respectively, when they are 100m apart. How long will it take for the runners to meet, and at what position will they meet if they maintain these speeds? It kinds looks like so: Runner A 4.5m/s--->>>______________100m______________<<<----Runner B 3.5m/s
Sep 11th 2008, 03:07 AM   #2
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 Originally Posted by tcrocRE hey, i am gonna just write the problem just as it is in the textbook. I have thought about this one for a while, and yet have no idea where to start. I figured out the answer (I think) by guess and check, but i would really like to UNDERSTAND why the answer is what it is. Thanks, here it is: Two runners approaching each other on a straight track have constant speeds of 4.50m/s and 3.50m/s, respectively, when they are 100m apart. How long will it take for the runners to meet, and at what position will they meet if they maintain these speeds? It kinds looks like so: Runner A 4.5m/s--->>>______________100m______________<<<----Runner B 3.5m/s
Consider one of the runners standing still and the other approaching at 8 m/s. Calculate the time they meet.

Now consider both runners approaching with their given speeds and calculate the distance travelled by each in that time.

Sep 11th 2008, 06:06 AM   #3

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 Originally Posted by tcrocRE hey, i am gonna just write the problem just as it is in the textbook. I have thought about this one for a while, and yet have no idea where to start. I figured out the answer (I think) by guess and check, but i would really like to UNDERSTAND why the answer is what it is. Thanks, here it is: Two runners approaching each other on a straight track have constant speeds of 4.50m/s and 3.50m/s, respectively, when they are 100m apart. How long will it take for the runners to meet, and at what position will they meet if they maintain these speeds? It kinds looks like so: Runner A 4.5m/s--->>>______________100m______________<<<----Runner B 3.5m/s
Another way. Say runner A starts at the origin and let x be the position that they meet at. Then we know
$\displaystyle x = 4.5t$
and
$\displaystyle 100 - x = 3.5t$

Solve the system of equations for t and x.

-Dan
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