Physics Help Forum Finding final speed without initial speed

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Jan 25th 2013, 07:56 PM #1 Junior Member   Join Date: Jan 2013 Posts: 1 Finding final speed without initial speed My homework problem reads: a truck slows down uniformly with acceleration of -5.1m/s2 for 4.20s, making skid marks 58.7m long the end of a tree. With what speed does the truck then strike the tree? I know that v-v0= at which would give me -21.42m/s And I also know v2-v02= 2ad which would give me -598.74m/s. I tried taking the square root of a positive 598.74 thinking it may would give me the 21.42 answer but they don't seem to come out even. I've just hit a road block with this one. Thanks! *I do realize the problem does contradict itself in that it reads 'acceleration' and then has a negative number. I think it was just a typo in the problem.
Jan 25th 2013, 09:05 PM   #2
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 Originally Posted by KARH4 My homework problem reads: a truck slows down uniformly with acceleration of -5.1m/s2 for 4.20s, making skid marks 58.7m long the end of a tree. With what speed does the truck then strike the tree? I know that v-v0= at which would give me -21.42m/s
yes, this is correct
 And I also know v2-v02= 2ad which would give me -598.74m/s.
yes
 I tried taking the square root of a positive 598.74 thinking it may would give me the 21.42 answer but they don't seem to come out even. I've just hit a road block with this one. Thanks!
you could rewrite that equation as v_0^2 -v^2 = 598.74 but when you take the square root of both sides, sqrt {v_0^2 -v^2} is not the same as v - v_0, so you shouldn't expect to get the same result. Instead, try using one of the other kinematic equations, like d = v_0t + 1/2at^2, to solve for v_0 directly, then use your first equation to solve for v.
 *I do realize the problem does contradict itself in that it reads 'acceleration' and then has a negative number. I think it was just a typo in the problem.
there is no contradiction, the minus sign implies that the car is slowing down, not speeding up.

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