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## Homework Statement

The U of a 4kg object is given by U= 3x^2-x^3 for x smaller than or equal to 3. U is equal to 0 for all numbers larger than 3. If the total energy of the object is 12J what is its speed at 2 meters?

## Homework Equations

K(x)=E-u(x) E=mechanical energy

K(x)=1/2mv^2

## The Attempt at a Solution

It seems like i would plug 2 into the u equation and get the value of U at that particular moment, then use the E, which would be 12??? and subtract the U from it.

I ended up with the answer 2m/s but I could have sworn doing something with integrals for a problem similar to this so it seems like this would be too easy.

Any advice?

thanks