Practice Efficiency in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

The ratio of useful output energy (or power) to total input energy, expressed as a percentage — always less than 100% due to energy losses.

What fraction of the energy you put in actually goes where you want it to go, rather than being wasted as heat.

Showing a random 20 of 50 problems.

Example 1

medium
A 60% efficient motor must deliver 1200 J of useful work. How much input energy is required?

Example 2

medium
An LED bulb is 35%35\% efficient and produces 77 W of visible light. Find the electrical power input.

Example 3

medium
A two-stage process: stage 1 is 60%60\% efficient, stage 2 is 70%70\% efficient. Find overall efficiency.

Example 4

medium
A motor delivers 720720 J of useful work and wastes 480480 J as heat. Find efficiency.

Example 5

easy
A machine uses 500 J500 \text{ J} of input energy and produces 350 J350 \text{ J} of useful output. What is its efficiency?

Example 6

easy
Two engines: A is 30% efficient, B is 45% efficient. Which wastes a larger fraction of input energy?

Example 7

medium
A coal power plant burns fuel releasing 1,000,000 J1{,}000{,}000 \text{ J} and generates 350,000 J350{,}000 \text{ J} of electrical energy. What is the efficiency?

Example 8

easy
Fill in the blank: efficiency = useful energy output / total energy input, expressed as a ______.

Example 9

challenge
A two-stage system: stage 1 is 80% efficient, stage 2 is 50% efficient. Find the overall efficiency.

Example 10

easy
Express an efficiency of 0.650.65 as a percentage.

Example 11

medium
A pulley system lifts a 4040 kg mass 33 m using 15001500 J of input work. Use g=10g = 10 m/s2^2. Find the efficiency.

Example 12

medium
An electric motor is 70%70\% efficient. It must do 14001400 J of useful work. How much input energy is required?

Example 13

challenge
A car gets 25%25\% engine efficiency, 90%90\% transmission efficiency, and 95%95\% wheel-to-road efficiency. Find overall efficiency.

Example 14

medium
An electric motor with 85%85\% efficiency lifts a 200 kg200 \text{ kg} load by 10 m10 \text{ m}. How much electrical energy is consumed? Use g=10 m/s2g = 10 \text{ m/s}^2.

Example 15

challenge
A 40% efficient engine produces 600 J of useful work each cycle. How much heat does it dump to the surroundings per cycle?

Example 16

medium
A crane lifts a 50 kg load 4 m (use g=10g = 10) using 2500 J of input energy. Find its efficiency.

Example 17

medium
A car engine takes in 50005000 J of chemical energy and converts 17501750 J to kinetic energy. Find efficiency.

Example 18

hard
A pulley system requires 500500 N of effort to lift a 15001500 N load through a height of 11 m using 55 m of rope movement. Find efficiency.

Example 19

medium
Two machines do the same 600 J of useful work. Machine X uses 800 J input, machine Y uses 1000 J. Which is more efficient?

Example 20

hard
A coal plant burns coal at 90009000 MJ/kg and is 33%33\% efficient. How much electrical energy does 11 tonne (10001000 kg) of coal produce?
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