What a learner can do afterwards
- Uses P = Fv for a vehicle held at constant speed against a stated resistive force
- Moves between the work-over-time and force-times-velocity forms and says when each is easier
- Calculates efficiency as a ratio and names where the rest of the input went
1 · Read
Power tells how fast work gets done, not how much work there is in total. It is work divided by time, and the unit is the watt. One watt is one joule per second. A powerful engine and a weak one can shift the same load, but the powerful one finishes sooner.
For a push along the motion, instant power is force times speed. A cyclist holds 8 metres per second against 30 newtons of drag. Multiply 30 by 8 to get 240 watts. At steady speed the driving force matches the drag, so drag times speed gives the power.
No machine is perfect. Efficiency is useful output over total input, always below one. The rest is wasted, mostly as heat and sound. Muscles run near a quarter, and the rest warms the skin. Name where the waste went to turn a bare ratio into a diagnosis.
Work over time suits whole jobs with times and energies. Force times velocity suits a snapshot with known speed and thrust. Both are the same physics, since work is force times distance and distance over time is velocity.
Read power off work over time or force times speed, and score machines by useful output over input.
More to read
7.7 Power
Power tells you how fast work gets done, not how much work there is in total. It is work divided by time, and the unit is the watt, one joule per second. A powerful engine and a weak one can both lift the same load to the same height, but the powerful one finishes sooner. For a vehicle pushing with force F while moving at speed v, the instant power is P = Fv, as long as force and velocity point the same way. Hold a car at steady speed against drag and the engine power must exactly equal drag force times speed.
7.2 Kinetic Energy and the Work-Energy Theorem
The two faces of power suit different questions. Work over time handles whole jobs, like the average power of a lift over a full climb. Force times velocity handles a snapshot, like the power needed at top speed on a motorway. They are the same physics seen two ways, since work is force times distance and dividing by time turns distance over time into velocity. Pick the form that matches your data: times and energies point to W over t, while speeds and thrusts point to Fv. Check they agree whenever you can calculate both.
7.8 Work, Energy, and Power in Humans
No machine is perfect, and efficiency measures how close it gets. Divide the useful energy or power out by the total energy or power in, and you always get less than one. The missing share is not destroyed but wasted, usually scattered as heat and sound into the surroundings. Human bodies show the same story: muscles convert chemical energy with an efficiency near a quarter, and the rest warms the skin. Naming where the waste went, friction, drag, resistance heating, turns efficiency from a bare ratio into a diagnosis.
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2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.