🔢 Problem 1: Basic Energy LossA person does 500 J of useful work. The body efficiency is 25%.
Find:
Total energy used
Energy lost
Solution:Efficiency formula:
\eta = \frac{W}{E_{in}}
[
E_{in} = \frac{W}{\eta} = \frac{500}{0.25} = 2000 , J
]
Energy loss:
[
E_{loss} = E_{in} - W = 2000 - 500 = 1500 , J
]
✅ Answer:
Total energy used = 2000 J
Energy lost = 1500 J
🔢 Problem 2: Lifting WorkA worker lifts a 20 kg object to a height of 1.5 m.
Efficiency of the body = 20%
Find:
Useful work done
Total energy used
Energy lost
Solution:Work done:
W = mgh
[
W = 20 \times 9.8 \times 1.5 = 294 , J
]
Total energy:
[
E_{in} = \frac{294}{0.20} = 1470 , J
]
Energy loss:
[
E_{loss} = 1470 - 294 = 1176 , J
]
✅ Answer:
Work done = 294 J
Energy used = 1470 J
Energy lost = 1176 J
🔢 Problem 3: Power and Energy LossA person works at a power of 100 W for 10 minutes.
Efficiency = 25%
Find:
Total energy used
Useful work
Energy loss
Solution:Time:
[
t = 10 \times 60 = 600 , s
]
Total energy:
E = P \times t
[
E_{in} = 100 \times 600 = 60,000 , J
]
Useful work:
[
W = 0.25 \times 60,000 = 15,000 , J
]
Energy loss:
[
E_{loss} = 60,000 - 15,000 = 45,000 , J
]
✅ Answer:
Total energy = 60,000 J
Useful work = 15,000 J
Energy lost = 45,000 J
🔢 Problem 4: Running ActivityA runner uses 300,000 J of energy. Only 30% is converted into motion.
Find:
Useful work
Energy loss
Solution:[
W = 0.30 \times 300,000 = 90,000 , J
]
[
E_{loss} = 300,000 - 90,000 = 210,000 , J
]
✅ Answer:
Useful work = 90,000 J
Energy lost = 210,000 J
🔢 Problem 5: Efficiency CalculationA person uses 2500 J of energy and performs 500 J of work.
Find efficiency.
Solution:[
\eta = \frac{500}{2500} \times 100 = 20%
]
✅ Answer:
Efficiency = 20%
🔢 Problem 6: Continuous Work (Advanced)A laborer works for 2 hours at 150 W power.
Efficiency = 25%
Find:
Total energy used
Useful work
Energy loss
Solution:Time:
[
t = 2 \times 3600 = 7200 , s
]
Total energy:
[
E_{in} = 150 \times 7200 = 1,080,000 , J
]
Useful work:
[
W = 0.25 \times 1,080,000 = 270,000 , J
]
Energy loss:
[
E_{loss} = 1,080,000 - 270,000 = 810,000 , J
]
✅ Answer:
Total energy = 1,080,000 J
Useful work = 270,000 J
Energy lost = 810,000 J
📌 Final InsightFrom all problems, we observe:
Human efficiency is low (20–30%)
Most energy (70–80%) is lost
Loss mainly occurs as:
Heat
Internal resistance
Metabolic activity