alleryStart learning →A motor receives 100 J; 30 J becomes heat, with no other losses. Mechanical output?
- 30 J
- 130 J
- 70 J
- 100 J
Show answer and solution
Correct answer: C. 70 J
Conservation of energy leaves 100 − 30 = 70 J as mechanical output.
ICSE Class 8 Work, Energy and Power — practice questions — page 2
Explore 42 free Work, Energy and Power questions in ICSE Class 8 Physics. Open each question or exercise to read its content, reference answers and available explanation.
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Showing questions 31–42 of 42.
A motor receives 100 J; 30 J becomes heat, with no other losses. Mechanical output?Read the answer and worked solution →A machine does 900 J of work in 1.5 minutes. Average power?Read the answer and worked solution →Motors A and B do equal work in 2 s and 6 s. Average power ratio A:B?Read the answer and worked solution →A device runs at unchanged power for three times as long. Energy transferred?Read the answer and worked solution →A 50 W device runs for 3 minutes at that power. Energy transferred?Read the answer and worked solution →Mass becomes three times as large and speed halves. Kinetic energy becomes?Read the answer and worked solution →Bodies of 4 kg and 1 kg have equal kinetic energy. Speed ratio heavy:light?Read the answer and worked solution →Unequal masses have equal GPE above ground. Dropped from rest without air, faster at ground?Read the answer and worked solution →A body drops from rest through 5 m without air resistance. Final speed? g = 10 m/s².Read the answer and worked solution →A ball has KE 20 J and GPE 30 J. With no losses, GPE falls to 10 J. New KE?Read the answer and worked solution →A gives 100 W for 5 s; B gives 40 W for 20 s. Which transfers more energy?Read the answer and worked solution →B steadily lifts twice A's mass through half A's height in twice A's time. Useful lifting power B:A?Read the answer and worked solution →
Worked practice questions
A machine does 900 J of work in 1.5 minutes. Average power?
- 600 W
- 10 W
- 90 W
- 15 W
Show answer and solution
Correct answer: B. 10 W
The time is 90 seconds, so average power is 900 ÷ 90 = 10 W.
Motors A and B do equal work in 2 s and 6 s. Average power ratio A:B?
- 2:3
- 3:1
- 1:3
- 1:1
Show answer and solution
Correct answer: B. 3:1
For equal work, power is inversely proportional to time, giving A:B = 6:2 = 3:1.
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