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## Homework Statement

A 3.00kg object has velocity 7.00j m/s. Then, a total force 12.0i N acts on the object for 5.00s.

Find the final kinetic energy from 1/2mvf^2 = 1/2mvf . vf

## Homework Equations

## The Attempt at a Solution

i got 1/2 * 3 * (20i+7j)(20i+7j)

then 600j^2 + 210ij + 73.5j^2

but the answer is 673.5 J, why?

since j.j =i.i=1 and i.j=0?

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