A block and tackle pulley system has a movable block with a mass of 10 kg. The effort applied is 70 N. Calculate the efficiency of the system.

The load is the weight of the $10 \text{ kg}$ mass.

$$L = m \times g$$ $$L = 10 \text{ kg} \times 10 \text{ m/s}^2$$ $$L = 100 \text{ N}$$

The velocity ratio for a block and tackle system is equal to the number of rope segments supporting the movable block. In this diagram, there are 6 segments of rope supporting the lower block (which carries the load).

$$VR = 6$$

Mechanical advantage is the ratio of the load to the effort.

$$MA = \frac{\text{Load}}{\text{Effort}}$$ $$MA = \frac{100 \text{ N}}{70 \text{ N}}$$ $$MA = \frac{10}{7}$$

Efficiency is the ratio of mechanical advantage to velocity ratio, expressed as a percentage.

$$\eta = \frac{MA}{VR} \times 100\%$$ $$\eta = \frac{\frac{10}{7}}{6} \times 100\%$$ $$\eta = \frac{10}{7 \times 6} \times 100\%$$ $$\eta = \frac{10}{42} \times 100\%$$ $$\eta = \frac{5}{21} \times 100\%$$ $$\eta \approx 0.238095 \times 100\%$$ \eta \approx \boxed{\text{23.81%}}