A block and tackle is used to raise a load of 250N through a vertical distance of 30m. What is the efficiency…

Analyze the question: A load of 250 N is lifted through 30 m. Some energy is wasted overcoming friction (1500 J). We need the efficiency of the pulley system.

Concept: Efficiency = (useful work output / total work input) × 100%

• Useful work output = work done on the load = load × distance
• Work input = useful work + work done against friction

Solution:

1. Useful work output = 250 N × 30 m = 7500 J

2. Work done against friction = 1500 J

3. Total work input = 7500 + 1500 = 9000 J

4. Efficiency = (7500 / 9000) × 100% = 0.8333 × 100% = 83.3%

Answer: C. 83.3%

Tip: In efficiency questions involving friction, remember that the work input is not just the useful work. Always add the wasted work (friction) to the useful work to get the total input:

Efficiency = useful work / (useful work + wasted work)

A common mistake is dividing by 7500 − 1500 = 6000 J, which gives 125%. Efficiency can never exceed 100%, so use that as a quick check on your answer.