Average Power, Machines, and Efficiency
Average power is Delta W/Delta t; efficiency is useful output divided by input and cannot exceed one for an ordinary passive conversion accounting.
Why this shows up in the exam
Motors, lifts, and pumps · Human work-rate estimates · Generators and efficiency
Learn the idea
Average power is energy transferred per elapsed time. Machines are compared by how quickly they deliver useful work, while efficiency records what fraction of input becomes useful output.
🧠 Memory hook: Useful out over total in, over the same interval.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- P_avg = Delta W/Delta t — average power
- eta = P_useful/P_input — power efficiency
- eta = E_useful/E_input — energy efficiency
How to approach it
- 1Identify useful output work
- 2Divide by the stated time
- 3Include losses before applying efficiency
Common slip-ups that cost marks
- •Mixing energy with power
- •Using percent efficiency without dividing by 100
- •Ignoring frictional load in motor problems
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A 2 kg body speeds up from 3 m/s to 7 m/s. What net work is done on it?
More from Work, Energy and Power
Work and its calculation
Work is the energy transferred by a force acting over a distance, and can be calculated using the dot product, area under a force-displacement graph, or for variable and constant forces.
Conservation of energy
The law of conservation of energy states that energy cannot be created or destroyed, only transformed, including cases with energy loss and efficiency considerations.
Work-energy theorem
The work-energy theorem states that the net work done on an object equals the change in its kinetic energy, and applies to both constant and variable forces.
Conservative and non-conservative forces
Conservative forces, like gravity and spring force, conserve mechanical energy, while non-conservative forces, like friction, dissipate energy as heat.
Elastic potential energy
Elastic potential energy is the energy stored in a stretched or compressed spring, proportional to the square of its displacement.
Power
Power is the rate at which work is done or energy is transferred, and can be calculated as the product of force and velocity at any instant.