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Old Jan 25th 2013, 07:56 PM   #1
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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.
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Old Jan 25th 2013, 09:05 PM   #2
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Originally Posted by KARH4 View Post
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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