Chapter 8 - Potential Energy and Conservation of Energy - Problems - Page 209: 77a

$F_x = 4N $

$U(x) = -W_x$ Potential Energy of Work Done by horizontal force ($F_x$) through a distance in the x-direction ($d_x$) $U(x) = -F_x d_x$ $U(2) = -(F_x \times 2m)$ $-8J = -(F_x \times 2m)$ $8J = F_x \times 2m$ $F_x =\frac{8J}{2m} $ $F_x = 4N $