I have a problem I solved using kinematics/Newton's 2nd law.
It gives the mass of a walker as 55kg. It then says she starts from rest and walks 20m is 7s. It wants to know the horizontal force acting on her.
From kinematics for constant acceleration, I know $\vec{r}=\frac{a}{2}t^2\hat{i}$. Plugging in the known time and the known distance I solved for the acceleration and then I could get the force by multiplying the acceleration by the walker's mass. So I got the problem right... but then I got to wondering: Was there a way to do this problem using energy? I have in mind $\vec{F}\cdot\Delta\vec{r}=\Delta K$. I tried but I don't know the final velocity (from the given information).
Edit: I realized after looking at some of the feedback that I do know the final velocity (because the linear dependance of velocity on time means the average velocity must be half the final velocity). Therefore, you can see below, that I have posted the answer I was hoping to write back when I wished I knew the final velocity.