W = F · d
W = F d cos θ
Where θ is the angle that separates the vectors. The second form of the equation is the expanded form of the "dot product" in the first equation. In physics, the dot product "a · b" (read "a dot b") can be rewritten as "a b cos θ".
Work is a transfer of energy; if W is positive, there is a transfer of energy to the system, and if W is negative there is a transfer of energy from the system.
- Serway and Beichner, Physics for Scientists and Engineers, Fifth Edition