A mechanical device performs useful work at a rate of $X$ and loses energy at a rate of $Y$. Which expression represents the efficiency of this device?
- A$\frac{X}{Y}$
- B$\frac{(X - Y)}{Y}$
- C$\frac{X}{(X + Y)}$
- D$\frac{(X - Y)}{(X + Y)}$
Physics 9702 · AS & A Level · Energy conservation